The Promotion of Poly(Methyl Methacrylate

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1997

The Promotion of Poly(Methyl Methacrylate) Depolymerization by The Promotion of Poly(Methyl Methacrylate) Depolymerization by

Metal-Based Initiators Metal-Based Initiators

Jason andrew Macko College of William & Mary - Arts & Sciences

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Recommended Citation Recommended Citation Macko, Jason andrew, "The Promotion of Poly(Methyl Methacrylate) Depolymerization by Metal-Based Initiators" (1997). Dissertations, Theses, and Masters Projects. Paper 1539626107. https://dx.doi.org/doi:10.21220/s2-r0c4-3n85

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THE PROMOTION OF POLY (METHYL METHACRYLATE)

DEPOLYMERIZATION BY

METAL-BASED INITIATORS

A Thesis

Presented to

The Faculty of the Department of Chemistry

The College of William and Mary in Virginia

In Partial Fulfillment

Of the Requirements for the Degree of

Master of Arts

by

Jason Andrew Macko

1997

APPROVAL SHEET

This thesis is submitted in partial fulfillment of the

requirements for the degree of

Master of Arts

Author

Approved, August 8 , 1997

...K.obert A? Orwoll

Robert D. Pike

William H. Starnes, Jr.

TABLE OF CONTENTS

Acknowledgements vii

List of Tables viii

List of Figures ix

List of Schemes xi

Abstract xiii

I. Introduction

A. General 2

B. Thermal Degradation of PMMA 6

C. PMMA Thermal Degradation : Modification by High

Energy Radiation 17

D. PMMA Thermal Degradation : Promotion by Metal-Based

Compounds 22

II. Experimental

A. Instrumentation

1. Gas Chromatography/Mass Spectroscopy (GC/MS) 4

2. Gas Chromatography (GC) 40

3. Infrared Spectroscopy (IR) 40

4. Melting Point Determinations 41

5. Nuclear Magnetic Resonance (NMR) 41

6 . Thermogravimetric Analysis (TGA) 41

B. Materials

1. Chemical Suppliers 42

C. Methods

1. Synthesis of Copper(II) p -Toluenesulfonate 43

2. Synthesis of Copper(II) Methacrylate 43

3. Synthesis of Copper(II) Benzoate 44

4. Synthesis of Copper(II) Malonate 44

5. Synthesis of [Cu(0 2 GCH3 )(PPh3 )3] 44

6 . Synthesis of Copper(II) Glutarate 45

7. Synthesis of [Cu(0 2 0 CH3)] 46

8 . Preparation of Dimethyl 2,2-Dimethylglutarate 46

9. Preparation of Dimethyl 2,2,4-Trimethylglutarate 47

10. Preparation of Dimethyl 2,2,4,4-

Tetramethyl glutarate 52

D. Depolymerization Experiments 63iv

E. Analysis of Products of Depolymerization Experiments by GC

1. Standardization Curves for Depolymerization

Product 65

2. Analysis of Depolymerization Products 65

F. Preparation of Polymer Films for IR Analysis 70

G. Reactions of Dimethyl 2,2,4,4-Tetramethylglutarate

1. Experimental Procedure 71

2. Recrystallization of Benzophenone for GC/MS

Analysis 71

H. Analysis of Depolymerization Reactions by

Thermogravimetric Analysis 73

III. Results and Discussion

A. General 74

B. Analysis of Gas Chromatography Results 76

C. Depolymerization Results 78

D. Factors Influencing Depolymerization 80

E. Copper(I) Acetate vs. Copper(II) Acetate 84

F. Role of Free Radicals 87

G. Copper(II) Acetate Coordination to PMMA 92

H. Model Compound Studies

I. Mechanism of Copper(II) Acetate-Promoted

Depolymerization

J. Solvent Effects

IV. Conclusions 108

References 110

ACKNOWLEDGEMENTS

I would first like to thank The College of William and Mary for it’s

commitment to educating undergraduate students in general, and myself in

particular. I know of very few colleges which could rival this institution

for price and quality. Secondly, I wish to acknowledge the Department of

Chemistry for it’s outstanding faculty and the personal interest taken in the

education of chemistry students, both undergraduate and graduate. Those

faculty members I would like to personally recognize are Dr. Pike and Dr.

Starnes, both of which were invaluable to my maturing as a chemistry

student and research scientist. The lessons I have learned from these

gentlemen within the classroom and the laboratory during my time at

William and Mary will stay with me forever.

List of Tables

1. Transition Metal Halides and Effects When Combined With

PMMA 24

2. Volatile Products Evolved During the Degradation of

Co(acac)3 -PMMA Blends 29

3. Stock Solutions 6 6

4. Standards and Individual Component Concentrations 6 6

5. Detection Limits for Major Depolymerization Components 68

6 . Component Concentrations in GC Injections 6 8

7. GC Parameters 6 8

8 . Results of Depolymerization Experiments 79

viii

List of Figures

1. Production of Plastics from 1935 to 1985 3

2. Major Anticipated Disproportionation Products 16

3. Disproportionation Products Formed After Addition of

Monomer 16

4.TVA Curves for PMMA Samples 18

5. Methyl Methacrylate Yields in Degradation of PMMA andSilver(I) Acetate 38

6 . ^H-NMR Spectrum of Dimethyl 2,2-Dimethylglutarate 48

7. l^C-NMR Spectrum of Dimethyl 2,2-Dimethylglutarate 49

8 . FTIR Spectrum of Dimethyl 2,2-Dimethylglutarate 50

9. GC/MS of Dimethyl 2,2-Dimethylglutarate 51

10. ^H-NMR Spectrum of Dimethyl 2,2,4-Trimethylglutarate 53

1 l.^C-NM R Spectrum of Dimethyl 2,2,4-Trimethylglutarate 54

12. FTIR Spectrum of Dimethyl 2,2,4-Trimethylglutarate 55

13. GC/MS of Dimethyl 2,2,4-Trimethylglutarate 56

14. GC/MS of Dimethyl 2,2,4,4-Tetramethylglutarate/

Dimethyl 2,2,4-Trimethylglutarate Mixture 57

15. ^H-NMR Spectrum of Dimethyl 2,2,4,4-Tetramethylglutarate 59

16. l^C-NMR Spectrum of Dimethyl 2,2,4,4-Tetramethylglutarate 60

ix

17. FTIR Spectrum of Dimethyl 2,2,4,4-Tetramethylglutarate 61

18. GC/MS of Dimethyl 2,2,4,4-Tetramethylglutarate 62

19. Depolymerization Apparatus 64

20. Calibration Curves for Major Depolymerization Components 67

21. Apparatus for Model Compound Studies 72

22. Typical Gas Chromatograph for Depolymerization Experiment

Analysis 77

23. Results of the Pressure Study Experiments 81

24. Results of the Time Study Experiments 81

25. Results of the Concentration Study Experiments 83

26. Thermal Decomposition of Copper(II) Acetate. 90

27. Coordination of Copper(II) Acetate to PMMA Carbonyl

Oxygens 93

28. GC/MS of Model Compound Experiment Products 95

29. FTIR Spectrum of PMMA Film - Virgin Polymer 95

30. FTIR Spectrum of PMMA/Copper(II) 2-Ethylhexanoate

Film 97

31. FTIR Spectrum of Copper(II) 2-Ethylhexanoate 99

32. Results of TGA Experiments 106

x

List of Schemes

1. Synthesis of Methyl Methacrylate 9

2. PMMA Thermal Degradation from Unsaturated Chain Ends 11

3. Macroradicals Both Leading to MMA Production 12

4. Production of a Stable Methallyl-Terminated PMMA 13

5. Homolytic Scission of a Methoxycarbonyl Side Group 14

6 . PMMA Side-Group Scission and Subsequent Radical Chemistry 20

7. Postulated Mechanism for Generation of Methyl Halide

for MnCl2 25

8 . Proposed Interaction of C1CI3 with PMMA 27

9. Production of a Possible PMMA Backbone

Irregularity by Co(acac)2 31

10. Proposed Interaction of SnCl4 with PMMA 32

11. Thermolysis of Phenyltin Chlorides with Formation

of Corresponding Tin-Based Radicals 34

12. Proposed Interaction of PhSnCl3 with PMMA 35

13. Silver(I) Acetate Degradation and Subsequent Radical

Chemistry 37

14. Radical Quenching by Copper(II) Acetate 89

15. Copper-Assisted Depolymerization Quenching 94

xi

16. Copper(II) Acetate Coordination to PMMA and Subsequent

Reactions 102

17. Methyl Radical Reactions with PMMA Chain Ends 103

18. Hydrogen Abstraction from the PMMA Chain 104i

19. Radical Scavenging by Benzophenone 106

xii

ABSTRACT

In recent decades, the disposal of plastic wastes has become an

enormous problem. Poly(methyl methacrylate) (PMMA) has a natural

tendency to degrade almost quantitatively to monomer unassisted at

~300°C. The depolymerization of PMMA in solution in the presence of

various metal-based additives was studied. Copper(II) carboxylates were

found to be effective depolymerization promoters at 200°C. A mechanism

for this reaction is proposed.

THE PROMOTION OF POLY (METHYL METHACRYLATE)

DEPOLYMERIZATION BY

METAL-BASED INITIATORS

I. INTRODUCTION2

A. General

Over the last 50 years, plastic usage in the United States has grown at a

rapid pace. In 1935, the U.S. produced slightly more than 95 million lb of

plastics. By 1987, according to the Society of the Plastics Industry,

polymer output, including plastics, synthetic fibers, adhesives, coatings, and

synthetic rubbers, reached as high as 57 billion lb (Figure 1). This

translates into an average annual growth between 2.5 and 3 times as fast as

the nation’s gross national product. Total business in polymers surged to

more than $70 billion in the U.S. in 1987. According to the U.S.

