Indian Journal of Chemistry Vol. 40A, December 200 I, pp. 1282-1287

Polymerization of vinyl acetate with styrene and a-methylstyrene under high oxy­gen pressure

Priyadarsi De & D N Sathyanarayana*

Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560 012, Ind ia

Received 9 July 2001; revised 19 September 2001

The polymerization of vinyl aceta te wi th styrene and a-methyl styrene of various compositions has been studied by the free radical-initiated oxidative polymerization. The compositions of the resultant polymers obtained from 1H and 13C{ H1

}

NMR spectra have been utilized to determine the reac tivity ratios of the monomers. The reactivity ratios reflect the tenden­cies of the two monomers towards consecutive homopolymerization. The NMR studies reveal irregularities in the chain due to the cleavage reactions of the propagating peroxide radical. The thermal degradation study by differential scanning calo­rimetry (DSC) supports alternating peroxide units in the polymer. The activation energy for the thermal degradation sug­gests that the degradation is controlled by the di ssoc iation of the peroxide ( -0-0-) bonds of the polymer.

Polymerization of vinyl monomers in the presence of oxygen is known as oxidative polymerization 1

• The main products of oxidative polymerization, namely, th e polyperoxides, are alternating copolymers of vinyl monomer and oxygen. They find importance as spe­cial fuels, thermal and photo initi ators , curators in coating and moulding2

·5

. Further, during commercial polymerization of vinyl monomers, the interaction with oxygen is unavoidable and as a result, thermally labile peroxy linkages are incorporated into the poly­mer backbone thus affecting their thermal stability. To improve the quality of polymeric coatings and ad­hesives, the use of monomer mixtures and cross­linking agents has attracted much attention6

. Co­polymerization of styrene-vinyl acetate system is not efficient, since styrene radical is too unreactive to add to the unreactive vinyl acetate monomer. It is inter­esting to study their copolymerization at high pres­sures of oxygen.

The term oxidative copolymerization is applied when two monomers (R 1 and R2) in the presence of oxygen at high pressures result in the formation of copolymers of the general formula -[-(R 1-0-0-)x-­(Rr0-0-)y-]-. Compared to simple vinyl polyperox­ides obtained from the copolymerization of a vinyl monomer and oxygen, those obtained by the co­polymerization of a mixture of vinyl monomers with oxygen have been less studied 1• The oxidation of two monomers can be considered as a special case of ter­polymerization, where the monomers (R) are not ho­mopolymerized. The uniqueness of thi s polymeriza­tion is that it approximates to a binary copolymeriza-

tion in terms of -R02" units . The rate of polymeriza­tion may then be described in terms of the copolym­erization equation and the reactivity ratios7

•8

.

In the present work, the oxidative polymerization of two systems, vinyl acetate-styrene and vinyl ace­tate-a-methylstyrene have been investigated. The study aims chiefly at the determinati ~n of the reactiv­ity ratios of the monomers by nuclear magnetic reso­nance spectroscopy (NMR). In spite of the higher oxygen flux that is needed for the copolymerization process, the choice of a-methylstyre ne is advanta­geous since it is not homopolymerized due to the close proximity of the polymerization temperature to the ceiling temperature9

.

Materials and Methods Styrene (STY or S) and a-methylstyrene (AMS or

A) (Rolex, India) were freed from inhibitor by wash­ing with 5% NaOH and then with water repeatedly . After drying over anhydrous Na2S04, they were dis­tilled under reduced pressure. Vinyl acetate (V Ac or V) (Rolex, India) was freed from inhibitor by drying over CaC12, and fractional disti llation. 2,2'­Azobis(isobutyronitrile) (AIBN) (Koch Light, Eng­land) was recrystallized twice from methanol. High purity oxygen was used. Reagent grade solvents like petroleum ether, CH2Cb etc. were pu ri fied by stan­dard procedures.

The required amount of the monomers and AIBN (0.01 mol L' 1

) were placed in a 300 mL Parr reactor (Parr Instruments Co., USA) and pressu rized to 100 psi with oxygen. The reactor is equipped with a digital

DE eta/. : POLYMERIZATION OF VINYL ACETATE WITH STYRENE & a-METHYLSTYRENE 1283

pressure transducer, temperature controller and a me­chanical stirrer. The polymerization was carried out at 50± 0.5°C with stirring for about 6~84 h. Conversion was maintained below 15 %. The feed ratio was var­ied to get polymers of various compositions. The polymers were isolated and purified by repeated pre­cipitation from CH2CI2 followed by the removal of the solvent by vacuum drying. Details of synthesis, identi­fication of the polymers, etc. are given in Table l.

