Polymerization of methyl acrylate using a heterocyclic ylide as an initiator and degradative chain transfer agent

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1 Indian Journal of Chemical Technology Vol. 4, January 1997, pp Polymerization of methyl acrylate using a heterocyclic ylide as an initiator and degradative chain transfer agent Rita Vasishtha & A K Srivastava" Department of Chemistry, H B Technological Institute, Kanpur , India Received 13 February 1996; accepted 21 August 1996 The free radical polymerization of methylacrylatc has been investigated at 80 ± 1 C for 60 min in l,4-dioxan using p-bromophenacyl dimethyl sulphonium ylide as initiator and degradative chain transfer agent up to 23% conversion. No gelation occurred due to auto acceleration leading energy and the value of il, were calculated as 72 J mol-i and 18 x 10-2 Ls'-I, respectively. The radical mode of polymerization was confirmed by ESR and the effect of hydroquinone. A possible mechanism of the system is also proposed. A substance which decreases the rate of polymerization as well as the average degree of polymerization of the polymers is called a degradative chain transfer agent ana the effect is referred to. as degradative chain transfer'. It is found that dehydromycene and isopropylbenzene act as degradative chain transfer agents for the polymerization of vinyl acetate". Dimethylsulphides and diazoaminobenzene have been used as initiators and degradative chain transfer agents, respectively='. It is well-nown+' that bul polymerization of methyl acrylate (MA) exhibites a large auto-acceleration of the overall rate of polymerization. referred to as Trommosdorff.or gel effect, with associated increase in the molecular weight of the polymer. The present wor shows that the heterocyclic ylide p-bromophenacyl dimethyl sulph onium ylide (p-bpdsy) could be used in low concentration as a radical initiator to carry out polymerization runs up to 23% conversion without gelation due to autoacceleration. Experimental Procedure Methyl acrylate (Ranbaxy) and l,4-dioxan (Qualigen) were purified by standard procedure while p-bromophenacyl dimethyl sulphonium ylide (p-bpdsy) [Br-C 6 H 4 -CQ-CHS(CH 3 h) was prepared by a reported method", Polymerization reactions were carried out at 80 ± 1 C for 60 min using dilatometric technique. The dilatomer was filled with the required amount of solvent, monomer, initiator and flushed with purified nitrogen gas, stopped and placed in Author to whom correspondence should be addressed. a thermo stated bath. The progress of the reaction was monitored with the help of cathetometer. The rate of polymerization (Rp) was calculated from the.slop of percent conversion vs time plots. The intrinsic viscosity ('I) of polymethyl acrylate determined in benzene at 30 C with an Ubbelhode viscometer was used to calculate the average viscosity molecular weight (Mv) using the equation (['I] in dug); ('1)= 4. x 10- Mv... (1) Results and Discussion The results of inetic investigations of the polymerization of methyl acrylate at 80 ± 1 C initiated by p-bpdsy in 1,4-dioxan for 60 min are summarized in Tables 1-3 and Figs 1-4. The ylide (p- BPDSY) initiates the polymerization in which no gelation occurs up to 23% conversion and the resulting polymers has a high molecular weight. Variation of the rate of polymerization with temperature- In order to study the effect of temperature on the rate of polymerization, the poly- Table 1- Effect of [MA] on the polymerization of methyl acrylate initiated by p-bpdsy at 80 ± l C' [MA], Conversion, Rp x los, Mn mol L-I % mol L-I S-I , , , ,820 'Polymerization condition: [p-bpdsyj=4.66 x 10-3 mol L-I; Time =60 min.

