Cross-linked polypropylene prepared by PP copolymers containing flexible styrene groups

This paper discusses an effective method to prepare cross-linked isotactic polypropylene (PP-X) with high purity (almost 100% gel content and no contamination) and high melting temperature and crystallinity. The reaction scheme involves a linear poly(propylene-co-p-(3-butenylstyrene)) copolymer (PP-BSt) containing few pendent styrene groups and the subsequent thermal treatment without any external reagent. The intermediate PP-BSt copolymers were prepared by a specific rac-CH ₂(3-tert-butyl-Ind) ₂ZrCl ₂/MAO catalyst that not only performs iso-specific propylene insertion but also incorporates BSt units with a preferred α-olefin insertion over styrene moiety. The combined features permit the preparation of linear PP-BSt copolymers with high molecular weight and high catalyst activity, without the presence of hydrogen. The resulting PP-BSt copolymers having some flexible pendent styrene moieties are completely soluble in xylene at elevated temperatures, and the solution case films show active cross-linking activity at temperatures >160 °C by engaging in a Diels - Alder [2 + 4] interchain cycloaddition reaction between the pendent styrene units. Evidently, the flexibility of styrene units is important, which enhances the interchain cyclization to form a complete 3-D network, even with a very small amount of BSt units. In contrast, the corresponding poly(propylene-co-p-divinylbenzene) (PP-DVB) prepared by the same metallocene catalyst only shows moderate cross-linking efficiency under similar conditions.