Is isoprene capable of being a comonomer under metallocene catalysis to reach side chain-unsaturated isotactic polypropylene?

Isoprene (2-methyl-1,3-butadiene) was attempted as a comonomer in metallocene-catalyzed propylene polymerization to prepare side chain-unsaturated isotactic polypropylenes (i-PP). Hydrogen was added into the sluggish copolymerization of propylene and isoprene mediated by a highly isospecific metallocene complex rac-Me2Si(2-Me-4-Ph-Ind)2ZrCl2/MAO. Catalyst activity was restored due to breaking by hydrogen of the stable π-allyl zirconocene species caused by 1,4-isoprene insertion. Stunningly, it was found that the generated Zr-H species had been able to take the accompanying vinylidene-terminated polymer chain for insertion as a preemptive mode of re-initiation. With a consecutive action of chain release followed by in situ reinsertion, the hydrogen-induced catalyst reactivation was not accompanied by chain termination. With the smooth occurrence of the copolymerization, the 1,2-insertion of isoprene led to the preparation of i-PP bearing pendant vinylidene groups.