Ester Functionalization of Polypropylene via Controlled Decomposition of Benzoyl Peroxide during Solid-State Shear Pulverization

Abstract

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 (PPg-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. ■ INTRODUCTIONPolyolefins are nonpolar by nature which leads to excellent chemical resistance but poor compatibility with polar molecules. When functionalized with polar moieties 1−15 such as hydroxyl groups, 2,3 itaconates, 4 methacrylate esters, 4,5 and maleic anhydride, 6−12 the modified polyolefins are useful as compatibilizers in blends, 16−18 composites, 19,20 and nanocomposites. 21−23 Commercial, functionalized polyolefins are based on covalent addition of functional monomers to polyolefins in the presence of a radical initiator; the major categories of this type of functionalization are discussed in detail in a recent publication. 12 The most common method used for functionalization is melt extrusion; under the high temperature (T) conditions utilized, functionalization is usually accompanied by undesirable side effects, including chain scission, branching, and cross-linking. 24−28 For this reason, there is ongoing interest to develop improved, commercially scalable methods for postpolymerization functionalization of polyolefins in the presence of both a radical initiator, such as an organic peroxide, and a functional monomer. 12 An alternative method for direct functionalization of polyolefins in the absence of functional monomer was described in a 2005 review by Boaen and Hillmyer, 29 who stated, "An interesting approach that is related but could prove to be more efficient is the direct functionalization with organic peroxides alone." At...

Novel synthesis of branched polypropylene via solid-state shear pulverization

“…During SSSP, functionalization was made possible by both thermal and mechanochemical decomposition of BPO. Under the near-ambient temperature conditions associated with SSSP, the extent of decarboxylation of the benzoyloxy radicals formed during BPO decomposition is significantly decreased, allowing for effective functionalization of PP with benzoates and resulting in grafting yields of 36e87% [57]. Contrary to the results obtained via SSSP, when PP and BPO were processed by melt mixing at 200 C, no benzoate grafting was achieved [57].…”

“…We demonstrated recently that SSSP can be used to graft ester functional groups (benzoates) onto PP at high yields using BPO alone [57]; the chemical structure of the so-formed ester functionalized PP (PP-g-ES) is shown below. During SSSP, functionalization was made possible by both thermal and mechanochemical decomposition of BPO.…”

“…Under the near-ambient temperature conditions associated with SSSP, the extent of decarboxylation of the benzoyloxy radicals formed during BPO decomposition is significantly decreased, allowing for effective functionalization of PP with benzoates and resulting in grafting yields of 36e87% [57]. Contrary to the results obtained via SSSP, when PP and BPO were processed by melt mixing at 200 C, no benzoate grafting was achieved [57]. Upon heating to high temperature, the extent of benzoate decarboxylation is extremely high and radicals from BPO are lost long before the PP melts, preventing PP-g-ES synthesis.…”

“…Upon heating to high temperature, the extent of benzoate decarboxylation is extremely high and radicals from BPO are lost long before the PP melts, preventing PP-g-ES synthesis. We demonstrated that SSSP provides a new platform of chemistries, which is not attainable by melt processing, for direct functionalization of PP with benzoates [57].…”

“…Compatibilization and intimate mixing of immiscible blends [47e51] and effective dispersion and exfoliation of filler in composites and nanocomposites can result from SSSP [52e55,58e61]. Recently, we showed that SSSP can be used to synthesize maleic anhydride grafted PP with strongly suppressed MW reduction [56] and ester functionalized PP with moderate MW reduction using a symmetric organic peroxide alone [57]. In the current study, we show that by taking advantage of the near-ambient temperature conditions associated with SSSP, LCB PP can be synthesized during SSSP or in post-SSSP melt extrusion.…”