Silane-functionalized isotactic polypropylene: A direct route to advanced polyolefin materials

Alkenylsilanes represent promising comonomers for the synthesis of functional polyolefins through direct catalytic copolymerization, preventing catalyst deactivation due to the weak interaction between the silicon atom and the metal center. However, their potential has been largely overlooked due to their tendency to undergo β-elimination, which leads to the formation of low-molecular-weight polymers with compromised material properties. We show that isotactic polypropylene-based copolymers with incorporated silane groups can be obtained by stereoselective Hf-catalysed copolymerization of propylene and allyltrimethylsilane (ATMS). These materials possess a random microstructure with ATMS contents ranging from 5.5 to 35.0 mol%. Contrary to the previously reported copolymerization of ATMS with α-olefins using metallocene catalysts, no involvement of ATMS in enhanced chain transfer rates could be detected and samples with high molecular weights were obtained (Mn > 600 kg/mol). The incorporation of bulky ATMS comonomer units leads to a significant reduction in the crystallinity of the copolymers compared to the iPP homopolymer, resulting in a markedly different mechanical response, characterized by enhanced flexibility and ductility compared to the brittle iPP. The possibility to tune the crystallinity, combined with the presence of functional comonomeric units, allows to obtain copolymers with a broad spectrum of properties, with glass transition temperatures ranging from subambient values to Tgs approaching room temperature. Notably, at high ATMS contents, the copolymers exhibit exceptional elastic properties, which have been rationalized on the basis of the structural transformations occurring during deformation that involve the formation of a novel mesomorphic phase of iPP, which has not been previously reported in the literature.