NMR Study of Grafting Polystyrene on Low MolecularWeight Polybutadiene in Thermal Polymerization

In this study styrene was thermally polymerized by solution method in the presence of low molecular weight polybutadienes (PBDs) at 100, 175, 250ºC. Two types of PBD with various contents of 1, 2-vinyl and cis/trans isomers were employed. Using PBDs of low molecular weight allowed the study of grafting mechanisms by solubleژ grafted products obtained for liquid nuclear magnetic resonance (NMR) characterization. The effect of polymerization temperature and the type of polybutadiene on grafting degree were investigated. It is shown that grafting of polystyrene (PS) on PBD increases with increasing 1, 2-vinyl content and temperature of polymerization. Accordingly, PBD with high 1, 2-vinyl content and polymerization at 250ºC gave the highest grafting efficiency. However, grafting was identical for cis and trans isomers in 1, 4-PBD. Liquid 13C {1H} NMR spectroscopy using distortion enhancement by polarization transfer (DEPT) techniques were performed and a mechanism of grafting was proposed based on addition reaction with reducing the unsaturated olefinic double bonds without formation of any quaternary olefinic or aliphatic carbons. With quantitative analysis of triad sequences of 1, 2-PBD methylene carbon before and after grafting, it was shown that polystyrene chains mainly graft on syndiotactic sequences of 1, 2 PBD.