The FTIR spectrum was recorded on a Bruker Equinox 55 FTIR spectrometer. The thermal analysis was carried out using a Perkin-Elmer DSC-2C differ­ential scanning calorimeter (DSC) under nitrogen at­mosphere at heating rates of 5, I 0, 20, 40 and 80°C/min with sample sizes of 1-5 mg. Electron im­pact mass spectra (El-MS) in the positive mode were obtained at 70 eV in a JEOL JMS- OX 303 mass spectrometer. The samples were introduced by direct inlet probe and heated from 25°C at a heating rate of 64°C/min. The molecular weights were obtained us­ing a Waters HPLC/GPC instrument (refractive index detector) with THF as a mobile phase at a flow rate of 1.0 mL min-1 at 30°C using polystyrene standards. The 1H and 13C { H 1) NMR spectra were recorded at room temperature on a Bruker ACF 200 MHz spec­trometer in CDCI3 and CH2CI2 (020 internal Jock), respectively using tetramethylsilane (TMS) as refer­ence. The 13C( H1

) NMR spectra were obtained under

inverse gated decoupling with 6 s delay between the pulses.

Results and Discussion Polymers obtained are stic!y solids. The number­

average molecular weight ( Mn ) and polydispersity index (PO-I) of some polymers are presented in Table l. The polymers have low molecular weight due to various chain transfer reactions occurring during oxy­gen copolymerization10

• The polymers should be stored in the dark and in a refrigerator to minimize degradation.

FT-IR spectra The strong band in the FT-IR spectra of polymers

near 1020 cm·1 is assigned to the peroxide bond stretching vibration. The very intense band appearing at 1755 cm-1 is assigned to the carbonyl groups pres­ent in vinyl acetate unit and it shows increased inten­sity as the vinyl acetate content in the polymer is in­creased. Other carbonyl groups present in the various end groups also show infrared absorption in this re­gion and hence they are not distinguishable.

The broad absorption centered at 3480 cm-1 is due to the hydroxyl and hydroperoxide end groups. The formation of these end groups via various chain trans­fer mechanisms has been reported 11

·12

• The absorption at 1600 cm·1 in the polymers is due to the stretching

Table 1-Results of the oxidative polymerization, initiated by A IBN at 50°C, of vinyl acetate wilh styrene and a-methylstyrene

Mole fraction of vinyl acetate Polymer Reaction Yield Feed [V] Co12Qly~rQ2Sidc Mol.wt. PD-1

time (h) (%) 1HNMR 13C{H 1} NMR ( Mn)

Vinyl acetate/styrene

PSP 15 10.4 0.000 0.000 0.000 3600 1.66 VSI 20 12.3 0.554 0.027 VS2 25 7.5 0.789 0.080 0.088 VS3 50 7.4 0.897 0.129 0. 124 VS4 70 5.9 0.939 0.237 0.244 3290 1.20 vss 74 4.2 0.972 0.390 0.407 VS6 80 4.4 0.978 0.596 0.623 PVAcP 84 9.5 1.000 1.000 1.000

Vinyl acetate/a-methylstyrene

PAMSP 6 12.6 0.000 0.000 0.000 3370 1.87 VAl 10 10.0 0.609 0.020 VA2 20 8.8 0.781 0.033 0.038 VA3 40 5.4 0.908 0.1 16 0.112 VA4 70 4.2 0.946 0.163 0.164 3000 1.41 VAS 75 3.6 0.973 0.402 0.362 VA6 75 2.9 0.982 0.559 0.573 PVAcP 84 9.5 1.000 1.000 1.000

1284 INDIAN J CHEM, SEC A, DECEMBER 2001

of C=C bond present as chain ends. There are evi­dences for the presence of C=O and C=C end groups in vinyl polyperoxides 12

•

Copolyperoxide compositions The compositions of copolyperoxides were deter­

mined from their 1H and 13C{ H 1} NMR spectra. Both

1H and 13C{H 1} NMR spectra reveal that the mono­

mers do not homopolymerize under high pressure of oxygen employed here for polymerization. For all the polymers, downfield shift of the main chain CH2 and CH protons is observed due to the adjacent electro­negative oxygen atoms to which they are bonded. Figure l depicts 1H NMR spectra of VS4 and VS5, along with the spectra of the homopolyperoxides, PSP and PVAcP. The 1H NMR spectrum of PSP shows signals at 8=4.03, 5.31 and 7.22 ppm due to methyl­ene, methine and aromatic protons respectively 11

