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

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1 Indian Journal of Chemistry Vol. 4A, December 2 I, pp 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 56 12, Ind ia Received 9 July 21; revised 19 September 21 The polymerization of vinyl acetate 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 1 H and 13 C{ H 1 } NMR spectra have been utilized to determine the reactivity 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 ssociation of the peroxide ( ---) 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 initiators, curators in coating and moulding 2 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 attention 6. 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 R 2 ) in the presence of oxygen at high pressures result in the formation of copolymers of the general formula -[-(R 1 ---)x- (Rr--)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 copolymerization in terms of -R 2 " units. The rate of polymerization may then be described in terms of the copolymerization equation and the reactivity ratios 7 8. In the present work, the oxidative polymerization of two systems, vinyl acetate-styrene and vinyl acetate-a-methylstyrene have been in vestigated. 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 temperature 9. 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 Na 2 S 4, they were distilled under reduced pressure. Vinyl acetate (V Ac or V) (Rolex, India) was freed from inhibitor by drying over CaC1 2, 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, CH 2 Cb etc. were puri fied by standard procedures. The required amount of the monomers and AIBN (.1 mol L' 1 ) were placed in a 3 ml Parr reactor (Parr Instruments Co., USA) and pressurized to 1 psi with oxygen. The reactor is equipped with a digital

2 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 5±.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 CH 2 CI 2 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, 2, 4 and 8 C/min with sample sizes of 1-5 mg. Electron impact mass spectra (El-MS) in the positive mode were obtained at 7 ev in a JEOL JMS- OX 33 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. ml min- 1 at 3 C using polystyrene standards. The 1 H and 13 C { H 1 ) NMR spectra were recorded at room temperature on a Bruker ACF 2 MHz spectrometer in CDCI 3 and CH 2 CI 2 ( 2 internal Jock), respectively using tetramethylsilane (TMS) as reference. The 13 C( H 1 ) 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 numberaverage 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 copolymerization 1 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 12 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 348 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 The absorption at 16 cm 1 in the polymers is due to the stretching Table 1-Results of the oxidative polymerization, initiated by A IBN at 5 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) (%) 1 HNMR 13 C{H 1 } NMR ( Mn) Vinyl acetate/styrene PSP VSI VS VS VS vss VS PVAcP Vinyl acetate/a-methylstyrene PAMSP VAl VA VA VA VAS VA PVAcP

3 1284 INDIAN J CHEM, SEC A, DECEMBER 21 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 determined from their 1 H and 13 C{ H 1 } NMR spectra. Both 1 H and 13 C{H 1 } NMR spectra reveal that the monomers do not homopolymerize under high pressure of oxygen employed here for polymerization. For all the polymers, downfield shift of the main chain CH 2 and CH protons is observed due to the adjacent electronegative oxygen atoms to which they are bonded. Figure l depicts 1 H NMR spectra of VS4 and VS5, along with the spectra of the homopolyperoxides, PSP and PVAcP. The 1 H NMR spectrum of PSP shows signals at 8=4.3, 5.31 and 7.22 ppm due to methylene, methine and aromatic protons respectively 11 In the 1 H NMR spectrum of PVAcP, the signals at 8 = 2.7, 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 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 different methyl group signals present in the two different monomers. ll L(a) ppm ppm 2 Fig. I- 1 H-NMR spectra of (a) PSP, (b) VS4, (c) VS5 and (d) PV AcP in CDCI 3. Fig. 2-1 H-NMR spectra of (a) PAMSP, (b) VA4, (c) VAS and (d) PV AcP in CDCI 3.

4 DE et al. : POLYMERIZATION OF VINYL ACETATE WITH STYRENE & a-methylstyrene 1285 Figure 3 displays the 13 C{H 1 } NMR spectra of YS4 and YS5 along with the spectra of the homopolyperoxides. The 13 C {H 1 } NMR spectra recorded under in verse 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 13 C{H 1 } NMR spectrum of PSP exhibits signals at 75.75, 82.81, and ppm; they are assigned correspondingly to methylene, methine, aromatic and aromatic ipso carbon. The signals in the spectrum of PV AcP at 8 = 2.7, 72.7, 95. and 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 13 C{H 1 } NMR spectra (Fig. 4). The 13 C{H 1 } NMR spectrum of PAMSP exhibits signals at 21.85, 78.53, 84.97, and ppm; they are assigned respectively to - CH 3, -OCHr, --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 13 C { H 1 } NMR spectra are in good agreement with those determjned from 1 H NMR spectra. The 13 C{H 1 } NMR spectra for the methylene region are complicated due to chajn irregularity. The polymers show two weak peaks around ppm in the 1 H NMR spectra due to two types of O=CH- groups occurring as chajn ends. There is a signal near 92.7 ppm in the 13 C{H 1 }NMR spectra, which may be assigned to the inclusion of methylene group with peroxy (--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-CH 2 --) in the polyperoxide chrun has been reported for the oxidation of AMS at very low pressures of oxygen 13. (d' J I.JlL- -'-----""''-- (b)-....a.-._,,....._. }LA.,...J PPM Fig C-NMR spectra of (a) PSP, (b) VS4, (c) VS5 and (d) PV AcP in CH,CI, rzo 1 8 PPM Fig C-NMR spectra of (a) PAMSP, (b) VA4, (c) VAS and (d) PV AcP in CH2Cl2.

