Die Angewandte Makromolekulare Chemie 144 (1986) 207-218 (Nr. 2351)

Institute of Macromolecular Chemistry, Czechoslovak Academy of Sciences, 162 06 Prague 6, Czechoslovakia

The Diels-Alder Reaction and Copolymefization of Halogenated N-Phenylmaleimides with Dienes

Jan Lokaj, Hana Pivcovii, and FrantiSek HrabAk

(Received 24 January 1986)

SUMMARY: The course of the Diels-Alder reaction of N-(2,4,6-tri-bromophenyl)maleimide (I)

with 2-methyl-I ,3-butadiene (Ip) and of N-(2,4,6-trichlorophenyl)maleimide (11) with Ip, 2-chloro-I ,3-butadiene (Cp), and butadiene (Bt) in CHCI, was investigated. For the I-Ip addition, the reaction order was determined with respect to both com- ponents, and the activation energy was calculated. The Djels-Alder adducts thus obtained were characterized by elementary analysis, by 'H- and 13C-NMR spectra, specific volume contractions, melting points, solubilities, and polymerization abili- ties. Copolymers of I and I1 with Ip, Cp, Bt, and 2-methylpropene (Ib) were pre- pared, and their composition was determined. The copolymerization and Diels-Alder addition of I-Ip and 11-Ip occurring in parallel were investigated, and the participa- tion of these two processes in the total reaction was discussed.

ZUS AMMENFASSUNG: Der Verlauf der Diels-Alder-Reaktion von N-(2,4,6-Tribromphenyl)maleinimid (I)

mit 2-Methyl-I ,3-Butadien (Ip) und von N-(2,4,6-Trichlorphenyl)maleinimid (11) mit Ip, 2-Chlor-1,3-Butadien (Cp) und Butadien (Bt) in CHCI, wurde untersucht. Bei der Addition von I-Ip wurde die Reaktionsordnung in bezug auf beide Komponenten be- stimmt und die Aktivierungsenergie berechnet. Die Produkte der Diels-Alder-Reak- tion wurden durch Elementaranalyse, ' H- und j 3 C-NMR-Spektroskopie, spezifische Volumenkontraktionen, Schmelzpunkte, LBslichkeiten und Polymerisationsfilhigkei- ten charakterisiert. Es wurden die Copolymeren von I und I1 mit Ip, Cp, Bt und Iso- buten (Ib) hergestellt und ihre Zusammensetzung bestimmt. Copolymerisation und parallel verlaufende Diels-Alder-Addition von I-Ip, 11-11> wurden untersucht und die Teilnahme der beiden Reaktionen an der Gesamtreaktion diskutiert.

0 1986 Hiithig & Wepf Verlag, Base1 0003-3146/86/$03.00 207

J. Lokaj, H. PivcovB, and F. HrabBk

I . Introduction

In our earlier study we described the preparation of copolymers of halo- genated N-phenylmaleimides '*' and determined their copolymerization characteristics 3. After a preliminary investigation of their properties, co- polymers of N-(2,4,6-tribromophenyl)maleimide (I) and N-(2,4,6-trichloro- pheny1)maleimide (11) with 2-methylpropene (isobutene-Ib) appeared to be promising, due to the high halogen content and high heat stability4. At the same time, however, these copolymers were found to be very brittle, which is an obstacle preventing their application. As we expected the polymer chain containing double bonds to possess higher mobility, we press-moulded sheets of alternating copolymers I and I1 with 1,3-butadiene (Bt), 2-methyl- 1,3-butadiene (isoprene-Ip), and 2-chloro-l,3-butadiene (chloroprene-Cp) at 250 - 260 "C. These, too, were very brittle. Only sheets of copolymers I-Cp and 11-Cp containing more than 70 mol-Yo Cp (with Bt and Ip, even if present in excess, only alternates are formed) were compact and flexible, but they were dark in colour, and hydrogen chloride was released from them during the press-moulding. This could be explained by the presence of 1,2- additions of Cp units in the chain, and also by the formation of a Diels- Alder adducts from Cp and I or 11, followed by the copolymerization of this adduct with the starting monomers. In this study we have therefore tried to verify the course of the Diels-Alder addition in the copolymerization of I and I1 with Cp and Ip, to characterize the new adducts (Ad) of the type N-(2,4,6- trihalogenophenyl)-4-Y-4-cyclohexene-1 ,Zdicarboximide, and to determine the kinetic characteristics of both parallel reactions.