• In the 1H NMR spectrum of PVAcP, the signals at 8 = 2.07, 4.1 and 6.45 ppm are assigned to the methyl, methylene and methine protons, respectively. The methylene region of the spectra of PV AcP and the

ppm

Fig. I- 1H-NMR spectra of (a) PSP, (b) VS4, (c) VS5 and (d) PV AcP in CDCI3.

copolyperoxides show complex pattern, which could be due to the excess methylene groups present in the polymer chains as defects. This is due to the fact that during the oxidation of VAc, the peroxy radical un­dergoes preferential cleavage reactions rather than addition reactions with V Ac molecules. The compo­sitions given in Table 1 were obtained from the ratio of the integrated intensity of the signals of aromatic protons of styrene units to that of the CH protons of VAc units. Similar spectra were obtained for V Ac units in vinyl acetate-a-methylstyrene copolymer series as described earlier. In the spectrum of PAMSP, the signals at 8 = 1.46, 4.19 and 7.2 ppm are assigned to a-methyl, methylene and aromatic pro­tons, respectively (Fig. 2). Due to the increased cleav­age reactions of the peroxide radicals, the chain ir­regularities in this series were found to be higher compared to those of the other series because of the excess methylene groups present in PAMSP. The co­polyperoxide compositions were obtained from the ratio of the intensities of the two di fferent methyl group signals present in the two different monomers.

ppm

lL

2

L(a)

0

Fig. 2- 1H-NMR spectra of (a) PAMSP, (b) VA4, (c) VAS and (d) PV AcP in CDCI3.

DE et al. : POLYMERIZATION OF VINYL ACETATE WITH STYRENE & a-METHYLSTYRENE 1285

Figure 3 displays the 13C{H 1} NMR spectra of YS4

and YS5 along with the spectra of the homopolyper­oxides. The 13C {H 1

} NMR spectra recorded under inverse gated decoupling also permits the determjna­tion of the copolyperoxide compositions from the in­tegrated intensities of the appropriate resonance sig­nals. Considerable downfield shift of the main chain carbon is observed due to the adjacent electronegative oxygen atoms. The 13C{H 1

} NMR spectrum of PSP exhibits signals at 75.75, 82.81 , 127.15-128.40 and 137.76 ppm; they are assigned correspondingly to methylene, methine, aromatic and aromatic ipso car­bon. The signals in the spectrum of PV AcP at 8 = 20.7, 72.7, 95 .0 and 169.6 ppm are attributed to methyl, methylene, methine and carbonyl carbons, respectively. The ratio of the signal intensities of the two different methylene carbons in the two different monomer units (V Ac and STY) yields directly the compositions of the copolyperoxides. In the case of V Ac-AMS polymers, the presence of V Ac units in the copolymer finds support from the 13C{H1

} NMR spectra (Fig. 4). The 13C{H 1

} NMR spectrum of PAMSP

(d' J I .JlL- -'-----""''--

(b)-...___ __ _.A.-._,,...._ __ ___.._.___}LA.,..___.J._ __ .......__

160 140 120 100 80 60 40 20 PPM

Fig. 3- 13C-NMR spectra of (a) PSP, (b) VS4, (c) VS5 and (d) PV AcP in CH,CI,.

exhibits signals at 21.85, 78.53, 84.97, 126.05-127.98 and 142.06 ppm; they are assigned respectively to -CH3, -OCHr, -0-C-, aromatic and aromatic ipso car­bon atoms. The ratio of the intensities of the two dif­ferent methylene carbons present in the two different monomer units (V Ac and AMS) gives the copolymer compositions (Table 1 ). The compositions obtained from the 13C { H 1

} NMR spectra are in good agreement with those determjned from 1H NMR spectra. The 13C{H 1

} NMR spectra for the methylene region are complicated due to chajn irregularity.

The polymers show two weak peaks around 9.6-10.1 ppm in the 1H NMR spectra due to two types of O=CH- groups occurring as chajn ends. There is a signal near 92.7 ppm in the 13C{H 1}NMR spectra, which may be assigned to the inclusion of methylene group with peroxy (-0-CHrO-) groups on either side in the chain. It indicates that the cleavage reactions occur to a considerable extent during the oxidative polymerization. The inclusion of ( -O-CH2-0-) in the polyperoxide chrun has been reported for the oxidation of AMS at very low pressures of oxygen 13

.

160 140 rzo 100 80 PPM

60 40 20

Fig. 4- 13C-NMR spectra of (a) PAMSP, (b) VA4, (c) VAS and (d) PV AcP in CH2Cl2.