Department of Commerce, that surge made the plastics and resin industry

the 14th largest manufacturing enterprise in the country. The major

markets for polymers in 1987 were packaging (30%), building and

construction (23%), and consumer products, including electrical equipment

and furnishings (20%). * But this increase in usage has inevitably fostered

an increase in accumulated waste. Plastics represented 11.5% of the waste

stream in 1986. Concerns over this problem have resulted in mandatory

recycling laws and demands for degradable polymers. The common plastic

waste contains 61% polyethylene, 25% poly(vinyl chloride), 11%

polystyrene, and 3% other plastics .2

Home scrap and immediate scrap from polymer manufacturing and

9 999 9 \ 9 9 9 9 9*9999* * * * * * *

S M 99 9 9* \ \ 0 \ \ X n v t f t f f 9 9

1935 1940 1945 1950 1955 I960 1967 1971 1974 1977 1981 ' 1985

Figure 1

Production of Plastics from 1935 to 1985

4

compounding can be mixed and reformulated into materials of downgraded

specification and product application as a form of recycling. Therefore,

post-consumer waste in the form of final products is the most difficult

material in the recycling process to separate and prepare because plastics

are formulated, compounded, and fabricated with a range of chemical and

physical properties.^ The conversion of a polymeric structure to produce

chemicals and energy is one possible recycling route. The recycling process

involved in treating poly(ethylene terephthalate) (PET), as in beverage

bottles, is another possibility. This process involves the depolymerization

of PET to the constituent monomers of dimethyl terephthalate and ethylene

glycol with supercritical methanol at a temperature of 573 K and pressures

from 2 to 23 MPa.^ However, few polymers can be recycled effectively in

this manner. Incineration is a common form of waste disposal. Yet the

production and emission of noxious pollutants into the atmosphere, the loss

of useful forms of carbon and nitrogen, and the cost involved have

increasingly made this method impractical. Oxidation and photo-oxidation

of plastics have also been considered, yet these alternatives see little use.

A third, more environmentally sound, possibility is biodegradation. All

living systems are capable of degrading natural polymers to some extent,

some examples of which are regenerated cellulose (cellophane), and chitin.

These polymers may be degraded by organisms through mechanisms

involving hydrolysis and oxidation. But enzymatic degradation requires a

5

close fit of a flexible polymer chain to the active site of the enzyme. Those

polymers with a high crystallinity are degraded more slowly, if at all.

Also, the presence of additives may affect biodegradation rates. Plasticizers

increase the rate, while inorganic additives may act as microbicides

retarding biodegradation.^ Some synthetic biodegradable plastics include

polycaprolactone, polyacetic acid, and polyanhydride.

What makes synthetic polymers attractive is what also makes them

difficult to recycle; their stability. Polymers are manufactured in large

quantities simply because they are so stable. However, a failure of the

mechanical properties of some polymer product does not translate into a

failure at the molecular level. The overall structure remains intact, which

means that the polymer continues to resist degradation. This is the reason

why many researchers have turned their attention to the degradation of

synthetic polymers.

B. Thermal Degradation of PMMA6

A polymer molecule can be broken into fragments in three general

ways: by random chain scission, by which bonds are broken in a random

fashion (Reaction 1);

I I 33S“C •f - C H j C H j - h — 3— C H j = C H j + C j - C * , alkanes and aikenes ( 1 )

'n 450 C

by weak link degradation, whereby chain scissions occur at weak links

distributed throughout the chain at random, which are more easily broken

than the regular bonds (Reaction 2);

ci a

- c h 2 - c h - c h 2- c - c h 2-

c h 2 - c h - c h 2 - c h 2ci

- c h 2 - c h - c h 2- c - c h ; xHCl

c h 2- c h - c h 2- c h -

- CH2- C H = C H - CH- CH2 -

txHCl | + Internal double bond ( 2 )

7and by depoiymerization, in which chain scission is the reverse of addition

polymerization and monomer units are released successively from a chain

end or at a weak link of a homopolymer in an “unzipping” process

(Reaction 3).

c h 3 CH3

4 -C H ,-c 4 - ■ A „ > C H ,= C (3)f I " 300 C 2 I

^ C\O OCH3 o o c h 3

Poly(methyl methacrylate) Methyl methacrylate

These types of reactions may occur separately or in conjunction with one

another.

Poly(methyl methacrylate) (PMMA) has a very strong tendency to

unzip to produce an almost quantitative yield of monomer upon thermal

degradation due to a very long zip length. The zip length for a

depolymerization is defined as the ratio average of the number of

depolymerization steps to the number of chain-transfer and termination

reactions. Thus, the zip length is the average number of monomer units

8removed due to each radical formed via initiation or intermolecular

transfer. When a thermal degradation reaction produces a chain fragment

shorter than the zip length, the entire chain fragment depolymerizes to

monomer before chain transfer occurs. In this case, no small fragments are

left which decrease the average molecular weight of the residue. When the

zip length is small, random chain scission predominates. The ratio for

PMMA has been calculated from kinetic studies to be approximately 60.^

Poly(methyl methacrylate) is a clear, colorless transparent plastic.

Injection molded acrylic articles include automotive lenses, reflective

devices, instrument and appliance covers, optical equipment, and home

furnishings. Acrylic sheeting is used for signs, glazing (as in aircraft

windows), furniture, partitions, and lighting-fixture diffusers. It also has

reasonable resistance to alkalis and acids, so it finds many uses in

laboratories (especially in biochemistry). About 0.5 billion lbs of acrylic

plastics were sold in 1982. The monomer building block, methyl

methacrylate (MMA), is made by heating acetone cyanohydrin (from the

addition of hydrocyanic acid to acetone) with sulfuric acid to form

methacrylamide sulfate .The latter is reacted (without isolation) with water

and methanol to give MMA which is then polymerized by a free radical

process in bulk or suspension polymerization (Scheme 1).

For additive-free PMMA of moderate molecular weight, a two-step

thermal decomposition is typical. The first stage consists of degradation

initiated at an unsaturated chain end. This process occurs at relatively low

9

CO

Xu -

coXu8 = 0U

Xu

Xo5

oC/D

CMas

f*Dn:U-

CM

EZ -U=0uIICNECu

oc/i

sa>-GoC/D

ECO.

zuI

■ u -

ICO

Xu

CO

X■u

10temperatures (Scheme 2). As the temperature is increased, the polymer

begins to undergo random chain scission, the second stage of degradation.

The products formed from the saturated polymer main chain scission are,

however, a subject of debate. In Scheme 3, a random chain scission is

indicated as the initiation step, producing two macroradicals. These radicals

both give rise to unzipping. Kashiwagi et al.^ have proposed an alternative

mechanism wherein, rather than both initial macroradicals unzipping, one

macroradical eliminates a methoxycarbonyl radical, producing a stable

methallyl-terminated PMMA, as shown in Scheme 4.

The assumption in both of the above mechanisms is that backbone chain

scission occurs initially to produce radical species. L. E. Manring^ has

suggested that main-chain scissions are kinetically inhibited relative to side-

group or chain-end scissions due to the efficient recombination of caged

radicals. He proposed an alternative mechanism whereby random chain

scission is initiated by homolytic scission of a methoxycarbonyl side-group

(Scheme 5). Using extreme values^ for the heats of formation of the

COOCH3 radical, Manring suggested that the (CH2 )(CH3)C-COOCH3 bond

is either considerably weaker or only slightly stronger than the main chain

linkages. The kinetic factor thought to favor side-group over main-chain

scission is the faster diffusion out of the cage of the monomeric radical,

relative to that of a polymeric radical. Support for this mechanism is

dependent on the details of formation and the subsequent reactions of small

Scheme 2

c h 3 c h 3

< 250°C ^CH2^ C - C H = C H ------------------------- ~ C H 2 - + • C - C H = C HI I I I

n^ S ^ c\O o O O O o O O

I I I ICH, CH, CH, CH,

CH, CH, CH, CH,

V I'- C CH, - C™ ^ ,, ^ < 250°C I IIc h 2 — c — c h 2 — c -► —c h 2 - c - + c h 2~ c

_ ^ c \O O O O o o oI I I ICH3 c h 3 c h 3 c h 3

CH, CH3

I I*vvv CH2 — C — CH2 • CH -----------------------No Degradation

12

Scheme 3

+ MMA + MMA

13

Scheme 4

/ /

o

f

/

Q

+ MMA

/

o o

f

/

o

+ *co 2c h 3

14

Scheme 5

15radicals formed during degradation. Radicals may be lost from the system

in a number of ways, but most importantly in this case, through second-

order reactions which generate products through disproportionation. Major

products of the disproportionation of anticipated radicals are indicated in

Figure 2. The expected radicals formed when a monomer binds one of the

radicals in Figure 2 may also disproportionate to form six other significant

products (Figure 3). All of these structures have been detected and

support the mechanism in Scheme 5.

The primary evidence used by Kashiwagi et al. to support their

mechanisms was the observation of trace products attributed to the

degradation of the radical produced from side-group scission (Reaction 4).

oII

c h 3c o c h 3 + co2

R -H

CHj,

or c h 3o + CO (4)

However, they observed much more carbon monoxide and carbon dioxide

than that predicted by either their mechanism or that of Manring.

16

c o 2c h 3 c o 2c h 3 c o 2c h 3

Figure 2

Major Anticipated Disproportionation Products

h 3c o 2c

o 2c h 3

h 3c o 2c

c o 2c h 3

h 3c o 2c

o 2c h 3

h 3c o 2c

c o 2c h 3

h 3c o 2c

c o 2c h 3

h 3c o 2c

c o 2c h 3

Figure 3

Disproportionation Products Formed After Addition o f Monomer

17C. PMMA Thermal Degradation : Modification by High Energy

Radiation

The effects of high energy radiation on PMMA have been investigated.

Ultraviolet and gamma radiation have sufficient energy to break chemical

bonds and act as initiators for complex chemical reactions. M. A. Rauf and

I. C. McNeill^ studied the effects of gamma irradiation on PMMA. When a

polymer is subjected to gamma irradiation at normal temperatures, the

changes induced by the radiation may have very significant effects on the

thermal stability and the way in which the polymer breaks down at elevated

temperatures. Little is known about the changes in the polymer structure

upon irradiation which produces structural defects. However, these defects

can have very important effects on the stability and polymer decomposition

mechanisms, even at defect levels which are difficult to detect

spectroscopically. In order to characterize thermal degradation products,

Thermal Volatilization Analysis (TVA) was employed in this research.

TVA allows volatile product fractions to be analyzed. Rauf and McNeill

found that the thermal degradation of PMMA is significantly altered by

prior exposure of the polymer to gamma irradiation. As the polymer is

subjected to increasing doses of gamma rays, the shape and characteristics

of the TVA trace are changed as indicated in Figure 4. The initial peak was

attributed to trapped volatile material, mainly the monomer. The polymer

exists as a glass at lower temperatures; therefore, as the temperature was

100 200 300 400 500

temperature, °C

Figure 4

TVA Curves for PMMA Samples (vacuum, 10°C/min). (a) Unirradiated; (b) After Initial Dose (70 kGy);

(c) After 4.3 Times Initial Dose (300 kGy).