X ru . X Y Ad -.'Z \

CH Br CH3 Ad 1 II CY Cl CHB A d 2

CH2 C1 Cl A d 3

Cl H A d 4

2. Experimental

2.1 Chemicals

N-(2,4,6-tribromophenyl)maleimide (I), m. p. 142 "C, and N-(2,4,6-trichloro- pheny1)maleimide (11), m. p. 131 "C, were prepared by procedures described earlier1P3. Butadiene (Bt) (Kauhk, Kralupy) and isobutene (Ib) (Slovnaft, Bratislava)

208

Diek-Alder Reaction and Copolymerization

were of 99.8% purity (chromatographically), isoprene (Ip) was “purum” (Fluka, A. G., Switzerland), and chloroprene (Cp) (KauEuk, Kralupy) was distilled at reduced pressure prior to use (b. p. 33 “W39.9 kPa). 2,2’-Azobisisobutyronitrile (AIBN) was recrystallized from ethanol, m. p. 105 “C. The solvents used (Lachema, Brno, Czechoslovakia) were of “p. a.” or “purum” grade.

2.2 Methods

The melting points of imides were determined with a “Bo&ius” apparatus (F. Kastner, Dresden, GDR). The purity of liquid monomers and solvents was checked with a gas chromatograph CHROM 5 (Laboratory Instruments, Prague). Conventio- nal ‘H-NMR spectra of 10% solutions of imides in CDCl, were recorded with a PS- 100 JEOL spectrometer at 100 MHz and room temperature. FT-I3C-NMR spectra of the same solutions were recorded with an XL-200 Varian spectrometer at 50 MHz and room temperature (acquisition time 1.63 s, pulse repetition time 4.83 s, 900 scans). Hexamethyldisiloxane was the internal standard. The viscosity numbers [q] of the copolymers were determined in tetrahydrofuran solutions using an Ubbelohde viscometer at 20°C. Sheets of copolymers I, 11, and hydrocarbon monomers were press-moulded with an electrically heated press FONTIJN 3 S at 250 OC and 9.15 MPa for 10 min in stainless steel circular moulds, 1 mm thick and 25 mm in diameter. All reactions described below with the exception of temperature dependences took place at 50°C in an inert atmosphere.

Copolymers of I and I1 with Ib, Bt, Ip, and Cp destined for the qualitative charac- terization of the properties were prepared by heating the hydrocarbon monomer (3.25 mol * 1-I), imide (2.5 mol . l-l), and AIBN (0.04 mol * 1-‘) for 24 h in ampoules in dimethylformamide solution. They were then precipitated from the polymerization mixtures and reprecipitated with ethanol from tetrahydrofuran solutions.

The course of Diels-Alder reactions between 0.5 mol.l-’ of I or I1 and 0.65 mol e l - ’ of diene in chloroform was investigated in dilatometers, ca. 8 c d in volume, with 0.1 Qo hydroquinone as inhibitor, with respect to the diene. On reaching a constant volume (100% conversion), the solvent was removed from the reaction mixture by distillation, and the addition product was recrystallized from benzene and dried in the vacuum of an oil pump. The specific volume contraction of formation of the given adduct (AVsp)ad was calculated from the ratio of the observed volume contraction and the weight of the isolated adduct, and used further to calculate the initial rate of addition (q,) in mol 1-1 s-l from the time dependence of volume con- traction.

The copolymerization rate of I and I1 with Ip in chloroform was determined by the polymerization of identical starting mixtures in five ampoules, ca. 15 cm3 in volume. The starting concentration of imide was 0.5 mol 1-‘ , of diene 0.65 mole 1-’ , and of AIBN 0.02 mole 1-’ . The amount of the copolymer was determined gravimetrically while the ampoules were successively opened in five time intervals.

