Preparation of Polystyrene-Polyolefin Multiblock Copolymers by Sequential Coordination and Anionic Polymerization

Abstract

Block copolymers composed of polyolefins (PO) and polystyrene (PS) are attractive materials that are not synthesized directly from the olefin and styrene monomers. A strategy for construction of PS-b-PO-b-PS triblock units directly from the olefin and styrene monomers is disclosed herein. PO chains (ethylene/1-octene or ethylene/1-pentene copolymers) were grown from dialkylzinc species bearing the -methylstyrene moiety (i.e., [4-(isopropenyl)benzyl]2Zn) by the so-called 'coordinative chain transfer polymerization (CCTP)' using a standard ansa-metallocene catalyst, rac-[Me2Si(2-methylindenyl)2]ZrCl2, activated with modified-methylaluminoxane (MMAO). Afterward, PS chains were grown from the Zn-alkyl sites and from the -methylstyrene moieties of the resulting PO chains by switching to anionic polymerization. When nBuLi(tmeda)2 was fed into the system as an initiator in a quantity fulfilling the criterion [Li] > [Zn] + [Al in MMAO], nBuLi(tmeda)2 successfully attacked the -methylstyrene moieties to generate the desired PS-b-PO-b-PS triblock units. However, in model studies, the attack of nBuLi(tmeda)2 on -methylstyrene in the presence of (hexyl)2Zn consumed two molecules of -methylstyrene per nBuLi to afford mainly R-CH2C(Ph)(Me)-CH2C(Ph)(Me)Li, where R is either an n-butyl or hexyl group originating from nBuLi or (hexyl)2Zn, respectively. This observation suggests that the block copolymer does not simply comprise the PS-b-PO-b-PS triblock, but instead comprise a multiblock containing PS-b-PO-b-PS units. The molecular weight of the polymer increased after performing anionic polymerization. Formation of multiblock copolymers leads to less regular phase separation of the PS and PO blocks.