19

increased, the polymer is expected to soften, whereby any trapped

molecules may escape. An increase in gamma dose changed the relative

areas of the degradation peaks in the TVA trace. The first of these (Tmax

about 325°C) gradually decreased and merged at higher doses with the

second peak. The temperatures of the rate maxima in the TVA traces were

not significantly altered by irradiation.

The product volatility indicated in the TVA traces of all of the

irradiated samples of the polymer suggested that the monomer may be the

main product in every case, but the presence of other materials was also

indicated. A Sub-Ambient Thermal Volatilization Analysis (SATVA) of

condensible volatile products resulting from thermal decomposition

indicated carbon dioxide to be the main component. Other products found

to vary with irradiation dose included methyl formate at lower doses and

isobutene and methanol at higher doses. The non-condensible thermal

degradation products were methane and carbon monoxide. The formation

of these products, apart from isobutene, was explained by scissions at the

side-groups. It was concluded that methyl formate is the primary

degradation product formed by gamma irradiation.

Raman and ESR investigations of gamma-irradiated samples of

PMMA showed the presence of main chain radicals formed by side group

scission. The proposed mechanism of degradation is shown in Scheme 6 .

Scission of an entire side-group produces a radical which, by hydrogen

abstraction, may form methyl formate. The polymer chain radical thus

20

Scheme

— CH2 -

CH, CH,

C - CH2 - C—I I

c o 2c h 3 c o 2c h 3

CH, CH,- c o 2c h 3

CH, CH, - O

c o 2c h 3

c o 2c h 3

Hydrogen Abstraction

CO + • OCHc h c o 2c h 3

Hydrogen Abstraction

HOCH,

21

produced may give rise to subsequent backbone scission. The unsaturated

end structure formed is believed to provide the source for the minor

product isobutene. Scission of the side group can also occur to give

methoxy radicals which may then abstract hydrogen to produce methanol.

D. PMMA Thermal Degradation : Promotion by Metal-Based

Compounds

Depolymerization may be induced in the presence of suitable

promoters, especially metal-based compounds. C. A. Wilkie and I. C.

McNeill^" 12 have independently studied the effects of transition metal

halides on PMMA. These compounds included MnCl2 , C1CI3 , NiCl2 ,

Q 1CI2 , FeCl2 , and FeCl3 (studied by Wilkie using TGA-FTIR data), and

ZnBr2 and CoBr2 , studied by McNeill using TVA, IR, visible

spectroscopy, and mass spectrometry. None of these additives, however,

catalyzed the generation of monomer as the primary product. The principal

effect of these additives was to cause the thermal degradation of PMMA to

become substantially more complex than simple depolymerization to

monomer. The range of products obtained included methane, carbon

monoxide, methanol, methyl halide, and MMA. Through the use of IR

spectroscopy, McNeill postulated that the initial step was the coordination

of the metal ion to the carbonyl oxygens of the polymer as indicated by 1 -

23Transition metal compounds were found to react initially in one of three

ways: coordination to PMMA with elimination of methyl halide,

coordination without elimination of methyl halide, or no coordination.

Some transition metal halides stabilized the polymer against thermal

degradation while others had a destabilizing effect, the results of which are

summarized in Table 1.

The proposed mechanism for metal chloride exchange with the polymer

is indicated in Reaction 5.

c h 3 c h 3 CH3 CH3

CH2 c CH2 C — *■ CH2-----c --------CH2-------c

c - o c h 3 c - o c h 3 ------- ► C = O ' c = o + 2 CH3x (5 )

ov , o o o/ M \ M

X X

The postulated mechanism for the generation of methyl halide for MnCl2

was an exception, as indicated in Scheme 7. Whenever methyl halide was

generated, destabilization of the polymer was observed.

The gaseous products were identical for ZnBr2 and MnCl2 reactions,

yet the order of appearance of these products was not the same. With

ZnBr2 , the initial product was methyl bromide, released at 160°C. On the

24

Additive Stabilizing DestabilizingGeneration of Methyl Halide Coordination

ZnBr2 ------- yes yes yesCoBr2 ------- yes yes yesFeCI2 yes yes yesFeCI3 ------- yes yes yesMnCI2 yes yes yesCrCI3 yes ------- no yesNiCI2 yes ------- no yesCuCI2 yes ------- no yesCuCI ~ — ~ no no

Table 1

Transition Metal Halides and Effects When Combined With PMMA

25

Scheme 7

c h 3 c h , c h 3 c h 3 c h 3

— C H j— C — CH2— C — — nCI; p- — CH2 — C — CHj— C — ♦ CHj ♦ — CH2— <j!— CH2— <j!— ♦ -OCH,

r r r r -/V rL. T T_ CH. CH3a I c h 3o h

C = 0 C = 0 H

c h 3 c h , c h 3 3 C»3

j-H3 ^H 3

Degradation Products (monomer)

j:H3•CHj — C— CHj—

CO

r ??CH.

= o

Degndation Products (monomer)

fHj P*»CHj— y — CH2— <j:~~

r o r o

r 9CH,

f Ha < 3■CHj— y — CHj— y —

r° r°i. rCHj H

jIH j CH3

C H j — y — C H j — y —

<jr=o y= o

T TCH3 MnCl

Degradation Products (monomer and add)

Hj p 3CHj — <j:— CHj— y —

<j:=o y= o

?CHj?M n + *C1

1"HC1

3CHj— y—CHj— (j:—

r° r°M nOj *

Degndation Products

T ?CH, Mn

? T'f °“ f' ( j :— C H j— y — CHj

C H j C H j

26other hand, methyl chloride from MnCl2 was not observed until

temperatures of above 220°C were reached. The initial reaction in this case

was the formation of monomer.

Three metal chlorides, CrCl3 , NiCl2 , and CuCl2 , had a stabilizing

effect on the polymer. For the CrCtyPMMA system, Wilkie claimed that

CrCl3 was thermally reduced to CrCl2 with the formation of a chlorine

atom. Methyl chloride was not observed and instead, Wilkie postulated the

transfer of a methyl group from the ester to the main chain, as indicated in

Scheme 8 .

NiCl2 and CuCl2 were postulated to stabilize the polymer against

thermal decomposition as a result of the metal-halogen bond strengths.

These bonds were assumed to be stronger than the other coordinating

additives and therefore, methyl halide would not be eliminated as readily

and decomposition would occur at a slower rate. Another transition metal

chloride additive considered was CuCl. This compound, unlike the others,

did not coordinate at all. Wilkie theorized that, because the carbonyl

oxygen of PMMA is a hard base and Cu(I) is a soft acid, coordination

would not be favored. As a result, CuCl had no effect on the rate of the

degradation process.

Non-halogenated transition metal additives have also been combined

with PMMA. I. C. McNeill and J. J. Liggat^ studied PMMA degradation

with cobalt(III) acetylacetonate [Co(acac)3]. Using TVA, IR, and UV

27

Scheme 8

-H Q

CH3 c h 3i i

— c h 2 - c - c h — c-~

Cl2Cr. A JCH3 \ = 0o No '

Ic h 3

c h 3 c h 3 c h 3

CKCr

C H ,-C — C H — C~

/ o\A>c= o

CH,

28techniques, McNeill and Liggat observed four distinct stages of degradation

of Co(acac)3 with PMMA. Stage one commenced around 135°C and was

comprised of mostly monomer with traces of non-condensible gases.

Monomer was also the primary product during stage two commencing at

190°C and during stage three at 270°C. At stage four, around 360-380°C,

no monomer was observed but the production of non-condensible gases was

abundant. Overall products are indicated in Table 2. The general effect of

this additive was to induce the low-temperature evolution of monomer, but

also to stabilize PMMA toward decomposition in the normal

depolymerization temperature region.

McNeill and Liggat speculated that at the lowest temperatures,

degradation to monomer commenced as a result of decomposition of the pi-

complex formed between the acetylacetonate (acac) salt and the unsaturated

chain ends in the polymer. They believed that Co(acac)3 does not directly

complex with the ester groups in PMMA but that its presence, even in

small amounts, greatly modified the degradation route of the polymer. The

thermal decomposition of Co(acac)3 was believed to produce Co(acac)2

and acac radicals as shown in Reaction 6 .

C o(acac)3 C o(acac)2 + acac- (6)

29

Up to end o f Stage 2 Up to end o f Stage 4

Acetylacetone M ethano l (above 200°C)

Acetylacetone Methanol Carbon dioxide A ketene Methyl acetate Cyclic anhydride C arbon monoxide Isobutene (trace) Dimethyl ether (trace)

Table 2

Volatile Products Evolved During the Degradation of Co(acac)3-PMMA Blends, In Addition to Monomer

30Either of these species could potentially initiate depolymerization.The

Co(acac)2 was speculated to complex with ester groups, promoting side

group scission. The metal complex may also decompose to form cobalt

carboxylate structures which are introduced into the polymer as backbone

irregularities which may prevent unzipping (Scheme 9). The production of

anhydride rings in the backbone, as detected by IR, was also postulated as a

reason for the high-temperature stabilization observed.

The degradation of PMMA in the presence of some tin compounds was

studied by C. A. Wilkie. 14-15 These compounds included SnCl4 , PhSnCl3 ,

Ph2 SnCl2 , Ph3SnCl, and Ph4Sn. SnCl4 undergoes virtually no degradation

when heated alone at 375°C, but is completely consumed when similarly

heated in the presence of PMMA. The FTIR and GC/MS data demonstrated

that methyl methacrylate was not the primary product of degradation, but

rather that the degradation was a complicated process producing water,

methacrylic acid, isobutyric acid, and methyltin trichloride as major

products. Minor products included MMA and methyl isobutyrate. When the

amount of SnCl4 was increased, both the monomer and methyl isobutyrate

were reduced to trace amounts. Wilkie suggested that the interaction begins

with the coordination of the additive with the carbonyl oxygens of the

polymer, followed by reactions such as those proposed in Scheme 10.

Coordination to the carbonyl oxygens of PMMA was the proposed initial

step in this reaction, the evidence for which was

Scheme 9

32

Scheme 10

CH,

■CH2— C — ♦ • SnClj

c= oOCH,

CH,

■CH,— C«

L o llo

I-

CHjSnClj

cn,■ CH,— C —

— OH

33the production of CH3CI.