209

J. Lokaj, H. Pivcovh, and F. Hrabhk

The copolymerization and Diels-Alder reaction of I and I1 with Ip proceeding in parallel were followed dilatometrically. On reaching constant volume of the reaction mixture, the total volume contraction (AV,) was read off, the polymer was precipi- tated with an excess of ethanol, and the Diels-Alder adduct was isolated by removing the solvents from the filtrate by distillation. Using the determined amounts of the copolymer (m,), adduct (m& and the theoretically available amount of the alter- nating copolymer or adduct (m), the specific volume contraction (AV,,), was calcu- lated for the transformation of 1 g of comonomers into an alternating copolymer by means of relations (1) through (3),

here, vo denotes the starting volume of the reaction mixture with 0.5 mole 1-' imide, and Y, respectively, are the relative molecular weights of diene and imide. The

time dependences of copolymerization and of the Diels-Alder addition proceeding in parallel were constructed using data on the isolated amounts of the copolymer (q) in an independent copolymerization in ampoules and on the time dependence of AV, in dilatometers. The amount of the adduct (qd) for a given time interval was calculated from Eq. (4):

AVg - mp*(AVsp&

vspkd mad= (4)

The rates of copolymerization (RJ and addition (Kd) were read off from the slopes at the beginning of these dependences (Fig. 2).

3. Results

3.1 Diels-Alder Reactions of I and 11 with Dienes

a) Characteristics of Diels-Alder adducts: Tab. 1 presents the character- istics of the adducts prepared in this study, namely N-(2,4,6-tribromo- phenyl)-4-methyl-4-cyclohexene-l,2-dicarboximide (Ad l) , N-(2,4,6-tri- chlorophenyl)-4-methyl-4-cyclohexene-l,2-dicarboximide (Ad 2), N-(2,4,6- trichlorophenyl)-4-chloro-4-cyclohexene-l ,Zdicarboximide (Ad 3), and N- (2,4,6-trichlorophenyl)-4-cyclohexene-l ,Zdicarboximide (Ad 4). They readi- ly dissolve in chloroform, acetone, tetrahydrofuran, benzene, less readily in

210

Tab.

1.

Cha

ract

eris

tics o

f D

iels

-Ald

er a

dduc

ts.

b g

C

H

N

Hal

ogen

k

(rel

. MW

) (Y

o) OC

(c

d g

-1)

(mol

-1-1

*s-1

) $ P 9

(478.02)

(37.69

2.53

2.93

50.15)

Ad'

3 G

4w

wQ

46-22

2.47

3.90

37.97

150

0.093

0.57

9 (365.05)

(46.06

2.49

3.84

38.85)

x z A

d 4

G,H

,,CbN

Q

50.68

3.09

4.24

30.65

154

0.121

5.00

(330.60)

(50.86

3.05

4.24

32.17)

2.

Add

uct

Form

ula

Foun

d (c

alcu

late

d)

mP

(Avs

p)ad

kd

*

Ad

1 C,

H12

Br, N

Q

37.51

2.46

3.01

49.01

144

0.082

2.85

6'

3

Ad 2

C,,H

l,C&

NQ

52.18

3.52

4.06

30.65

138-9

0.108

5.90

(344.64)

(52.28

3.51

4.06

30.86)

8 B

f? k

d : ra

te o

f the

Die

ls-A

lder

add

ition

in c

hlor

ofor

m a

t 50 O

C an

d in

itial

con

cent

ratio

n 0.5

mol

e 1-

I im

ide a

nd 0.65 m

ol *

1-I

dien

e;

(AV

sp)a

d : sp

ecifi

c vol

ume

cont

ract

ion

at fo

rmat

ion

of 1

g of th

e ad

duct

.

