New organocatalytic approaches for the synthesis of functional polyesters and polycarbonates [electronic resource]
Keywords: Chemistry.

Aliphatic polyesters and polycarbonates are sustainable alternatives to petroleum-based polymers owing to their excellent biodegradability and renewable feedstocks. Ring-opening polymerization (ROP) is one of the most efficient methods for synthesizing polyesters and polycarbonates with a high degree of control over macromolecular and microstructural features. Organocatalytic ROP has proven particularly useful for the synthesis of metal-free polymers suitable for biomedical and microelectronic applications. The work described herein aims at developing new families of organocatalysts with higher reactivities and selectivities for ROP as well as exploiting organocatalytic ROP for the synthesis of functional materials. The precise control of ROP on the polymer structure relies on the use of a selective catalyst. The selectivity criteria encapsulate many aspects of the polymerization: the selectivity for chain propagation over chain transfer, the selectivity for a particular topology over another, and regio-/stereoselectivity. Catalyst developments pertaining to these aspects are presented. 1) We developed a class of isothioureas as effective nucleophilic initiators for the zwitterionic ROP of lactide to synthesize high molecular weight cyclic polylactides. Unlike the previously reported amidine catalyst which generates a mixture of linear and cyclic chains, these isothioureas exhibited significantly higher selectivities for producing cyclic polymers, confirmed by a combination of NMR, MALDI-TOF mass spectrometry, and intrinsic viscosity studies. The reaction pathway that leads to the formation of linear contaminants for amidine catalysts is identified. 2) We discovered thioimidates as a class of efficient, tunable, and operationally simple catalysts for ROP. The combination of high reactivity and selectivity (for propagation over chain transfer) of these catalysts distinguishes them from most previous systems. Mechanistic studies revealed that the high selectivity arises from their higher binding affinity to monomers over polymer chains. 3) We investigated the regio-/stereoselectivity of various organocatalysts for the ROP of lactide and methyl substituted trimethylene carbonate and identified a thiourea-phosphazene catalyst that exhibits the highest regioselectivity for the carbonate monomer among reported organocatalysts. Our versatile organocatalytic platform allows a wide range of monomers and functional groups to be enchained to elicit novel properties and function. Inspired by the rich biological function of lipoic acid, we synthesized a class of ABA triblock copolymers with hydrophilic poly(ethylene glycol) B block and hydrophobic A blocks bearing pendant 1,2-dithiolanes via expedient organocatalytic ROP. We then prepared dynamic covalent hydrogels from these polymers via a cooperative self-assembly of triblock copolymers and reversible crosslinking based on thiol-initiated ring-opening cascade of dithiolanes. The properties of these gels are sensitive to the polymer architectures, substitution pattern of ditholanes, and various environmental factors such as pH, temperature, the concentration of thiol, and the presence of "capping agents" like maleimide. The structure-property relationship of these gels was illustrated based on the investigation on the thermodynamics and kinetics of dithiolane crosslinking. In addition to hydrogels, the self-assembly behavior of these amphiphilic polymers was exploited to form micelles that can function as nano-carriers for hydrophobic drugs and stabilizing ligands for gold nanoparticles. We also observed that the fast drying of the micellar solutions of these triblock copolymers led to the formation of nanoparticles with unusual cubic shapes, which was proposed as a crystallization-driven aggregation process of micelles. This provides a template-free strategy for the fabrication of non-spherical polymeric nanoparticles.