Synthesis of isotactic polypropylene containing a terminal Cl, OH, or NH₂ group via metallocene-mediated polymerization/chain transfer reaction

This paper discusses a direct (one-pot) polymerization process to prepare isotactic polypropylene (i-PP) having a terminal functional group including Cl, OH, and NH₂. The chemistry involves metallocene-mediated propylene polymerization using rac-Me₂Si[2-Me-4-Ph(Ind)]₂ZrCl₂/MAO complex in the presence of styrene derivatives (St-f), carrying a Cl (St-Cl) or a silane-protected OH (St-OSi) or a silane-protected NH₂ (St-NSi₂), followed by hydrogenation. Apparently, the propylene propagating chain end engages in a facile consecutive chain transfer reaction, reacting with St-f and then hydrogen, with high catalyst reactivity under the proper St-f and hydrogen concentrations. The polymer molecular weight was inversely proportional to the molar ratio of [St-f]/[propylene] with a chain transfer constant (k_tr/k_p) of 1/21 for St-Cl, 1/34 for St-NSi₂, and 1/48 for St-OSi, respectively. Both silane protecting groups were hydrolyzed in acidic aqueous solution during the sample workup step to obtain the desirable i-PP polymers with a terminal OH and NH₂ group (i.e., PP-t-St-OH and PP-t-S-NH₂). The terminal functional group was confirmed by end group analysis and chain extension reaction. Despite the high molecular weight, the terminal functional group in PP engages a coupling reaction with polycapolactone (PCL) in solution and melt to form PP-b-PCL diblock copolymers that are very effective compatibilizers in PP/PCL polymer blends.