2 14 INDIAN J. CHEM. TECHNOL., JANUARY 1997 Table 2- Effect of concentration on the polymerization of methyl acrylate initiated by p-bpdsy at 80 ± 1 C Additive Additive, Conversion, e, x 10, Mn mol L- 1 % mol L- 1 S - 1 a Benzene ,680 b ,810 c ,800 a DMSb ,680 b ,690 c ,910 a Hydroquinone ,680 b 1.1 x c 3.03 x 10 2 Inhibited =Polyrnerization condition: [MAj= 3.48 mol L-I; [p-bpdsy]= 7.77 x 10-3 mol L-I; Time=60 min. z o 0 iii > Z o U ~>- Z III U n TIME,h TIME,h Fig. l++conversicn. vs time plots, for the polymerization of methyl acrylate at 80 C in [MAl = 348 mol L- 1 [p-bpdsy] = 0.43 (0),1.31 (e), 1.74 (Ll), 2.18 (A), 2.62 (x), 3.0 (0), 3.93 x 10-6 (.) mol L-I merization runs were performed at different temperature, i.e., 70, 7, 78 and 80 C using 3.4 mol L-] of MA and 4.66 x 10-3 mol L- ] of (p- BPDSY). R; increases with polymerization temperature. The overall energy of activation was calculated from the Arrhenius plot (Fig. 3) as 72.0 J mol-]. This value also confirms the radical mode of polymerization. Effect of additives-table 2 shows the effect of polar and non-polar solvents. It is clear that nonpolar solvents lie benzene decrease the Rp and polar solvents lie DMSO favour the polymer formation. There results can be explained on the basis of the fact that benzene (non-polar) suppresses the decomposition of initiator and DMSO being a polar solvent enhances the decomposition of initiator, thereby, increasing the Rp. Variation of the rate of polymerization with ylide concentration-the results of polyrnerization of MA in 1,4-dioxan, carried out at 80 ± 1 C varying in (n) (p-bpdsy) from 1.01 x 10-3 to x 10-3 mol L- I while eeping [MA] constant at 3.48 mol L-1, are presented in Table 3 and Fig. 1. The polymerization was associated with varying induction periods. It is observed that the ylidc initiates the radical polymerization of MA and Rp increases when ylide concentration is varied at low concentration (from 1.01 x 10-3 to 1. x 10-3 mol L- 1). The order of reaction, calculated from the slope of the plot of log 10 Rp vs log 10 (p-bpdsy), is 0., as expected for ideal inetics. This is further supported by the facts that the line of the plot of limn vs p-bpdsy passes through the origin and the hydroquinone inhibits the polymerization reaction at 3.03 x 10-3 mol L- 1 (Table 2). Besides, the value of i I.; calculated from the slope of the plot between limn and R/(M)2 is 18xlO- 2 L mol s-i, which matches well with those reported in literature 7.

3 VASISHrnA & SRNASTAVA: POLYMERIZATION OF MElHYL ACRYLATE a. 0.7 co.s.;, \og [MAl Fig. 2-Plot between loglo Rp and log 10 [MA) for the polymerization of methyl acrylate at 80 C [p-bpdsy] = 4.66 x 10-3 mol L-I Table 3-Effect of the concentration of p-bpdsy on the polymerization of methyl acrylate initiated by p-bpdsy at 80 ± 1 C' [p-bpdsy] X 10 3, Conversion, Rp x 10, M n moll-i /0 mol L-I S-I apolymerization condition: [MAl = 3.48 mol L- 1 ; Time = 60 min. 0.9 a. : F 0.8 It) Fig. 3-Arrhenius plot, for the polymerization of methyl acrylate [MA)=3.48 mol L-I, [p-bpdsy]=4.6x 10-3 mol L-' 40 However, when inetic studies were extended at higher [ylide] from 3.1x 10-3 to 12.43X 10-3 mol L- 1 (Table 3) Rp is an inverse function of [ylidel. This might be due to mutual anhilation of initiator radicals formed beyond 1.x 10-3 mol L-I of [ylide] The order of reaction was calculated to be 1.6, which shows that the system follows non-ideal inetics. Table 3 also. reveals that Mn of the polymers decreases as [ylide1 increases. The non-ideality is further confirmed by the fact that the plot of limn vs (p-bpdsy) (beyond 1. x 10-3 mol L-I) does not pass through the origin and some additional mode of termination is operating with usual bimolecular termination. The additional initiator dependent termination process may be of two inds as shown below: (A) Primary radical termination: MA +H ~ polymer co ~ ().8 + en [p- BPDSVl x 16 [MAl (B) Termination via degradative transfer, either (i) With reinitiation: MA +p-bpdsy ~ MA + p- BPDSY...:.:!!L.. Polymer +p-bpdsy Polymer (initiator transfer) (chain termination) Fig. 4-Relationship between log RiI[p-BPDSy] = [p-bpdsy]![ma] to analyse degradative initiator transfer with reinitiation effect, in the polymerization of methyl acrylate at 80 C, [MAl = 3.48 mol L-I. MA+ p-bpdsy..::!!l... or MAt (reinitiation)