Next to be considered were PhSnCl3 , Ph2 SnCl2 , and Ph3 SnCl. The

condensible gases produced by PMMA in the presence of these three

compounds consisted of a variety of methylated benzenes, carbon dioxide,

methyl chloride, hydrogen chloride, water, monomer, methyl isobutyrate,

methacrylic acid, and isobutyric acid. The likely initial reaction in the

thermolysis of the phenyltin chlorides is the cleavage of a phenyl group

with the formation of the corresponding tin-based radicals (Scheme 11).

Wilkie suggested that these radicals, as they were present in the greatest

concentrations, would be the species most likely to react with the polymer.

The proposed mechanisms of reaction of PhSnCl3 and Ph2 SnCl2 with

PMMA were similar, giving similar products (Scheme 12). The reaction

involving Ph3 SnCl was somewhat different. In this case, triphenyltin

chloride decomposed to give much less SnCl2 than was obtained from the

other systems. The final compound studied by Wilkie was Ph4 Sn. The five

major products observed were benzene, toluene, MMA, methyl isobutyrate,

and methyl 2-methylbutyrate. Also detected were 11 minor products, as

well as many trace compounds. Wilkie suggested that with Ph4 Sn, the

initial step of the additive degradation was the cleavage of a phenyl radical.

But the degradation process, he surmised, was a mutually assisted process,

with PMMA thermally degrading independently to produce radicals. These

species then reacted with the tin-based radicals to initiate complicated

34

Scheme 11

PhSnCl3

Ph2SnCl2

PhjSnCl

• Ph + SnCl3

• Ph + PhSnCl2

•Ph + Ph2SnCl

35

Scheme 12

CH, CH,

•CH,— C — ♦ -P h ------► — CH,— C — ♦ PhCH,

=o C — O

A--t—

1“ °OCH,

HydrogenAbstraction

PhSnQ ,

PhSnCl,

•P h .

PhSitCl,

2 SnCl,

•Ph ♦

• a +

PhH

•Ph «•

SnCl,

• S nO ,

P hS nQ ,

SnCl,

♦ SnCl4

36chemistry giving rise to the large number of products observed.

Metal acetates represent a little-studied class of compounds which may

affect the thermal stability of PMMA. Jamieson and McNeill^ examined

the effects of silver acetate on the degradation of PMMA. The degradation

behavior of the polymer at silver acetate/polymer ratios of 1:1, 1:5, and

1:10 were studied using TVA as the principal technique. Degradation of the

metal salt itself was also examined and found to give a variety of products.

This degradation was best explained by a series of reactions resulting from

an initial cleavage of CH3CO2 radicals and silver atoms (Scheme 13).

When silver acetate was present during PMMA degradation, a severe

destabilization resulted, producing a high yield of monomer at

temperatures as low as 200°C. This particular additive has a very different

effect upon PMMA than any of the previously discussed additives. In the

present case, methyl methacrylate was the only major product of the

degradation reaction. The most important volatile product was acetic acid,

although acetic anhydride, water, carbon dioxide, and ketene were also

present. The products of degradation of these mixtures were identical to

those of the constituent materials when degraded separately. The yield of

monomer was greatly dependent on the degree of mixing of the metal salt

and the polymer as indicated in Figure 5. McNeill explained this effect on

the basis of diffusion of radicals from silver acetate decomposition into the

polymer phase where they initiate chain scission and depolymerization.

Thus, this metal additive served principally as a source of radical initiators.

37

Scheme 13

• CH3 + • COOAg

CH3COOAg------- ► CH3COO- + Ag

+ CH3COO

CH3COOH + CH2COOAg

+ COOAg

CH2= C = 0 + AgO- C30 2 + Ag20 + H20

•COOAg --------► COz + Ag

2- CH2COOAg --------► CH3COOH + C + C 02

CH2====C===0 --------► dimer, etc

C + AgO- --------► CO + Ag

2* CH3 ■ ■' ^ C2H$

2 Ag

38

40C«owVa2®’>*<22

o 20 60 80 12040 100

t im e , min

Figure 5

Methyl Methacrylate Yields in Degradation o f PMMA andSilver(I) Acetate at 207°C (100 mg o f each) in unmixed

(squares) and mixed (circles) condition.

39Only a small number of radicals would be required to initiate rapid

depolymerization of PMMA. In this case, the degradation process was not

altered fundamentally because the effect of radical attack was to cause chain

scission and to permit the normal depolymerization to monomer to occur.

Is is apparent from these studies that a majority of the additives studied

only served to initiate complicated decomposition reactions when heated in

the presence of PMMA. The exception was silver acetate. This compound

allowed PMMA to undergo depolymerization to monomer. The additive

merely lowered the temperature at which the monomer was evolved.

Because the depolymerization reaction was so clean in the presence of

silver acetate, the present research project focused on the depolymerization

of PMMA in the presence of those metal-based compounds that most

resembled silver acetate.

II. EXPERIMENTAL40

A. Instrumentation

1. Gas Chromatographv/Mass Spectroscopy (GC/MS)

A Hewlett Packard 5890 Series II GC with a crosslinked methyl

siloxane 12m x 0.2mm x capillary column was used coupled with a Hewlett

Packard 5971A Mass Selective Detector. Sample data were analyzed by

using Hewlett Packard G1034B software for the MS ChemStation (DOS

series). All injection volumes were one 1 pL.

2. Gas Chromatography (GC)

A Hewlett Packard 5890A gas chromatograph was utilized with a 6 ’ x 4

mm glass, Varian 3700 column. The packing was 3% OV-1 on 80/100

Chromosrob W-HP. Data were plotted on a Hewlett Packard 3396A

integrator. All injection volumes were one 1 pL.

3. Infrared Spectroscopy (IR)

All samples were examined on a Perkin-Elmer 1600 Series FTIR

instrument. Liquid samples were prepared as thin films on NaCl, solid

samples as KBr pellets, and polymer films were scanned as is.41

4. Melting Point Determinations

Melting points were determined using a UniMelt (Thomas Hoover)

capillary melting point apparatus.

5. Nuclear Magnetic Resonance (NMR)

The NMR spectra were acquired using a General Electric QE-300

spectrometer. Data processing was performed using Tecmag MacNMR 5.4

software. Chemical shifts are reported in ppm ( 6 ) with Me4 Si as an

internal reference ( 6 = 0 .0 0 ).

6. Thermogravimetric Analysis (TGA)

All experiments were conducted using the Seiko SSC 5040 thermal

analysis system. This included a TG/DTA 200 simultaneous

thermogravimetric / differential thermal analyzer with version 2 .0 system

software. All samples were pre-ground powders. Sample masses varied

from 15 to 20 mg. The flow rate for nitrogen was 50 mL/min.

B. Materials42

1. Chemical Suppliers

Aldrich supplied the poly(methyl methacrylate) which had a molecular

weight of -120,000 with less than 5.0% toluene added as a stabilizer. Other

reagents from Aldrich included: p-toluenesulfonic acid monohydrate, 99%;

malonic acid, 99%; glutaric acid, 99%; triphenylphosphine, 99%;

dimethylglutaric acid, 98+%; iodomethane, 99.5%; lithium

diisopropylamide-THF, 1.5M solution in cyclohexane; benzophenone, 99%;

and methyl benzoate, 99%. Those reagents supplied by Fisher Scientific

Company included: benzoic acid; cupric carbonate; ammonium chloride;

and sodium bicarbonate. Mallinckrodt supplied the copper(II) acetate-fÔ.

Acetic anhydride was distilled previously and tetrahydrofuran (THF) was

freshly distilled from sodium benzophenone ketyl. Copper powder was

purified by etching with nitric acid followed by washings with water,

acetone and ether.

C. Methods43

1. Synthesis of CopperdI) v -Toluenesulfonate

In a 250 mL beaker with a stir bar, Q 1CO3 (3.50 g, 28.3 mmol) was

dissolved in water (100 mL). p -Toluenesulfonic acid (10.30 g, 54.1 mmol)

was then added and the solution allowed to react for 1 h. The resulting

suspension was filtered and the blue product solution heated to a slurry on

a hot plate to remove excess water. The slurry was allowed to evaporate to

dryness overnight, resulting in a product mass of 8.39 g, 18.9 mmol (70%

yield).

2* Synthesis of CopperdI) Methacrylate

In a 250 mL beaker with a stir bar, CUCO3 (5.00 g, 40.4 mmol) was

dissolved in water (100 mL). Methacrylic acid (7.00 g, 81.3 mmol) was

then added and the solution allowed to react for 24 h. The resulting

suspension was filtered and the blue product solution heated to a slurry on

a hot plate to remove excess water. The slurry was dried in a vacuum oven

at 60°C overnight, resulting in a product mass of 3.34 g, 14.3 mmol (35%

yield).

3. Synthesis of Copper(II) Benzoate44

In a 250 mL beaker with a stir bar, C11CO3 (3.30 g, 26.7 mmol) was

dissolved in water (75 mL). Benzoic acid (6.60 g, 54.1 mmol) was

dissolved in 95% ethanol (75 mL) and added to the C11CO3 solution. The

resulting mixture was allowed to react for 24 h. A blue slush resulted

which was dissolved in acetone. The solvent was removed under vacuum.

The wet blue product was then dried in a vacuum oven overnight at 60°C

resulting in a product mass of 3.87 g, 12.6 mmol (47% yield).

4. Synthesis of CopperdI) Malonate

In a 250 mL beaker with a stir bar, CUCO3 (5.00 g, 40.4 mmol) was

dissolved in water (100 mL). Malonic acid (4.21 g, 40.4 mmol) was then

added and the solution allowed to react for 48 h. The resulting suspension

was filtered and the blue product solution heated to a slurry on a hot plate

to remove excess water. The slurry was dried in a vacuum oven at 60°C

overnight, resulting in a product mass of 2.01 g, 12.1 mmol (30% yield).

5. Synthesis of rCu«>>CCHaUPPIn^ l l 7

Triphenylphosphine (2.35 g, 8.96 mmol) was dissolved in hot 95%

ethanol (40 mL) in a 250 mL round-bottom flask with a stir bar.

45Cu(0 2 GCH3)2 *H2 0 was added to the solution which was then stirred under

reflux for 15 min. During this time, the blue-green color discharged. The

resulting nearly colorless solution was then evaporated under vacuum. The

remaining pale-yellow oil was triturated with pentane causing the oil to

crystallize. The crystals were then suspended in diethyl ether and collected

on a fritted funnel. The crystals were dissolved in a minimum of CH2 CI2

and precipitated into pentane. The white crystalline product was then

filtered on a fritted funnel and dried on a vacuum line, resulting in a

product mass of 0.29 g, 0.3 mmol (15% yield).