J. Lokaj, H. Pivcovh, and F. Hrabhk

-

-

-

ethanol and sulfuric acid, little in hexane, and virtually not at all in water. Chemical shifts of the 'H- and 13C-NMR spectra are given in Tab. 2 and 3. The signals of the CH,, CH, and CH = groups in the IH-NMR spectra have a complicated structure due to spin-spin coupling of the proton nuclei; those of the CH3 groups and of aromatic protons are singuletts. Assignment in the 13C-NMR spectra was verified by recording additional spectra with no spin- spin decoupling of carbon and proton nuclei.

b) The reaction order and temperature dependence of the Diels-Alder reaction I-Ip: The initial rates k d at various starting concentrations of [I], [Ip], and various temperatures are given in Tab. 4. The slopes of logarithmic dependences of %d on [I] (0.998) and [Ip] (0.992) in Fig. 1 suggest a first- order addition with respect to both reaction components. The activation energy of addition, Ead = 53.5 k 3.0 kJ * mol-l, and the frequency factor, F = (4.0 f 0.1) - 104, were calculated from the linear dependence of log Rad on 1/T. After addition of the alternating copolymer I-Ip (0.25 mol * 1 - I of imide and Ip units) to the same starting mixtures, the following rates were mea- sured:

0.7 ::

E cn 0 + -

0.5

0.3

R'$ = 1.94 - and = 6.94 - mol 1-1 . S-I

1+log Ill 0.1 0.4 0.7

I I I

I I I I I

l+logIlpl 0.8 1.0 1.2

Fig. I. Order of the Diels-Alder reaction I-Ip at 50 "C in CHC1,.

212

Diels-A lder Reaction and Copolymerization

Tab. 2. * H chemical shiftsa of Diels-Alder adducts I and I1 with dienes. ~~~

Compound CH,- -CH2- -CH- = CH CH aromatic (PP@

Ad 1 1.72 (3) 2.29 (2) 3-26 (2) 5.60 (1) 7.73 (2) 2.56 (2)

2.56 (2)

2.79 (2)

Ad 2 1.70 (3) 2.27 (2) 3.26 (2) 5.61 (1) 7.38 (2)

Ad 3 - 2.56 (2) 3.36 (2) 5.95 (1) 7.38 (2)

Ad 4 - 2.46 (4) 3.26 (2) 5.93 (1) 7.37 (2) ~~ ~ ~~

a In ppm with respect to the internal standard HMDS; relative intensities are given in brackets.

Tab. 3. 13C chemical shifts of Diels-Alder adducts I and I1 with dienes.

Com- CH3- -CH,--CH- =CH CH =c- co pound (ppm) aromatic

Ad I 21.75 22.07 37.74 118.64 132.98 122.29 175.11 26.58 38.47 122.42 175.30

129.09 134.39

Ad 2 21.60 22.23 37.85 118.49 126.72 125.55 175.33 26.78 38.42 132.86 175.53

133.10 134.45

A d 3 - 23.09 36.72 121.11 126.74 128.90 173.91 29.46 38.69 132.81 174.37

133.17 134.62

133.22 134.37

A d 4 - 21.52 37.84 125.73 126.71 132.85 175.28

213

J. Lokaj, H. PivcovB, and F. HrabBk

Tab. 4. Effect of the concentration of components and the temperature on the rate of the Diels-Alder reaction ( K d ) I-Ip.

[I] - 10 tIPl * 10 T K d * (mol - 1-l) (mole 1-') (K) (mole I-' - s-I) 5.0 6.5 5.0 10.0 5.0 15.0 3.5 15.0 1.5 15.0 5.0 6.5 5.0 6.5 5.0 6.5

2.85 4.37

323.16 6.54 4.62 I .95

303.16 0.75 313.16 1.46 333.16 5.14

I 3.2 Copolymerization of I and 11 with Ib, Bt, Cp, and Ip

a) Characterization of properties of the copolymers: The copolymers, their composition, [q], and appearance of the sheets press-moulded from them are given in Tab. 5. They dissolve readily in DMF, tetrahydrofuran, 1 ,Cdioxane, and chloroform, less readily in benzene and tetrachlorometh- ane, and very little in acetone, alcohol, and aliphatic and aromatic hydrocar- bons. The copolymers of Cp assumed a deeper colour at the temperature of press-moulding while releasing hydrogen chloride at the same time.

