Synthesis of 3-carbonoyl acrylic acid-functionalized polystyrene and an insight in to its role in cross linking and grafting of polystyrene on to natural rubber

Abstract

Polystyrene (PS), partly substituted with 3-carbonoyl acrylic acid (ML-PS) was synthesized by reacting PS with maleic anhydride under Freidel-Crafts conditions·BF3 served as a catalyst. The product was thoroughly characterized. The acylation occurred to an extent of 10 mol% of styrene. The strong electron-withdrawing carbonoyl and carboxylic acid groups on either side rendered the π-bonds so electron-deficient that they readily formed a charge-transfer complex (CTC) with the electron-rich double bonds in natural rubber. This was very much evident from UV spectral analyses. The CTC led to an alternating copolymerization between the two unsaturated compounds leading also to cross linking. The formation of CTC and subsequent reaction to form alternating polymer was further confirmed from studies on model organic molecules viz: 2-Methyl 2-Butene (MB) and maleic anhydride (MAn). The same was extended to the CTC pairs PS-ML and MAn. MB represents polyisoprene units in natural rubber. When initiated by a free radical source, an alternating “copolymer” of the two (MB and MAN) with limited degree of polymerization (DP) was formed. A similar interaction and copolymerization was observed between MB and ML-PS too. Characterization of the resultant “polymers” confirmed formation of an alternating sequence between the two unsaturated compounds. Extrapolating the same mode of copolymerization to natural rubber (NR) and ML-PS, it is concluded that generation of the maleic type unsaturation on PS renders the latter capable of forming a charge transfer complex with natural rubber (NR). This CTC gives way to a polymer wherein, styrene segments of pre-determined dimension are grafted on to rubber. In contrast to a non-reactive blend where the rubber addition decreased the strength of the blend, the reactive blend gave a higher tensile strength which increased with increase in crosslinking. The reacted blend showed a single phase conforming to the Flory-Fox equation on Tg. This polymer provided enhanced fracture energy with increase in rubber part in contrast to a non-reacted blend which showed an overall decrease in property with rubber addition. Studies extended to the pair ML-PS and MB also confirmed a similar reaction pattern. This strategy of functionalization of PS could help form a copolymer with segments of desired configuration. Revelation of these findings might increase the recycling prospects of waste polystyrene and scrap rubber.