6. Synthesis of Copper(II) Glutarate

Cupric carbonate (6.14 g, 50.0 mmol) and glutaric acid (7.00 g, 50.0

mmol) were added to water (100 mL) in a 250 mL round-bottom flask

with a stir bar and refluxed for 24 h. In order to remove any unreacted

Q 1CO3 and CuO impurity, the light-green solid was found to be insoluble

in boiling water. It was washed with three 30 mL portions each of 100%

ethanol, acetone, and diethyl ether. The product obtained (7.17 g) was

dried on a vacuum line. Because traces of CUCO3 and CuO were probably

present in the product obtained, a percent yield was not determined.

7«Synthesis of fCufO^CCH^) ! ^46

Into a 250 mL round-bottom flask with a stir bar were placed

anhydrous CH3 CN (100 mL), Cu(0 2 CCH3)2 *H2 0 (4.00 g, 22.0 mmol),

and copper turnings (10.00 g, 157.5 mmol). To these components was

added a 4:1 mixture of acetic acid and acetic anhydride (25.0 mL). The

mixture was stirred at 25°C for 48 h under a nitrogen atmosphere. A color

change from the green of the copper(II) reagent to the clear, pale-yellow.

The solution was extracted out of the flask by syringe into degassed,

anhydrous diethyl ether (200 mL) under nitrogen. The mother liquor was

then removed via syringe and the solid was washed with another 200 mL

portion of ether using the syringe technique. After the crystals settled, the

ether was decanted off and the product was dried using a stream of

nitrogen into the flask. The very pale-green crystals were then collected

and dried on a vacuum line, resulting in a product mass of 2.07 g, 11.4

mmol (52% yield).

8. Preparation of Dimethyl 2.2-Dimethvlfflutarate

Into a 250 mL round-bottom flask with a stir bar was added 2,2 -

dimethylglutaric acid (30.00 g, 187.3 mmol) and methanol (200 mL).

Once the acid dissolved, the solution was cooled to 0°C with an ice bath, at

47

which point sulfuric acid (10.0 mL) was added dropwise. The mixture was

then refluxed for 24 h. The resulting solution was extracted with three

portions (100 mL each) of diethyl ether. The ether layer was washed with

NaH(X>3 until the acid was neutralized. The solution was dried over

MgSC>4 and the solvent removed under vacuum. A vacuum distillation

(113°C, 5 mm Hg) resulted in 22.27 g, 118.4 mmol colorless liquid

dimethyl 2,2-dimethylglutarate (63%yield). *H-NMR (CDCI3 ): 6 1.2 (s,

6 H), 1.9 (t, J = 8.2, 2H), 2.2 (t, J = 8.3, 2H), 3.6 (s, 6H). 13C-NMR

(CDCI3): 8 24.6, 29.7, 35.0, 41.4, 50.9, 51.2, 173.1, 176.9. The JH-NMR,

3C-NMR, IR, and GC/MS spectra are shown in Figures 6 - 9 .

9. Preparation of Dimethyl 2»2»4-Trimethvlglutarate

Into a 500 mL round-bottom flask with a stir bar and cooled to -78^C

were placed THF (350 mL), 1.5M LDA (58.0 mL, 87.0 mmol), and

dimethyl 2,2-dimethylglutarate (15.00 g, 79.8 mmol). These components

were stirred under nitrogen for 30 min. Methyl iodide (14.9 mL, 239.0

mmol) was dissolved in THF (50 mL) and injected into the reaction flask.

The flask was then warmed to 0°C and the reaction mixture allowed to stir

for 12 h. Next, the solution was added to aqueous ammonium chloride (300

48

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Figure 9

GC/MS of Dimethyl 2,2-Dimethylglutarate

52

mL). Three extractions (50 mL each) were carried out with diethyl ether.

The ether was then washed using saturated sodium bicarbonate and water.

After drying over MgSC>4 anc* evaporating over ether, the crude product

was vacuum distilled (109°C, 5 mm Hg) yielding 7.91 g, 39.1 mmol crude

dimethyl 2,2,4-trimethylglutarate liquid (49% yield). The GC/MS results

indicated that there was a roughly 5% impurity of starting material in the

product obtained (Figure 10). lH-NMR(CDCl3 ): 6 1.1 (m, J = 15.8, 9H),

1.5 (dd, J = 3.7, 1H), 2.0 (dd, J = 8.7, 1H), 2.4 (m, J = 2.7, 1H), 3.6 (s,

6 H). 13C-NMR(CDCl3 ): 6 19.0, 25.6, 26.0, 36.4, 41.9, 44.1, 51.1, 51.2,

176.6, 177.3. The *H-NMR, 13C-NMR, IR, and GC/MS spectra are shown

in Figures 10-13.

10. Preparation of Dimethyl 2.2.4.4-Tetramethvlglutarate

A second methylation was carried out on the product above using the

procedure outlined above. The raw product of this methylation was

subjected to GC/MS which indicated that the product ratio was

approximately 67% dimethyl 2,2,4,4-tetramethylglutarate and 33%

dimethyl 2,2,4-trimethylglutarate as shown in Figure 14. To drive the

product to completion, a third methylation reaction was undertaken. After

the crude product was isolated, it was vacuum distilled (103°C, 5mm Hg)

53

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GC/MS o f Dimethyl 2,2,4-Trimethylglutarate

57

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GC/MS of Dimethyl 2,2,4,4-Tetramethylglutarate/ Dimethyl 2,2,4-Trimethylglutarate Mixture

resulting in 3.81 g, 17.6 mmol of colorless crystalline dimethyl 2,2,4,4-

tetramethylglutarate (43% yield). GC/MS indicated that the product was

99+% pure. *H-NMR (CDCI3): 5 1.1 (s,12H), 2.0 (s, 2H), 3.6 (s, 6H).

13C-NMR(CDCl3): 6 26.1, 41.8, 49.8, 51.3, 178.2. The !H-NMR, 13C-

NMR, IR, and GC/MS spectra are shown in Figures 15-18.

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1 5 0 0 0 0 -15 7

100000 -

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20 80 120 1 4 0 1 6 0 1 8 06 0 1004 0

Figure 18

GC/MS of Dimethyl 2,2,4,4-Tetramethylglutarate

D. Depolymerization Experiments63

The reactor design is illustrated in Figure 19. Fora typical

experiment, a mixture consisting of benzophenone (2 0 .0 0 g), poly(methyl

methacrylate) (2.00 g), and sample additive (0.20 g) was used. In some

cases, the concentration of a sample additive was varied. The typical

depolymerization was carried out for 3 h under the reduced pressure of

approximately 65 mm Hg. Both the times and the pressures were varied for

certain experiments. Reactants for all experiments were placed in a 100 mL

round-bottom flask along with a stir bar which was then immersed in a

magnetically stirred silicone oil bath held at 200°C. Power to a nichrome

wire coil within the bath was supplied by a Variac. In every instance,

volatile products were collected on the walls of a condenser which was kept

at 0°C with recirculated ice water. Condensed products were then trapped

in a flask immersed in liquid nitrogen mounted at the end of the apparatus.

Upon completing a reaction, the Firestone valve was switched to isolate the

vacuum pump from the reaction system. This valve was then opened to a

tank of nitrogen gas which brought the system back to atmospheric

pressure. This method prevented atmospheric water vapor from

condensing in the product flask.

64

Dep

olym

eriz

atio

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ppar

atus

65E. Analysis of Products of Depolymerization Experiments

by GC

1. Standardization Curves for Depolvmerization Product

A set of standards was needed to characterize the components of the

depolymerization products. From the two stock solutions in Table 3, seven

standards with varying concentrations of methyl methacrylate, toluene,

benzophenone, methyl benzoate (an internal standard), and acetone were

mixed together and injected into the gas chromatography column (Table 4).

Peak areas were used to produce the calibration curves in Figure 20. Two

sets of standards were created. Products analyzed by the second standard

set are indicated (Table 8 ) by an asterisk. The detection limits for both sets

of standards are indicated in Table 5.

2. Analysis of Depolvmerization Products

From the calibration curves for the major components of a

depolymerization experiment, reliable measurements of the product

concentrations could be obtained. After diluting a sample as shown in

Table 6 , 1 pL was injected into the GC. GC parameters are listed in Table

7. From the peak areas of the depolymerization components and the

area of the internal standard, a ratio of product to internal standard was

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Figure 20

Calibration Curves for Major

Depolymerization Components

Calibration CurveMethyl Methacrylate

< 3< 2

1CO0)u .< 00 10 20 30 40 50 60 70 80

% MMA per Depolymerization Sample

Calibration CurveToluene

^ 0.5

I 0.4

$ 0 .3<DS 0.23 -

0.1oHca03

■ l —< 3020 400 10% Toluene per Depolymerization Sample

Calibration CurveBenzophenone

nj 0.8| 0.7 ^ 0.6 c 0.5 c 0.4I 0-3o 0.2NSi 0.1

(003 0 40 60 80 10020

% Benzophenone per Depoly. Sample

6 8

Standard Group MMA % Tol. % Bnzph. %

Group 1 1.00:50 standard 16.77798 0.804212 1.79670310.00:50 standard 150.6756 7.994666 17.95813

Group 2 *1.00:50 standard 15.31998 0.855738 unreliable*1.67:50 standard . . — — ------ — 0.52133*10.00:50 standard 148.5309 7.985664 6.059988

Table 5

Detection Limits for Major Depolymerization Components

Component gram s total grams

Unknown 0.05 0.05

Methyl Benzoate 0.015Acetone 0.185 0.2

Acetone 0.75 0.75

Table 6

Component Concentrations in GC Injections

GC Parameters

oven temp 35 Cinjector temp 200 Cdetector temp 250 Cinitial 5 min @ 35 Crate 10 min to 200 Cfinal 30 min @ 200 C

Table 7

GC Parameters

69determined. This ratio, when applied to the slope of the calibration curves,

gave the percent of components (x-axis) within the total depolymerization

sample.

F. Preparation of Polymer Films for IR Analysis70

Two polymer solutions were prepared: PMMA (1.0 g) in ethyl acetate

(20 mL) and PMMA (1.0 g) and copper(II) 2-ethylhexanoate (0.1 g) in

ethyl acetate (20 mL). The solutions were coated on the surfaces of clean

4”x4” glass plates. The plates were covered by watch glasses and solutions

allowed to slowly evaporate in air at room temperature. Once dry, the

films were peeled up and the clearer sections analyzed by FTIR. Film

thicknesses were measured with a micrometer. The PMMA film was ~120

p,m and the PMMA/copper(II) 2-ethylhexanoate film was ~ 40 \im .