b) Copolymerization and Diels-Alder addition I-Ip and 11-Ip proceeding in parallel in chloroform: The time dependences of formation of the alternat- ing copolymer and of the Diels-Alder adduct for I-Ip at 40, 50, and 60 "C are shown in Fig. 2. The initial rates of copolymerization & and & derived therefrom, the determined [q] and copolymer compositions are given in Tab. 6. The activation energy of addition, Ead = 44.0 f 1.5 kJ - mol-', and of copolymerization, E, = 101.4 5 3.0 kJ * mol-', were calculated from the linear dependences of log ILp and log Rd on 1/T. The specific volume con- traction for the alternating copolymerization I-Ip is 0.104 cm3 - g-' . With 0.5 mol * 1-' of I1 and 0.65 mol * 1-' of Ip, 32.3% of the alternating copoly- mer was obtained in the dilatometer within 30 h. At the same time, the time dependence of polymerization was determined gravimetrically in the same mixture. The specific volume contraction for the alternating copolymeriza- tion 11-Ip at 50°C (0.159 cm3 - g-') was calculated from Eq. (1) to (3).

21 4

Diek-Alder Reaction and Copoljlmerization

Tab. 5. Copolymerization of 2.5 mole 1-1 I and I1 with 3.25 mole 1-1 Ib, Bt, Cp, or Ip in DMF at 50°C within 24 h.

Comonomers Conversion Imide content [qlb Sheet in the copoly- mer

(Yo) (mol-Yo) (cn+ g-l) ~ ~~~~ ~

I-Ib 85 50.5 30.0 noncompact Bt 81 49.7 30.0 very brittle CP 76 42.7 43.0 brown, brittle CPa 76 28.0 44.2 brown, elastic IP 71 50.3 41.3 very brittle

11-Ib 86 52.0 30.8 noncompact CP 57 42.0 50.0 brown, brittle CPa 76 25.6 51.6 brown, elastic IP 32 50.4 37.7 very brittle

Initial ratio [Cp]/[imide] = 2.0; [q]: viscosity number of copolymers in tetrahydrofuran at 2 O O C .

a

0 1 3 5 Time Ih)

Fig. 2. Effect of temperature on the rate of formation of the Diels-Alder adduct (filled symbols) and alternating copolymer (open symbols) from 0.5 mole 1-1 I and 0.65 mol*l-' Ip in CHC1,; (A, A) T = 40°C, ( 0 , 0) T = SOOC, (a, U ) T = 60°C.

215

J. Lokaj, H. PivcovB, and F. Hrabhk

Tab. 6. Effect of temperature (T) on the simultaneous Diels-Alder addition and copolymerization of 0.5 mole 1-1 I and 0.65 mol * 1-1 Ip in CHCb.

Tempe- (1/T) * 103 &d - 16 % . i d [111 Content of rature I in the

copolymer ("C) (mol-To) (mole 1-' - s-') (mole 1 - 1 * s-') c d g-'

40 3.19 1.33 1.14 20.6 50 3.09 2.31 3.61 19.7 50.5 60 3.00 3.62 11.9 22.0

% : rate of copolymerization; the other symbols as in the preceding tables.

3.3 Copolymerization of the Adducts Ad 1 and Ad 3 with N-Phenylmale- imide (PMI), Methyl Methacrylate (MMA), and Styrene (St)

From a solution of 0.5 mol - 1-' Ad 1,0.5 mol 1-' PMI, and 0.02 mole 1-I AIBN in CHC1, heated to 50°C for 50 h, 34.9% of the polymer were precipitated with ethanol. After reprecipitation from the same solvents the polymer contained 55.86% C, which corresponded to 21.2 mol-To Ad 1 in the copolymer Ad 1 - PMI. The copolymer Ad 1 - MMA was found to contain 6.6 mol-% Ad 1. In analogous copolymerizations Ad 3 - PMI and Ad 3 - St the respective conversions were 37% and 12.6%; the copolymers contained 32.9 and 16.4 mol-Yo of Ad 3, respectively.

4. Discussion

It can be seen in Tab. 1 that the determined contents of C, H, N are in good agreement with the calculated elemental composition of adducts 1 to 4. The lower halogen content determined by the usual procedure according to Schoniger is a consequence of the more difficult combustion of compounds with a high content of halogens. The assumed composition and structure of the adducts are also confirmed by chemical shifts of the ' H and I3C nuclei and the relative intensities in the 'H-NMR spectra (Tab. 2, 3).

