Defining the Macromolecules of Tomorrow through Synergistic Sustainable Polymer Research.

Transforming how plastics are made, unmade, and remade through innovative research and diverse partnerships that together foster environmental stewardship is critically important to a sustainable future. Designing, preparing, and implementing polymers derived from renewable resources for a wide range of advanced applications that promote future economic development, energy efficiency, and environmental sustainability are all central to these efforts. In this Chemical Reviews contribution, we take a comprehensive, integrated approach to summarize important and impactful contributions to this broad research arena. The Review highlights signature accomplishments across a broad research portfolio and is organized into four wide-ranging research themes that address the topic in a comprehensive manner: Feedstocks, Polymerization Processes and Techniques, Intended Use, and End of Use. We emphasize those successes that benefitted from collaborative engagements across disciplinary lines.

Isotactic Poly(propylene oxide): A Photodegradable Polymer with Strain Hardening Properties.

Leakage and accumulation of highly stable commercial plastics has led to substantial contamination of the environment. Highly isotactic poly(propylene oxide) (iPPO) was investigated as a potential high-strength thermoplastic with greater susceptibility toward degradation under ambient conditions. Various stereoregular forms of iPPO including enantiopure, enantioenriched, racemic, and stereoblock were synthesized with a single catalyst architecture in the presence of chain transfer agents. These materials were found to possess the same approximate ultimate tensile strength (UTS) via uniaxial tensile elongation analysis (∼75 MPa). A serrated tensile response corresponding to stress oscillations was observed in all forms of iPPO. An investigation on strain rate dependence showed that an increase in strain rate results in the decay and disappearance of the serrated response. Further evaluation of iPPO revealed its dramatic strain hardening afforded an UTS comparable to that of nylon-6,6. Exposing iPPO to UVA light (365 nm) resulted in photolytic degradation. Following 30 days of continuous exposure at 250 µW cm-2, the Mn decreased from 93 kDa to 21 kDa, while samples not exposed to UVA light remained unchanged. Through selective stabilization with antioxidant additives, we believe iPPO could be a suitable replacement for nylon-6,6 in environmentally susceptible applications.

Mechanistic Study of Isotactic Poly(propylene oxide) Synthesis using a Tethered Bimetallic Chromium Salen Catalyst.

Initial catalyst dormancy has been mitigated for the enantioselective polymerization of propylene oxide using a tethered bimetallic chromium(III) salen complex. A detailed mechanistic study provided insight into the species responsible for this induction period and guided efforts to remove them. High-resolution electrospray ionization-mass spectrometry and density functional theory computations revealed that a µ-hydroxide and a bridged 1,2-hydroxypropanolate complex are present during the induction period. Kinetic studies and additional computation indicated that the µ-hydroxide complex is a short-lived catalyst arrest state, where hydroxide dissociation from one metal allows for epoxide enchainment to form the 1,2-hydroxypropanolate arrest state. While investigating anion dependence on the induction period, it became apparent that catalyst activation was the main contributor for dormancy. Using a 1,2-diol or water as chain transfer agents (CTAs) led to longer induction periods as a result of increased 1,2-hydroxyalkanolate complex formation. With a minor catalyst modification, rigorous drying conditions, and avoiding 1,2-diols as CTAs, the induction period was essentially removed.