Ester functionalization of polypropylene via controlled decomposition of benzoyl peroxide during solid-state shear pulverization

Polypropylene (PP) is a nonpolar polyolefin that is sometimes functionalized with polar molecules to allow for reactive compatibilization and improve interfacial adhesion. Functionalized PP is conventionally synthesized by melt extrusion at elevated temperature (T) (≥∼180 C) with a radical initiator and polar monomer, e.g., maleic anhydride. A drawback to high T functionalization is that β-scission, which leads to cleavage of C-C backbone bonds, is significant and results in major molecular weight (MW) reduction and property degradation. We present a novel method of functionalizing PP using benzoyl peroxide (BPO) alone by a process called solid-state shear pulverization (SSSP), resulting in ester functional groups (benzoates) grafted at high yield onto PP. Ester functionalized PP (PP-g-ES) is synthesized with limited MW reduction because SSSP is done at sufficiently low T to suppress β-scission. Characterization before and after grafting at 0.18 mol % (0.46 wt %) graft level indicates that functionalization (and subsequent purification) is accompanied by only one chain scission event per 12 400 PP repeat units, resulting in 17% and 36% reductions in number-average MW and weight-average MW, respectively. Benzoate grafting levels are tuned from 0.08 to 0.41 mol % (0.22 to 1.14 wt %) by varying the BPO feed level. In addition to limited MW reduction, PP-g-ES exhibits modified interfacial properties, the ability to undergo transesterification reactions consistent with reactive compatibilization, and little to no loss of physical and mechanical properties relative to neat PP.