The determined specific volume contractions (A v & d (Tab. 1) correlate well with the relative molecular weight of the adducts; they decrease with increasing molecular weight. The measured Rd (Tab. 1) are in agreement

216

Diels-Alder Reaction and Copolymerization

with the assumed effect of substituents on the electron density of double bonds in diene and dienophile. Chlorine atoms in dienophile I1 cause a larger decrease in electron density on the aliphatic C = C bond than bromine atoms in dienophile I; for this reason, &d is larger than &d 1. During the forma- tion of the adducts Ad 2, Ad 3, and Ad 4 from the same dienophile 11, &d

increases with the electropositivity of the substituent in the diene. Thus, Kd

for 11-Bt and 11-Ip is larger by an order of magnitude than with Cp. The activation energy of addition I-Ip (Tab. 4) determined in the presence

of the inhibitor of polymerization (53.5 kJ mol-') is obviously lower than that of copolymerization (Tab. 6) of the same pair (101.4 kJ * mol-I), which is basically determined by the sum of the activation energies of the propaga- tion and initiation reaction. The finding that Ead (44.0 kJ * mol-I) calculated from the data on the simultaneous copolymerization and Diels-Alder addi- tion I-Ip was lower than in the addition alone can be explained by an increase of the ratio of the copolymer to the adduct with temperature. Due to this, the effective concentrations of the reaction components in the initial time interval in which the slopes of the conversion curves were read off (Fig. 2) were considerably lower than the assumed initial concentrations. The identical activation energy values of the Diels-Alder reaction I-Ip carried out in the presence of the inhibitor both, with and without the copolymer I-Ip, suggest that the addition which takes place in the copolymerization may be characterized by using data on the addition alone in the presence of the inhibitor.

The conversion of comonomers into the copolymer is given in Tab. 5 as the percentage of the copolymer obtained by the first precipitation from the reaction mixture. Though it was distorted due to the admixture of adducts, which were removed from the samples used for the analysis and viscosity determination by precipitation, the rate of copolymerization of I and I1 with Cp can be regarded as comparable with that obtained with Bt and Ip. Since, moreover, &d is lower by an order of magnitude for Cp than for Bt and Ip, the fraction of the undesired adduct in the copolymerization of I and I1 with Cp is very much smaller than in the copolymerizations I-Bt and I-Ip.

The copolymers of I and I1 with Ib, Bt, and Ip (Tab. 5) preserved their alternating character and the undesirable brittleness connected with it, even if unsaturated hydrocarbons were present in excess in the starting polymeri- zation mixture. Only with an increase in the initial concentration of Cp in the mixtures with I and I1 the content of Cp units in the copolymers also increas- ed above 50 mol-Yo, and the sheets prepared from them became elastic. The observed release of hydrogen chloride from chloroprene units assumes that

217

J. Lokaj, H. Pivcovh, and F. Hrab&

chloroprene may be incorporated in the polymer chain by 1 ,Zaddition or by copolymerization of Ad 3 with the initial I1 or Cp. An investigation of the copolymerization of Ad 1 and Ad 3 showed that the adducts prepared in this study may copolymerize, predominantly with monomers having a reduced electron density on the C = C bond, similarly to cyclic olefins6. The nature of this copolymerization and the accompanying phenomena mentioned above and occurring in the heating of copolymers of I and I1 with Cp will be the subject of further investigation.

The authors wish to thank Dr. Z. KruliS for press-moulding of the poly- mer sheets.

Czech. 171593 (1976), Invs.: F. Hrabhk, K. Bouchal; C. A. 89 (1978) 10873Oq * Czech. 173355 (1976), Invs.: F. Hrabhk, K. Bouchal; C. A. 90 (1979) 104859m

V. Hynkovh, F. Hrabhk, J. Polym. Sci., Polym. Chem. Ed. 14 (1976) 2587 F. Hrabhk, I. Masdik, K. Bouchal, M. Slavizek, Eur. Polym. J. 13 (1977) 509 M. Furdik, V. Sutoris, J. Drhbek, S. Pospi$lovh, Chem. Zvesti 13 (1959) 581 S. Murahashi, S. Nozakura, K. Yasufuka, Bull. Chem. SOC. Jpn. 38 (1965) 2082

218