5 1286 INDIAN J CHEM, SEC A, DECEMBER 21 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 -R 2 " 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 copolymerization 7. However, in the oxidative copolymerization, since the monomers are not homopolymerized and the reaction -R 2 + 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 so2s (ksos)... (i) -so2 + v ---7-so,s (ksov)... (ii) - vo2 + s vo2s (kvos)... (iii) -vo2 + v vo2 v (kvov)... (iv) Applying steady state approximation separately for the reactive species, -S 2 " and -V 2 ", the copolymer composition could be expressed in terms of the feed composition and the reactivity ratios d [S] [S] (r 5 [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 ( -S 2 or -V 2 ") 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 homopolymerization 9. 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 mechanism 19, vinyl polyperoxides 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 t 5 " c rs rv Vinyl acetate/styrene 44.47±.17.23± ±.32.2 ±.3 Vinyl acetate/a-methylstyrene ± ± ± ± Tempuature ("c ) \ Fig. 5-DSC thermogram of VS3 at a heating rate of I C/min.

6 DE er a/.: POLYMERIZATION OF VINYL ACETATE WITH STYRENE & a-methylstyrene 1287 Table 3-DSC Data for the polymers investigated Table 4-The molecular ions identified in the El-MS of VS4 Polymer Structure m/z.,. Vinyl acetate/styrene PSP VS3 VS4 vss PVAcP II 43 CHrC C4H3 51 C6Hs 77 C6Hs-CH2 91 C6H 5 -CO 15 :-:: c <:) u 1 6 c ::>. <(....~ ~ 2 "' Vinyl acetate/a-methylstyrene PAMSP VA 3 VA4 VAS ~r t ~ m/z The base peak at m/z = 15 is derived from the styrene unit. The peak at m/z = 43 corresponds to the CH 3 - 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 (.2453, mole fraction of V Ac). The composition of VS4 thu s 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 C 6 H 5 -CH(CH 3 )- fragment. Fi g. 6- El-MS spectrum of VS4. the degradati on is exothermi c. The enthalpy changes for thermal degradation obtained from the DSC thermogram are given in Table 3. The acti vation energy ( 3 ) for the thermal degradation process was determined by the Ki ssinger's method 2. The slope of the 2 plot of In (<j)/t 111 ) against 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 va lues tabul ated in Table 3, compare we ll with the dissociation energy of the - 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 show n in the Fig. 6. The assignment of the molecul ar ion peaks found in the spectrum is given in Table 4. The primary degradati on 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 vin yl polyperoxides, for YS4 too the degradati on is ini tiated at the weak - bond 12. References I Mogelivich M M, Russ Ch em Rev, 48 ( 1979) Ki shore K & Mukundan T, Narure. 324 ( 1986) Shanmugananda M K, Kishore K & Mohan V K, Macromolecules, 27 ( 1994) Subramanian K & Ki shore K, Polymer, 38 ( 1997) Subramanian K & Ki shore K. Eur Polmz J, 33 ( 1997) T ager A, Physical chemisrrv of polvmers (Mir Publishers) Valvassori A & Sarton G. Adi'Cmces in polymer science (Springer-Verlag, New York) 5 ( 1967/ 1968) Jayanthi S & Ki shore K, Macromolecules, 29 ( 1996) Odian G, Principles ofpolymerizmion (Wil ey, Ne w York ) 3rd Ecln, I Mukundan T & Ki shore K, Prog polym Sci, 15 ( 199) 475. II Cais R E & Bovey FA, Macromolecules. I ( 1977) De P, Sathyanarayana D N. Saclasivamurthy P & Sridhar S. Polymer, 42 (2 I) Mayo F R & Miller A A. J Am cilem Soc. 8 ( 1958) Fawcell A H & Smyth U, Eur Polym J. 25 ( 1989) Mayo F R & Lewis F M, JAm c/iem Soc, 66 (194-1) Niki E. Kamiya Y & Ohta N, Bull chem Soc Japan. 42 ( I %9) Fineman M & Ross S D, J polym Sci, 5 ( 195) Kclen T & T udos F. J macromol sci Ch em, 9 ( 1975) l. 19 MillerAA&MayoFR../ AmchemSoc. 78(1956) Ki ss inger H E. Anal Ch em, 29 ( 1957) Scott G. ATmospheric oxidants and amioxidanrs, (EisCYicr. London), 1965, P 37.