71G. Reactions of Dimethyl 2,2,4,4-Tetramethylglutarate

1. Experimental Procedure

Experiments were carried out in a pressure tube equipped with a stir

bar as illustrated in Figure 21. Into this tube were added benzophenone

(2.00 g), dimethyl 2,2 ,4,4-tetramethylglutarate (0.20 g), and

Cu(0 2 CCH3 )2 *H2 0 (0.020 g). Experiments were conducted under argon

atmosphere at 200°C for 3 - 24 h.

2. Recrvstallization of Benzophenone for GC/MS Analysis

It was necessary to remove as much benzophenone from the product as

possible to avoid overloading the detector of the GC/MS. The crude

product was dissolved in 5 ml of methanol and placed in a freezer for 24 h

during which much of the benzophenone crystallized from the solution.

The purified product solution was decanted from the solid benzophenone

and injected into the GC/MS.

72

Pressure Tuben

200°C Oil Bath

Heated by Nichrome Wire

o o Magnetic Stirrer

Figure 21

Apparatus for M odel Compound Studies

73H. Analysis of Depolymerization Reactions by

Thermogravimetric Analysis

Polymer/additive blends were placed into a liquid nitrogen “mortar”

and hand ground to powder form. For the experiments, a measured mass

of the reactants (between 0.150 g and 0.200 g) was added to the platinum

pan of the TGA. Each experiment was conducted with a nitrogen flow rate

of 50 mL/min. For the temperature study, PMMA and Cu(02CCH3)2*H20

(10 weight %) were pyrolyzed at temperatures of 200 to 275°C. The

heating program consisted of an initial 50°C/min increase to the desired

temperature. A 1 h hold followed, after which the temperature was

increased at the rate of 50°C/min up to 900°C in order to volatilize any

remaining residue from the pan. For the concentration study, PMMA and

Cu(0 2 CCH3 >2 *H2 0 were mixed. The Cu(02CCH3)2’H20 concentration

was varied from 2.5 to 35.0 weight %. The heating program was identical

to that of the temperature study with the exception that the temperature was

held at 200°C for all experiments.

i n . RESULTS AND DISCUSSION74

A. General

The purpose of this research was to investigate those metal-based

compounds that would promote PMMA depolymerization at moderate

temperatures. Most of the previous studies of the interaction of PMMA

with metal-based compounds conducted by other researchers were designed

to investigate compounds which would stabilize PMMA in high-

temperature systems. Therefore, the additives chosen in those instances

were not selected with PMMA destabilization in mind. The work of

Jamieson and McNeill^ was the exception. Their research illustrated that

silver(I) acetate is a depolymerization promoter.

In order to determine the extent of depolymerization promoted by a

particular metal-based compound, the apparatus shown in Figure 19 was

employed. It was necessary to develop a method whereby monomer

produced during a reaction could be separated from the reaction mixture

and isolated for characterization. The experiments were conducted at

200°C, well below the ~300°C ceiling temperature of PMMA.

Polymerization is a spontaneous process at this temperature. Therefore

monomer had to be continuously removed from the system to allow

depolymerization. Benzophenone was chosen as the solvent for all solution-

phase reactions because of its high boiling point (305°C) and tendency not

to volatilize into the product fraction even under reduced pressures. In a

control experiment containing only PMMA and benzophenone at 65 mm

Hg and 200°C, no measurable amounts of monomer or benzophenone were

detected. It is known that benzophenone may be reduced to a radical anion

in some solvents.^ Therefore, the possibility existed that benzophenone

might interfere with any radical chemistry that may have been occurring

during the depolymerization experiments.

B. Analysis of Gas Chromatography Results76

A GC trace of the condensable products of a depolymerization

experiment is shown in Figure 22. The chromatogram is typical for the

depolymerization products, showing peaks of methyl methacrylate, toluene,

methyl benzoate (internal standard), and on occasion, benzophenone. The

toluene peak was due to its presence as a stabilizer in PMMA (< 5%).

Benzophenone was sometimes present in the collection flask due to small

leaks in the air-tight glass joints which allowed air to be drawn through the

reaction system. This flow allowed small amounts of benzophenone to be

carried into the product flask.

A series of standards containing various concentrations of methyl

methacrylate, toluene, and benzophenone was prepared, each containing a

constant concentration of the internal standard. These standards were

injected into the GC to determine calibration curves for the products of

depolymerization experiments. After a GC trace was obtained, the peak

areas were entered into a spreadsheet and converted into concentrations.

Although acetic acid was detected by GC/MS in depolymerization products

from metal acetate promoters, it coeluted with the solvent in the analytical

chromatography.

77

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T OT A L A R E A - 5 . 7 3 9 7 E * 0 3 t l ' JL F A C T O R - 1 . OOQOEt - QO

Figure 22

Typical Gas Chromatograph for Depolymerization Experiment Analysis, (a) Acetone; (b) Methyl Methacrylate; (c) Toluene;

(d) Methyl Benzoate; (e) Benzophenone.

C. Depolymerization Results78

Some of the depolymerization promoters were purchased and used as

received, while others were synthesized. Thirty different experiments were

conducted using various compounds in either pure form or as mixtures.

The results of these experiments are summarized in Table 8 . After analysis

of product fractions from the depolymerization experiments, copper(II)

acetate was found to induce the largest percentage of PMMA

depolymerization (24.5 weight %). The other depolymerization promoters

responsible for greater than 10 weight % depolymerization included, in

decreasing order: copper(II) 2-ethylhexanoate, copper(II) propionate,

silver(I) acetate, copper(II) trimethylacetate, copper(II) benzoate,

copper(II) glutarate, and copper® acetate.

It was observed that those metal-based compounds with carboxylate

ligands were responsible for the most significant amounts of

depolymerization. However,the identity of the metal component was

equally important. As is shown in Table 8 , lead(II) acetate and tin(II)

acetate did not promote depolymerization to a great extent. Likewise,

copper metal and copper(II) salts with ligands other than carboxylate did

not promote significant depolymerization.

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D. Factors Influencing Depolymerization80

Factors such as reaction vessel pressure, reaction time, and additive

concentration were studied to determine their effects on monomer

production. In one study, PMMA and copper(II) acetate were allowed to

react in a 10:1 weight:weight ratio for three hours at 200°C. The reaction

pressure was varied from approximately 45 mm Hg to 85 mm Hg through

the use of a controlled vacuum leak. The results of this study are shown in

Figure 23. It was apparent that varied pressures within the range studied

had little effect on the product mass obtained. Therefore, small variations

in pressure probably did not contribute to experimental error.

A time study was conducted in which PMMA and copper(II) acetate, in

10:1 ratio, were reacted at 200°C under a reduced pressure of

approximately 65 mm Hg. The reaction durations were varied from one to

nine hours; the results are shown in Figure 24. As expected, longer

reaction times led to more monomer evolution up to a maximum of about

26% depolymerization. The rate of MMA production over time was not

linear, however, but followed a pattern of initial rapid evolution followed

by a gradual leveling by about the six-hour point. About one half of the

total monomer produced in nine hours was evolved during the first hour of

reaction.

In the concentration study, copper(II) acetate was added in weight

percent values ranging from 2.5 to 35.0 in reactions lasting three hours at

81

Pressure Study ProfilePMMA Degradation vs. Pressure

<! 29 'Q- 27 - •

CN 25 -•

o 23 - • •m m • . • •

£ 2 1 -■5 19 - • •

•

CD 17 -0 u ^ 15 -0

•

35 45 55 65 75 85 95Pressure (mm Hg)

F igure 23

Results o f the Pressure Study Experiments

Time Study ProfilePMMA Degradation vs. Time

2 35 | 30 0 ) 2 5

CM * 2 0“ -15“o 10Q.& 5 ^ o

0 2 4 6 8 10Time (h)

F igure 24

Results o f the Time Study Experiments

200°C under the reduced pressure of approximately 65 mm Hg. The

results of this study are shown in Figure 25. A non-linear increase in

monomer evolution with increased promoter was observed. However,

when the copper(II) acetate concentration reached approximately 15 weight

%, the addition of more additive decreased the monomer output. Thus it

appeared that depolymerization was inhibited at higher additive levels.

83

Concentration Study ProfilePMMA Degradation vs. Cone. Cu(OAc)2

^ 45 - S 40 -

35 <n 30 <5 25 “ -20 - ^>15 - 8 .10 - Q 5 '

0 -Io'*c

• •

1 III

5 10 15 20 25 30 35 40 % Weight Cu(OAc)2 vs. PMMA

Figure 25

Results of the Concentration Study Experiments

E. Copper® Acetate vs. Copper(II) Acetate84

To better understand how the oxidation state of copper affected PMMA

depolymerization, both copper® acetate and copper(II) acetate were

studied under identical conditions. Unsupported copper® acetate is

extremely air-sensitive and undergoes oxidation and disproportionation in

the presence of moist air as indicated by a change in color from white to

green. Therefore, copper® acetate was synthesized and purified

immediately before it was used in the depolymerization mixture.

Copper(H) acetate hydrate, on the other hand, is stable to moisture and was

used without prior purification. It was observed that copper(II) acetate

promoted depolymerization (24.5%) more effectively than did copper®

acetate (10.5%).

A phosphine-supported copper® acetate, [Cu(O2 0 CH3)(PPH3 )3] ^ ,

was prepared and studied on account of its higher stability. However, it was

found to be an ineffective depolymerization agent; no measurable monomer

product was obtained. Inhibition of the depolymerization reaction may

have resulted from increased stability of the copper salt due to a change in

the redox potential of copper® by the triphenylphosphine ligands. It is

also possible that the triphenylphosphine could have acted as a radical

scavenger, inhibiting the depolymerization process. In addition, the size of

the triphenylphosphine ligands may not have allowed the compound to

react as readily with the polymer.

85To determine the nature of the interaction of copper(I) and copper(II)

acetates with PMMA, it was necessary to consider how these salts thermally

degrade at 200°C (the experimental depolymerization temperature).

During the depolymerization experiments, the initial green color (due to

copper(U)) or the colorless mixture (due to copper(I)) transformed into

similar reddish-brown mixtures. Thus, copper metal appeared to be present

in both cases. JuddÔ previously observed that copper(II) acetate yields the

metal on thermal decomposition in an inert atmosphere. Cupric acetate

exists as a hydrated dimer with the metal atoms surrounded in an

approximately square plane by the four oxygens of the bridging acetate

groups shown as 42 1 .

