On the structural and thermal features of novel 4-methyl-1-pentene copolymers with uncommon microstructures by C2 symmetric metallocenes

Copolymers of ethylene (E) and propylene (P) with higher 1-olefins, such as, 1-butene, 1-hexene, 1-octene, have major industrial importance in the world of the polyolefinic materials. The development of single site metallocene and post-metallocene catalysts capable of incorporating high amounts of bulky comonomers, with the possibility of tailoring copolymer properties by controlling chain composition and microstructure, has burst out a renewal of interest in 4-methyl-1-pentene (4M1P) copolymers. Indeed, polymeric materials based on 4M1P display a unique combination of physical properties such as, transparency, low density, high gas permeability, due to the presence of the bulky branched 4M1P units. Within the framework of a systematic study on the structure-properties correlations of polyolefinic copolymers we prepared various series of E/4M1P and P/4M1P copolymers by using different C2 symmetric metallocenes with the aim of tuning microstructure and, in turn, material properties [1-4]. A thorough 13C NMR characterization combined with a statistical elaboration of copolymerization reactivity ratios allowed us to assess their microstructure. For the first time, it was shown that non-living isospecific metallocene catalysts are capable of giving basically blocky E/4M1P copolymers presenting segments highly rich in E and 4M1P co-units. The length of the polyethylene and poly-4M1P sequences was correlated with the isoselectivity or, more correctly, the steric hindrance of the catalytic complex. Results of DSC and XRD analyses revealed the presence of crystallinities arising from both comonomers, even simultaneous in samples with almost equimolar composition. A reasonable forecast of comonomer distribution in analogous P/4M1P copolymer series appeared no possible on the basis of the results obtained for the E/4M1P systems, as testified by the attainment of different microstructures: from truly random to blocky. The specific and combined roles of the P/4M1P pair and of metallocenes with different isospecificity and steric hindrance in the melt crystallization behaviour of P/4M1P copolymers was investigated.