1286 INDIAN J CHEM, SEC A, DECEMBER 2001

Reactivity ratios While deriving the equation for the oxidation of

two monomers, oxygen is considered as a third monomer. The propagation reactions involving the addition of one or the other monomer to a second monomer radical are assumed to be negligible'. In the oxidative polymerization, the reactivity of the mono­mer radical (-R") with oxygen is very high compared to that of -R02" with the monomer (R). Since in the oxidation of monomers, the copolymer composition is not proportional to the feed composition, a penulti­mate effect could be expected 14. The problem of the penultimate group effect in copolymerization is rather complex owing to the existence of 27 possible propa­gation reactions, compared to eight in the case of ter­minal model copolymerization7. However, in the oxi­dative copolymerization, since the monomers are not homopolymerized and the reaction -R02 • + R occurs very fast, the important rate determining propagation steps involved in the oxidative polymerization of sty­rene (S) with vinyl acetate (V) may be written as fol­lows15:

- so2 • + s ---7- so2s· (ksos) . . . (i)

-so2· + v ---7-so,s· (ksov) . . . (ii)

- vo2 • + s ---7- vo2s· (kvos) . . . (iii)

-vo2· + v ---7- vo2 v· (kvov) .. . (iv)

Applying steady state approximation separately for the reactive species, -S02" and -V02", the copolymer composition could be expressed in terms of the feed composition and the reactivity ratios

d [S] [S] (r5 [S] + [V]) =------

d [V] [V] ([S] + rv [V]) ... (v)

where d [S]Id [V] denotes the ratio of styrene to V Ac irn the copolyperoxide, [S]/[V] is the corresponding feed ratio, and rs and rv are respectively the reactivity ratio for styrene and VAc, defined as 16

rate constant for the reaction of -so; + s ksos r.- ---s - rate constant for the reaction of -so; + v - ksov

rate constant for the reaction of - v o; + v kvov r.- ---v - rate constant for the reaction of - v o; + s - kvos

Equation (v) for the oxidative copolymerization of two vinyl monomers resembles the Mayo-Lewis e<Juation 15 for copolymerization of binary monomer systems, except in the definition of the reactivity ra­tio. The reactivity ratios give the relative tendencies

of the peroxide radicals ( -S02• or -V02 ") to add to a monomer of the same kind or the other. In a similar way, the rate equations can also be written down for the oxidative polymerization of V Ac-AMS.

The reactivity ratios have been determined from Finemann-Ross 17 and Kelen-Tudos 18 plots. The reac­tivity ratios for the two series of polymerizations cal­culated using these two methods are presented in Ta­ble 2. The reactivity ratio of the V Ac monomer is very low compared to that of the other two mono­mers, i.e., rs or rA>>rv (rs or rA>> land rv<<l) . It reflects the tendency of the two monomers towards consecutive homopolymerization9. Although the cal­culation of reactivity ratios from NMR data is straight forward, the assumption about the absence of cleav­age products may introduce some eiTor in the calcula­tions.

Thermal degradation According to Mayo mechanism19, vinyl polyper­

oxides generally undergo random thermal scission at the peroxy bond, followed by unzipping of the ~­peroxyalkoxy radicals, giving carbonyl compounds . For example, PSP on thermal degradation yields ben­zaldehyde and formaldehyde in equimolar quanti­ties19. A representative DSC thermogram of VS3 is given in Fig. 5. The DSC thermogram indicates that

Table 2-Reactivity ratios ror the oxidative polymerization or vinyl acetate with styrene and a-methylstyrene

Method

Fineman-Ross Kelen-Tudos

Fineman-Ross Kelen-Tudos

0

t 5 0

"0 c ... 10

15

20 40

rs rv

Vinyl acetate/Styrene

44.47±0.17 0.023±0.001 43.77 ± 0.32 0.020 ± 0.003

Vinyl acetate/a-methylstyrene

77.67 ± 0.61 0.014 ± 0.001 83.75 ± 0.28 0.024 ± 0.002

\

60 80 100 120 Tempuature ("c )

1.02 0.88

1.09 2.01

14 0

Fig. 5-DSC thermogram of VS3 at a heating rate of I 0°C/min.

.,.