Based upon thermal analysis data, Judd proposed a decomposition

mechanism wherein copper(II) acetate undergoes a redox reaction

involving the loss of acetic acid to form copper(I) acetate, followed by the

evolution of more acetic acid, carbon dioxide, hydrogen gas, carbon, and

h 3c c h 3

c c/ r \ / A

c c

h 3c c h 3

(4)

copper metal (Reaction 7). It86

Cu2( 0 2CCH3)4-2 H 20 ► Cu2( 0 2CCH3)4 + 2 H 20

Cu2( 0 2CCH3)4 ► Cu2( 0 2CCH3)2 + 2 CH3C 0 2H ( 7 )

Cu2( 0 2CCH3)2 ----- ► 2 Cu + CH3C 0 2H + C 0 2 + H2 + C

therefore seemed likely that the reduction of copper® and copper(II)

acetates to copper metal was occurring in the present system.

F. Role of Free Radicals87

Jamieson and McNeill^ suspected that the production of free radicals

from the thermal decomposition of silver(I) acetate and the subsequent

diffusion of these radicals into the polymer were responsible for the

induced depolymerization and monomer production observed. It was

possible that depolymerization was induced by the abstraction of hydrogen

from the polymer by radical species. The Judd mechanism, which allows

for the production of acetic acid and copper metal from the thermal

decomposition of copper(II) acetate, was probably operative in the present

depolymerization studies. The primary products of copper(II) acetate

decomposition are probably copper(I) acetate and the acetate radical,

according to Reaction (8 ). Homolysis of the

Cu(02CCH3)2 ► Cu(OzCCH3) + CH3C02 (8)

copper-acetate bond forms an organic radical species while reducing

copper(II) to copper(I). Subsequent homolysis of copper(I) acetate would

produce another acetate radical, thus giving a total of two acetate radicals

and one atom of copper metal. At low copper(II) acetate loadings, these

radicals would be produced at low concentrations. Thus, the chance that

radicals would encounter one another, after escape from the radical “cage”,

88before abstracting hydrogen from a PMMA chain, would be low.

Therefore, many free radicals would be available to diffuse into the

polymer and induce depolymerization. The addition of more radicals into

the system at higher copper(II) acetate loadings could cause quenching

reactions such as shown in Scheme 14. Copper(II) acetate induces

approximately 24.5% depolymerization while copper(I) acetate induces

approximately 10.5%. The redox homolysis of copper(II) acetate occurs at

a lower temperature than that for copper(I) acetate, possibly due to a

weaker coordinating bond than that of copper®. Therefore, this lower

activation barrier for copper(II) allows the release of radicals to be more

facile, producing more radicals than copper® at moderate temperatures.

Copper® acetate may be stabilized by the addition of more ligands (i.e.,

the carbonyl oxygens of PMMA). However, the production of copper metal

indicated that copper® acetate undergoes some homolysis as well.

PMMA certainly plays a role in the decomposition of copper(II) acetate

and, therefore, a role in the depolymerization mechanism itself. Previous

TGA results^ suggested that decomposition of copper(II) acetate is a two-

stage process, although DTA suggested that the process may be more

complicated as indicated in Figure 26. The first stage, attributed to loss of

water, occurred over the temperature range of 100-180°C. The second

stage commenced at approximately 200°C and proceeded slowly until

290°C when the reaction became extremely rapid, leading to a final mass

89

Scheme 14

c h 3 c h 3 c h 3 CH;

— CH2 - C - CH2 - G + Cu(OAc)2------------- -► — CH2 - C - C H = C12 - v . - \ . n 2

c o 2c h 3 c o 2c h 3 c o 2c h 3 <to2CH3

+ Cu(OAc) + HOAc

c h 3 c h 3 c h 3 c h 3

, -C- 'CH2- C - C H 2-C - + Cu(OAc)2 ► ~ C H 2- C - C H —C—OAcI I I I

c o 2c h 3 c o 2c h 3 c o 2c h 3 c o 2c h 3

Cu(OAc)

CH, CH3 CH3 CH3

G + Cu(OAc), — ► ~ c h , - < ! : - c h —CH2 - C - CH2 - c* + Cu(O Ac)2I I I Ic o 2c h 3 c o 2c h 3 c o 2c h 3 c o 2c h 3

Cu(OAc) + CH 3 CO 2

90

20

60

AT

iI

iUJtI

n mn......... 7 fin 300 *.00 S00Tem perature, *C

Figure 26

Theimal Decomposition of Copper(II) Acetate, (a) TGA curve; (b) DTA curve.

91loss of 70%. This weight loss was attributed to the formation of copper(I)

acetate and acetic acid. It is interesting to note that significant degradation

did not occur until the temperatures were well above that used in the

present depolymerization experiments (200°C). A conclusion which may

be drawn at this point is that PMMA was most likely assisting copper(II)

acetate decomposition into radical species that would not have otherwise

been produced, through the coordination of copper(II) to the carbonyl

groups of the polymer. PMMA may then scavenge radicals from the

radical cage produced by the induced decomposition of copper(II) acetate,

thereby inducing depolymerization.

G. Copper(II) Acetate Coordination to PMMA92

In McNeill’s studies^ of the interaction of transition metal halides with

PMMA, he determined that the metal centers of the additives initially

coordinated to the carbonyl groups of PMMA. A similar coordination

almost certainly occurred in the present experiments, as illustrated in

Figure 27. The carboxylate group of PMMA bears similar electronic

character to acetate ligand. It is very likely that this similarity leads to

facile coordination of copper(II) to PMMA. Once coordination occurs,

homolysis of acetate may be facilitated, producing acetate radicals. The

reduced copper(I) may remain coordinated to the PMMA. The reduction of

monomer yields at higher additive concentrations could be due to the

blocking of unzipping PMMA chains by the more strongly coordinated

copper® acetate. Copper® may scavenge the chain free radical and thus

become reduced to copper metal. However, because the copper is not

directly coordinated to a PMMA backbone site that transfers the chain free

radical, the coordination is unlikely to affect depolymerization in this

manner (Scheme 15).

Polymer films were made to better understand the mode of

coordination of copper(II) carboxylate to the polymer. One film was cast

from PMMA using ethyl acetate solvent and the other from PMMA and

copper®) 2-ethylhexanoate in a 10:1 weight:weight ratio. The IR spectra

of these films are shown in Figures 28 and 29. The saturated C-H stretches

93

CH, CH,

H 3CO

H ,C

N > o ^ C

o / \ oU ° 0c c

OCH,

CH,

CH,

CH, ~ C - CH,

H3C

H,C

o' ° >

(II)O t \ O

U O o ^ S c c

CH,

Figure 27

Coordination of Copper(II) Acetate to PMMA Carbonyl Oxygens

94

Scheme IS

CH, CH3

■ v ” Copper Metal

H3c o V

«, \ L S / \

i \ — *•

O /"OCH,

cI

h 3c

(I) Cu

Copper Metal

+ Quenched Depolymerization

Quenched Depolymerization

p e r k in e lp e r

1000sooo isoo2S004000 30003500

Figure 28

FTIR Spectrum o f PMMA Film - Virgin Polymer ( < 5 % Toluene)

P E R K IN ELP E R

20004000 30003500 1500 1000

Figure 29

FTIR Spectrum of PMMA/Copper(II) 2-Ethylhexanoate Film (10:1 w:w)

96( v - 2900 - 3000 cm" 1) and the carbonyl peaks (v = ~ 1600 cm~l) are

clearly visible in both spectra. From the IR spectrum of copper(II) 2-

ethylhexanoate itself (Figure 30), it is clear that the characteristic

frequencies of this compound were merely overlaid with those of the

PMMA. Therefore, no conclusive evidence for coordination was obtained.

97

P B tK I N EUCR

1—100020004000 3500 3000 2S00 1500

Figure 30

FTIR Spectrum of Copper(II) 2-Ethylhexanoate

H. Model Compound Studies98

A model compound for PMMA was synthesized in order to better

understand the nature of a possible coordination. This species, dimethyl

2,2,4,4-tetramethylglutarate (5), is essentially a dimer of methyl

methacrylate units.

Thermolysis reactions of 5 were conducted in the closed system of a

pressure tube under an inert atmosphere. Benzophenone, copper(II)

acetate, and 5 were added together in the same ratio as used in a typical

depolymerization experiment and heated to 200°C. Two experiments were

carried out: three hours and 24 hours. After removal of excess

benzophenone by recrystallization, the reaction mixtures were analyzed by

GC/MS as shown in Figure 31. The chromatograms indicated that no

model compound decomposition had occurred. Only benzophenone and 5

were detected. Nevertheless, the color of the reaction mixture changed

over the course of both experiments from green to reddish-brown,

indicating the presence of copper metal and, therefore, radical species.

However, it appears that the model compound was too stable to allow

carbon-carbon bond scission under these conditions. This result may

Abundance 1 6 0 0 0 0 0 -

TIC: MODCUOAC.D 10 35

1 4 0 0 0 0 0

1 2 0 0 0 0 0

5 . 3 31 0 0 0 0 0 0

8 0 0 0 0 0

6 0 0 0 0 0

4 0 0 0 0 0 -

2 0 0 0 0 0 -

2 . 0 0 4 . 0 0 6 . 0 0rime ->

Figure 31

GC/MS of Model Compound Experiment Products

100

indicate that, in typical depolymerization experiments, radical attack on the

polymer occurred at unsaturated chain ends. No such unsaturated sites were

present in the model compound. Also, if coordination did occur, the short

length of this compound may not have facilitated electron transfer in an

“unzipping” process.

I. Mechanism of Copper(II) Acetate-Promoted

Depolymerization

101

Copper(II) acetate may coordinate to the oxygens of the carboxylate

side-group of the polymer in a number of ways, as noted previously

(Figure 27). Upon coordination, the additive may degrade by the

mechanism proposed in Scheme 16. Copper(II) acetate first loses an acetate

radical, and in the process is reduced to copper(I). It is important to note

that the acetate radical itself is very unlikely to be the direct initiator of

radical chemistry. It is well-known^^ that this species very rapidly

decomposes (half-life of about 10~9) into carbon dioxide and methyl

radical, as indicated in Reaction (9). Therefore, any radical reactions

occurring are induced by the methyl radical. There are a number of sites in

PMMA which may be susceptible to attack by the methyl radical.