4 16 INDIAN J. CHEM. TECHNOL., JANUARY 1997 (ii) With little reinitiation: l MA +p--bpdsy ~ Polymer (chain termination) + non-radical or inactive radical byproduct The equation developed by Deb and Meyerhoff" was used to analyse the possibility of additional modes of termination in the following manner: Primary radical termination: &2 I lodor: log 10 (Y)(M)2 oglo, Termination: (1) bimolecular MAn + MAn!s.- polymer (2) by initiator MAn +p-bpdsy ~ polymer The rates of initiation (R j ), propogation (Rp) and termination (RI and R/) are given by: R, = d X IG lma] [p--bpdsy] Rp = p [MA] [MA] RI=2; [MA]2 R/ = / [MA] [p--bpdsy] Assuming steady state conditions: R, Overall rate of termination --= e, s, [ l, d' p' andz, part are rate constant of initiation, imtiator decomposition, propogation and termination, respectively, and Y is the initiator]. The plot of the left hand side of the above equation vs R/(M)2 gave a positive slope, indicating the absence of primary radical termination for the present system. Degradative initiation transfer with reinitiation ejfect- The equation derived by Deb? was used to study the degradative initiator transfer with reinitiation: where C y is the initiator transfer constant. The plot of the left hand side of the above equation vs (Y)XM) (Fig. 4) gives a negative slope indicating degradative initiator transfer with reinitiation. The value of (rty )/( IGry - p).x C, determined from the slope, is 8.62 x 10 -I Ls.mol" 1. Degradative initiation transfer with little reinitiotion+this was treated in the following manner: Initiation: d p-bpdsy=h MA+H -MJ\ Propogation: MA + nma.2l.. MA (n+ 1) + ; [p-bpdsy] Rp p [MAl _ 2~ R: = ; Rp dj p 2 [MA]3[p-BPDSy] p [MA]2 The plot of the left hand side of the above equation vs R![MA]2 gives a positive slope, ; / p > 0 (;) > 0 ;hich, therefore, excludes the possibility of the above aspect. ESR spectrum-the radical mode of polymerization of the present system is confirmed by the ESR spectrum which shows a characteristic freeradical absorption signal at 3,30 gauss. The gyromagnetic ratio (g) is calculated to be , which confirms the radical mode of polymerization.

5 VASISHTHA & SRIVASTAVA: POLYMERIZATION OF METHYL ACRYLATE 17 NMR spectrum-the NMR spectrum of PMA shows singlet at 3.60 which is due to the ester methoxy group and the methine proton appears at 2.3. The methylene protons appear as triplet at 1.63, 1.88 and 2.1. It has already been reported 11 that ylides, in general dissociate to form triplet carbene which combines with acrylates to form diradical and ultimately H". Although formation of diradical is hypothetical of H" has been confirmed by the ESR spectrum of the system. The H" is responsible for bringing out polymerization. The various steps may be outline below: References I Bevington J C, Radical polymerization (Academic Press, New Yor), 9(1961) Allen P W. Merrett F M & Scanlan J, Trans Faraday Soc. 0(19g4)76. 3 Ferington T & Tobolsy A V, J Am Chern Soc, so (198) Harward R N & Simpson W, Trans Faraday Soc, 47 (191) 212. Trommsdorff E, Kohle H & Lagally R, Maromol Chern, 1 (1948) R 0 -? Br--IJ-C-CH +!'LOCH -ar-d--c -CH-CH2 ~ 3 ~ J CH I HJC-O-C:O -OR OR Ir ~ /I C-CH-C~-CH - If ~ II C-CH=CH + H' I th (initialing C:O 1 specl ) I.., C =0 OCH3 Inactl"c specl.. I OCHJ 6 Ratts R W & Yao A N, J Org Chern, 31 (1966) Shula A K, Saini S, Kumar P, Nigam S K & Srivastava A K, Angew Maromol Chern, 141 (1986) Deb P C & MayerhoffG, EurPolymJ, 10(1974) Deb P C, Eur PolyJ, 11 (197) 3l. 10 Pryor W A, Free Radicals, (McGraw Hill, New Yor), 1966, Vasishtha R, Sain S, Nigam S K & Srivastava A K, J Macromolecular Sci, Rev Macromol Chern Phys, 21 (1 ) (1989)39..'

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Pelagia Research Library Available online at www.pelagiaresearchlibrary.com Der Chemica Sinica, 2012, 3(1): 10-23 ISSN: 0976-8505 CODEN (USA) CSHIA5 Anthracene Iodine Charge transfer complex as degradative chain transfer agent