DE er a/.: POLYM ERIZATION OF VINYL ACETATE WITH STYRENE & a-METHYLSTYRENE 1287

10 0

:-:: c <:) u

60 c ::> .0 <(

... .~

~ 20 "'

Table 3-DSC Data for the polymers investigated

Polymer

Vinyl acetate/s tyrene

PSP VS3 VS4 vss PVAcP

-211.9 -2 12.5 -225.3 - 198.9 -2 13.6

Vinyl acetate/a-methylstyrene

PAMSP VA 3 VA4 VAS

51

43

~r11 40 60

-1 92 .8 -200.5 - 187 .3 -226.8

77 10 5

7t~ 91

80 100 120 m/z

38.2

30.4

24.5

35.0

28.8

140 160

Fi g. 6 - El-MS spectrum of VS4.

180 2 00

the degradati on is exothermi c. The enthalpy changes for thermal degradation obtained from the DSC ther­mogram are given in Table 3. The acti vat ion energy (£3 ) for the thermal degradation process was deter­mined by the Ki ssinger' s method20

. The slope of the plot of In (<j)/T111

2) aga inst l iT"" where <P is the heating

rate and Tm (K), the peak temperature obtained from the DSC data, prov ides the £ .. va lue for thermal deg­radati on. The Ea values tabulated in Table 3, compare well with the dissociat ion energy of the 0-0 bond 2 1

• It shows PSP to be thermally more stable than the other polyperoxides and the Ea va lues of copolyperox ides lie in between that of the two respective homopoly­peroxides.

The EI mass spectrum of YS4 is shown in the Fig. 6. The assignment of the molecul ar ion peaks found in the spectrum is given in Table 4. The primary degra­dation products fo rmed from therma ll y lab ile perox­ide-containing polymers wi ll mi x in the spectrometer with the fragmented ions generated due to the electron impact. Like other vinyl polyperoxides, for YS4 too the degradati on is ini tiated at the weak 0 -0 bond 12

.

Table 4-The molecular ions identified in the El-MS of VS4

Structure m/z

0

II 43

CHrC

C4H3 51

C6Hs 77

C6Hs-CH2 91

C6H5-CO 105

The base peak at m/z = 105 is derived from the styrene unit. The peak at m/z = 43 corresponds to the CH3-

C=O fragment. Since these two fragments originate from the two monomers, styrene and vinyl acetate respectively, the ratio of the relative intensities of these two peaks provides the overall composition of the copolyperoxide (0 .2453, mole fraction of V Ac) . The composition of VS4 thus obtained from mass spectrum is in satisfactory agreement with that ob­tained from the NMR spectra (see Table I). The polymer VA4 also shows similar mass spectral frag­mentation pattern , only one extra peak at m/z = 121 is observed which may be due to C6H5-CH(CH3)-0 fragment.

References I Mogelivich M M, Russ Chem Rev, 48 ( 1979) 199. 2 Ki shore K & Mukundan T , Narure. 324 ( 1986) 130. 3 Shanmugananda M K, Kishore K & Mohan V K, Macromole-

cules, 27 ( 1994) 7 109. 4 Subramanian K & Ki shore K, Polymer, 38 ( 1997) 527. 5 Subramanian K & Ki shore K. Eur Polmz J, 33 ( 1997) 1365. 6 T ager A, Physical chemisrrv of polvmers (Mir Publishers)

1978. 7 Valvassori A & Sarton G. Adi'Cmces in polymer science

(Springer-Verlag, New York) 5 ( 1967/ 1968) 28 . 8 Jayanthi S & Ki shore K, Macromolecules, 29 ( 1996) 4846. 9 Odian G, Principles ofpolymerizmion (Wil ey, Ne w York) 3rd

Ecln, 199 1. I 0 Mukundan T & Ki shore K, Prog polym Sci, 15 ( 1990) 475 . II Ca is R E & Bovey FA, Macromolecules. I 0 ( 1977) 169 . 12 De P, Sathyanarayana D N. Saclasivamurthy P & Sridhar S.

Polymer, 42 (200 I ) 8587. 13 Mayo F R & Miller A A. J Am cilem Soc. 80 ( 1958) 2-180. 14 Fawcell A H & Smyth U, Eur Polym J. 25 ( 1989) 791. 15 Mayo F R & Lew is F M, JA m c/iem Soc, 66 (194-1) 159-1. 16 Niki E. Kamiya Y & Ohta N, Bull chem Soc Japan. 42 ( I %9)

23 12. 17 Fineman M & Ross S D, J polym Sci, 5 ( 1950) 269. 18 Kclen T & T udos F. J macromol sci Chem, 9 ( 1975) l. 19 MillerAA&MayoFR . ./ AmchemSoc. 78(1956) 1017 . 20 Ki ss inger H E. Anal Chem, 29 ( 1957) 1702 . 2 1 Scott G. ATmospheric oxidants and amioxidanrs, (EisCYicr.

London), 1965, P 37.