Unsaturated chain ends are probably most vulnerable to radical addition

(Scheme 17 A, B). As for hydrogen abstraction, the methylene of the

polymer backbone (Scheme 18 B) is most likely the most vulnerable

because a more stable secondary radical species is produced.

10 2

Scheme 16

c h 3 c h 3 c h 3 c h 3

I I I Ic c c c

h 3c o ^ o 5 ^ o c h 3 -------------► h 3c o ^ ^ O C H ,

p / \ o / \> ° ° 4 . ^ /

h 3c ' c h 3 cI

H,C

0 ^ 1c ► CO, + -CH,

CH,

c h3I

— c h 2- c - c h 2«

^ ai>o f \ o l ^ o o^W

.Q C,H,C CH,

c h 3I

— c h 2 - C - CH2'» s„\ y .Cu (I)

/ \

HjC

CH,c o 2 + c h 3

103

Scheme 17

CHj CHjCH=C ♦ -CH,

COjCHj CO,CH,

CH, CH,— CH, - C— CH— C - CH,

< t L . ^

CH, CH, CH,— c ^ c h 2- < ! : = c h — < J :-c h ,

d:o2CH,

•c o , c h ,

\CO, ♦ CH,

CH, X p , CH, —CHj-C- ♦ • CH, — C = CH—C —CH,

(tojCH, <[o,CK,Unzips

CH, CH,3 ~ C H , - i - C H , - l

COjCH, COjCH,CH,

CH, CH,CH,~ C H 2 - ^ t i r C H 2- 0

^0,CH, COjCH,

CH, CH,CH,CH,=C— CH, —^ y

CO,CH, CO,CH,

Unzips

CH, CH, CH, CH,— I I I 3 IP — CH2 ♦ C *- CHj - CH «■ -CH, ------------► — CH, — C — CH, — C-

CO,CH, CO,CH, CO,CH, ^0,CH,Unzips

CH,

Scheme 18 104

Hydrogen Abetmetfoa

C O jCH j

CHj CH,— CH, - c l ^ C H , - C ~

COjCH, COjCH,

Uiuipi

. Hydrogen Abstraction

CH, H CH,g ~ c h 2- c — < b -----

COjCH, COjCH,

CHj CHj— CH2 - Cj- CH C-~

COjCH, OjCH,

CH| CH, a ,

CO,CH, COjCH,

CH, CH,

<!:o2ch,

CH,— CH

CH,i I 1j-C - ♦ ■ CHj —C=CH —C01CH,

Uruip,

• co2ch,

COj + • CH,

CH, CH,I I-C H 2 - C - C H 2 - C ~

co2ch, co2ch2

Hydrogen AbatmetSon

n

/

Complicated Chemistry

105

J. Solvent Effects

It was possible that the benzophenone solvent was interfering with the

free radical processes. A control thermolysis experiment was conducted

using benzophenone and PMMA in a typical reaction ratio, not only to

determine if benzophenone would volatilize at reaction pressures, but also

to determine whether it promoted depolymerization. No depolymerization

was observed. It is known that benzophenone has the capacity to be reduced

reversibly to the anion radical in aprotic s o l v e n t s . 19 Therefore,

benzophenone may consume radicals formed by either the decomposition

of PMMA or that of copper(II) acetate, and therefore inhibit the

depolymerization process.

A solid-state depolymerization study was undertaken to determine the

extent of solvent interference. Thermogravimetric analysis was conducted

on solid samples of PMMA and copper(II) acetate in typical ratios. The

polymer was supplied in granular form so it was necessary to hand-grind

samples in a liquid-nitrogen-filled mortar to powder form before loading

them into the analyzer. The result of the studies is shown in Figure 32. As

the variability of repeat experiments demonstrated, the hand-grinding

method apparently did not ensure consistent sample composition. No

meaningful information was gained from the data obtained. If weight loss

had been higher than expected, then it could have been concluded that

benzophenone was trapping radical species, as shown in Figure 33, and

TGA Concentration Profile? % Weight Loss vs. [Cu(OAc)2]“ 60 -r -CSIo '50 - 9 -4 0 -y 3 0 -2 20 ;;

i 1 0 -•<08 0 4-

o 5 10 15 20 25 30 35Concentration Cu(OAc)2 (percent)

40

Figure 32

Results of TGA Experiments

O R

Figure 33

Radical Scavenging by Benzophenone

107preventing monomer evolution. If, on the other hand, weight loss had been

lower than expected, then one conclusion could have been that

benzophenone was an intermediate in a series of possible monomer

generating radical reactions. The control experiment ruled out the

possibility that benzophenone could induce depolymerization alone at

200°C. However, it indicated nothing about the tendency of benzophenone

to promote or retard the degradation. Until a better method of solid-phase

testing is utilized, the role of benzophenone must remain a matter of

speculation.

IV. CONCLUSIONS108

A customized depolymerization reactor was used to test the effect of

a number of metal-based compounds on the depolymerization of

poly (methyl methacrylate) (PMMA) in solution. One of these additives,

copper(II) acetate, was shown to induce a high percentage of monomer

(methyl methacrylate) production (approximately 26% after 3 hours) at

200°C. Six other compounds induced greater than 10% depolymerization

under similar conditions. Copper(II) compounds with carboxylate ligands

induced the greatest degrees of depolymerization.

During depolymerization reactions, color change of the copper(II)

acetate mixture from green to reddish-brown indicated liberation of copper

metal. As a result of this copper reduction, the organic radical species

produced were most likely responsible for the PMMA depolymerization.

The thermal degradation of copper(II) acetate is facilitated by the polymer.

This degradation was likely promoted by coordination of the metal center

to the PMMA. Upon coordination, acetate radical is probably released,

generating coordinated copper(I) acetate. However, it is well-known that

the acetate radical is very short-lived and rapidly degrades to carbon

dioxide and methyl radical. It was the methyl radical which most likely

initiated free-radical chemistry. This radical may interact with the polymer

either through addition or hydrogen abstraction.

The PMMA model compound dimethyl 2,2,4,4-tetramethylglutarate

109was synthesized and thermolyzed with copper(II) acetate in a closed system

to understand the nature of radical attack on the polymer. The GC/MS

indicated that the model compound was unaffected and that no

depolymerization had occurred, even though copper(II) acetate had

released free radicals, as the reaction mixture had changed to reddish-

brown. Possible addition sites in a typical depolymerization experiment

include two unsaturated PMMA chain ends. Hydrogen abstraction may

occur at a saturated chain end, at a methyl side group, at a methylene

within the chain itself, or at the methyl of the acetate side group. There

were no unsaturated sites in the model compound, a condition which

suggested that the unsaturated chain ends of PMMA were the active sites of

depolymerization.

The observation that monomer production decreased as copper(II)

acetate loading was increased can be attributed to free-radical quenching by

free copper(II) acetate. The benzophenone solvent may have played a role

in free-radical chemistry; however, the solid state depolymerization

experiments using thermogravimetric analysis failed to give definitive

results. Future research should include the use of better solid-state

techniques to determine the effects, if any, of solvent on the

depolymerization mechanism. Also, gel permeation chromatography should

be utilized to analyze the molecular weights of polymer residue in the

reaction vessel.

REFERENCES110

1. Alper, J.; Nelson, G. L. Polym eric M ateria ls / Chemistry fo r the

Future ; American Chemical Society: Washington D. C., 1989; p 5.

2. Huang, S. J. In Comprehensive Polymer Science ; Allen G., Ed.;

Pergamon: New York, 1989; Vol. 6 , p 598.

3. Sako, T.; Sugeta, T.; J. Chem. Engn. Jpn . 1996, 342-346.

4. Wall, L. A .S P E J . 1 9 6 0 ,16 , 810.

5. Kashiwagi, T.; Inaba, A.; Hamins, A. Polym . Degrad. Stab . 1989,

26,161.

6 . Manring, L. E. M a c ro m o l. 1991, 24 , 3304-3309.

7. Anthoney, M. E.; Carson, A. S.; Laye, P. G. Conf. In t. Thermodyn.

Chim . 4th, 1 985 ,1 , 99.

8 . Rauf, M. A.; McNeill, I. C. Polym. Degrad. Stab . 1993, 40 , 263-266.

9. McNeill, I. C. Polym. Degrad. Stab . 1984, 9 , 167-183.

10. Chandrasiri, J. A.; Roberts, D. E.; Wilkie, C. A. Polym. Degrad. Stab .

1994, 45 , 97-101.

11. Wilkie, C.A.; Leone, J. T. J. App. Poly. Sci . 1991, 42 , 1133-1141.

12. Wilkie, C. A.; Beer, R. S. J. F ire S c i . 1 993 ,11 , 184-192.

13. McNeill, I. C.; Liggat, J. J. Polym. Degrad. Stab . 1990, 29 , 93-108.

14. Chandrasiri, J. A.; Wilkie, C. A. Polym. Degrad. Stab . 1994, 45 , 91-

96.

15. Chandrasiri, J. A.; Wilkie, C. A. Appl. Organometal. Chem . 1993, 7 ,

Il l599-605.

16. Jamieson, A.; McNeill, I. C. J. Po ly. S c i.: Polymer Chemistry ed.

1978,6, 2225-2235.

17. Hammond, B.; Jardine, F. H. J. Inorg. Nucl. Chem . 1971, 33 , 1017.

18. Darensbourg, D. J.; Holtcamp, W. M. In o rg . Chim . Acta . 1994, 227 ,

223-232.

19. Svaan, M.; Parker, V. D. Acta Chem. Scan . 1982, B36, No. 6 , 365-

370.

20. Judd, M. D.; Plunkett, B. A.; Pope, M. I. J. Thermal A n a l. 1974, 6 ,

555-563.

21. Tsuchida, R.; Yamada, S. Nature . 1 9 5 5 ,176 , 1171.

22. Martin, J. C.; Dombchik, S. A. In Oxidation o f Organic Com pounds-I,

American Chemical Society: Washington D. C., 1968, p 269.

112

VITA

Jason Andrew Macko was bom in Richmond, Virginia March 11, 1974. He first attended Good Shepherd Episcopal School through 6 th grade after which he attended St. Christopher’s School from which he graduated in 1992. As an undergraduate at the College of William and Mary, a career in chemistry was bom. After graduating in 1996 with a B.S. in chemistry, he entered the M.A. program in chemistry at William and Mary, which he completed August 8 th, 1997. He was admitted to the Macromolecular Science Department at Case Western Reserve University to begin research in the fall of 1997 in pursuit of a Ph.D. in polymer chemistry.

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