Reaction mechanism of suspension graft copolymerization of styrene and acrylonitrile in the presence of ethylene propylene diene terpolymer
A sequential synthesis of ethylene propylene diene terpolymer-g-poly (styrene-co-acrylonitrile) (EPDM-g-SAN) graft copolymers was carried out via suspension graft copolymerization using benzoyl peroxide (BPO) as an initiator. After the synthesis, the final product was separated by Soxhlet extraction to obtain the styrene-acrylonitrile free copolymer (SANf), non-grafted EPDM and EPDM-g-SAN, which were subsequently characterized by 1H-NMR and FTIR analyses to verify their purity. The dependence of the reaction behaviors on the reaction time was systematically investigated. When the reaction time was varied from 20 min to 100 min, the GPC analysis of the obtained SANf showed a bimodal profile, due to the co-existence of SANL and SANH free copolymers, of low and high molecular weights, which were respectively formed by chain transfer termination and bi-radical termination of the free polymeric radicals. These results revealed that the graft site initiation was dominated by free polymeric radicals attacking the EPDM backbones at the beginning of the grafting process, accompanied by non-grafting copolymerization reactions. On the other hand, the GPC analysis of non-grafted EPDM and EPDM-g-SAN revealed that the reaction mechanism also involved the chain scission of the EPDM molecules which were partly incorporated into the backbone of EPDM-g-SAN and the incorporation of SANf into the grafted chains (g-SAN), resulting in added grafted EPDM and grafting ratio. Furthermore, a novel and effective method was developed to determine the number of g-SAN, and thus the number-average molecular weight of g-SAN was estimated.