ELSEVIER S0032-3861(96)00235-2 Polymer Vol. 37 No. 18, pp. 4141-4148, 1996 Copyright © 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0032-3861/96/$15.00 + 0.00 Kinetic investigation of the free-radical crosslinking copolymerization of styrene with a mixture of divinylbenzene isomers acting as the crosslinker S. Sajjadi*, S. A. M. Keshavarz and M. Nekoomanesh Polymer Research Center of Iran, PO Box 14185/458, Tehran, /ran (Received 2 November 1994; revised 3 July 1995) The objective of this paper is to study the crosslinking copolymerization of styrene (S) with a mixture of para- and meta-divinylbenzenes (mixed-DVBs). The crosslinking copolymerization, initiated with benzoyl peroxide (BPO), was performed at temperatures of 70, 80, and 90°C. The mole ratio of DVB to S was kept small, i.e. within 4-8%. At various stages of the polymerization the overall conversion of the monomers to sol and gel were measured by gravimetry. The sol and gel formations with time were also measured. The gels were characterized by equilibrium swelling measurements as a function of the overall degree of conversion. The experimental data were then incorporated into the kinetic model developed by Tobita and Hamielec. The kinetic parameters involved in the crosslinking copolymerization of S/DVB were also estimated. A satisfactory agreement of the results obtained from the experimental measurements with the predictions of the model was observed. Copyright © 1996 Elsevier Science Ltd. (Keywords: crosslinking; free-radical copolymerization; kinetics) INTRODUCTION Most modern ion-exchange resins consist of a synthetic polymer backbone to which is attached a functional group that gives each ion exchanger its specific properties. The polymer matrices are usually crosslinked to make them insoluble and to give them mechanical strength and stability. The most common matrices are styrene- divinylbenzene (S/DVB) copolymers in which DVB is the crosslinking agent, and is typically present in 4-12% concentration. The copolymerization of DVB with S was first studied by Staudinger and Heuer 1, who demonstrated that crosslinking does occur for S/DVB systems. Storey 2 compared the initial rates of the S/p-DVB copolymerization with those measured for S and p-DVB hompolymefization and concluded that crosslinking results in an increase in the reaction rate. Gel times were measured and it was concluded that the higher the content of crosslinker, then the less would be the conversion required to reach the gelation point. The overall first-order rate constants for S/p-DVB copolymerization over a broad range of compositions, initiated by 1% BPO at temperatures of 70 and 90°C, were calculated. The conversion-time curves obtained from dilatometric measurements for 4, 10, and 20% p-DVB at 70°C were also reported. It was postulated that all of the double bonds in the S/p-DVB system are of equal reactivity. *To whom correspondence should be addressed. Present address: Department of Chemical Engineering, Loughborough University of Technology, Loughborough Leicestershire LE11 3TU, UK Hild and Okasha 3 studied the kinetics of the free-radical copolymerization of S and commercial DVB (para- and meta-isomers) in the presence of carbon tetrachloride as a chain transfer agent. In further work they investigated the kinetics of free-radical crosslinking copolymerization of S with p-DVB or m-DVB in benzene solution at 60°C 4. The DVB proportions were kept small with respect to styrene, in order to reduce the importance of the DVB radicals in the reaction media. The consump- tions of S and DVB isomers were measured as a function of time. The radical reactivity ratios of S with respect to both m-DVB and p-DVB were also obtained. 5 Malinsky et al. evaluated the copolymer composition of S with p-DVB or m-DVB, and indicated that after one of the vinyl groups in the DVB isomers has reacted, the reactivity of the remaining vinyl group then falls. It was also concluded that the pendant double bonds in both the p-DVB and m-DVB units have the same reactivity. They also evaluated the effect of concentration of the DVB isomers on the conversion at the gel point. This conversion was found to depend on the concentration of DVB at lower values of the latter (< 20%). Wiley et al. 6 measured the reactivity ratios of S/m- DVB and S/p-DVB at low conversion by differential analysis. In further work, they used the integrated form of the copolymerization equation to determine the 7 reactivity ratios. This method was claimed to avoid errors encountered in the low-conversion data that was required for the differential analysis. Hild and Okasha 8 investigated the crosslinking copolymerization of S/p-DVB and S/m-DVB in the post-gel state, and determined the swelling degree and POLYMER Volume 37 Number 18 1996 4141