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.

Bimetallic Chromium Catalysts with Chain Transfer Agents: A Route to Isotactic Poly(propylene oxide)s with Narrow Dispersities.

Bimetallic chromium catalysts are investigated for the enantioselective polymerization of propylene oxide. The catalyst is composed of two salen chromium species linked by an alkyl chain, the length of which significantly impacts the rate of polymerization. While the use of a chloride initiator on the catalyst resulted in bimodal molecular weight distributions, switching to a trifluoroacetate initiating group and adding a diol chain transfer agent afforded polymers of controllable molecular weight with low, unimodal dispersities.

Isospecific, Chain Shuttling Polymerization of Propylene Oxide Using a Bimetallic Chromium Catalyst: A New Route to Semicrystalline Polyols.

Hydroxy-telechelic poly(propylene oxide) (PPO) is widely used industrially as a midsegment in polyurethane synthesis. These atactic polymers are produced from racemic propylene oxide using chain shuttling agents and double-metal cyanide catalysts. Unlike atactic PPO, isotactic PPO is semicrystalline with a melting temperature of approximately 67 °C. Currently there is no practical route to hydroxy-telechelic isotactic PPO using racemic propylene oxide as the monomer. In this paper, hydroxy-telechelic isotactic PPO is synthesized from racemic propylene oxide with control of molecular weight using enantioselective and isoselective bimetallic catalysts in conjunction with chain shuttling agents. The discovery of an easily accessible bimetallic chromium catalyst is reported for this transformation. Diol, triol, and polymeric chain shuttling agents are used to give hydroxy-telechelic isotactic PPO of varying functionality and structure. Detailed quantum chemical studies are used to reveal the polymerization mechanism and origin of stereoselectivity.

(2S*,5R*)-2,5-Dimethyl-1,4-bis-(pyridin-2-ylmeth-yl)piperazine.

The title compound, C18H24N4, resides on a crystallographic inversion centre, so that the asymmetric unit comprises one half-mol-ecule. The piperazine ring adopts a chair conformation, with the mean planes of the two equatorial pyridine rings parallel to each other and separated by 2.54 (3) Å. No classical hydrogen bonds are observed.

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.

Dual catalysis for the copolymerisation of epoxides and lactones.

A dual catalysis system was developed to synthesize hydrolyzable polyether-polyester copolymers from propylene oxide and cyclic esters such as γ-butyrolactone, δ-valerolactone, and ε-caprolactone. A bimetallic chromium catalyst active for the enantioselective polymerisation of propylene oxide and an organocatalyst active for the ring-opening polymerisation of lactones were used in conjunction with an alcohol chain shuttling agent to create new copolymers. The monomer and alcohol ratios were varied to yield a wide range of copolymers with varying monomer ratios, molecular weights, and crystallinities.