Functional Nitroxyls for Polypropylene Modification

Abstract

Chemistry for introducing a long-chain branch (LCB) architecture to linear, isotactic polypropylene (PP) is described, wherein degradation of the polymer matrix through β-scission of macroradicals is inhibited by an additive comprised of a nitroxyl that bears a polymerizable functional group. Trapping of alkyl macroradicals by the nitroxyl yields a macromonomer derivative that crosslinks when exposed to residual initiator-derived radicals. As such, formulations containing 4-acryloyloxy-2,2,6,6,-tetramethylpiperidine-1-oxyl (AOTEMPO) and a peroxide transform linear PP into a gel-free, long-chain branched derivative (LCB-PP) that exhibits exceptional strain hardening with minimal loss in matrix molecular weight. The chemistry is extended to produce PP bearing pendant anhydride and/or imide grafts. Formulations containing 4-vinyl ether-2,2,6,6-tetramethylpiperidine-1-oxyl (VETEMPO), maleic anhydride (MA), and peroxide produced anhydride-functionalized polypropylene (PP-g-MA). Following macroradical trapping by VETEMPO, pendant vinyl ether groups undergo a strictly alternating copolymerization with MA, thereby introducing anhydride without incurring large-scale matrix degradation. At a given melt viscosity, this technology provided PP-g-MA derivatives with higher graft content than standard peroxide-only reactions. Moreover, when combined with the difunctional monomer N,N’-1,3-phenylene-dimaleimide (PDM), VETEMPO enabled the production of highly crosslinked PP with an appreciable gel fraction.