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.
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 M n 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.
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.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.