Zwitterionic polymerization for the synthesis of high molecular weight cyclic polymers [electronic resource]
Keywords: Chemistry.

Topological isomerism in macromolecules represents a fascinating field of study. Linear polymers form the basis of much of the current scientific understanding of bulk polymer behavior due to their simple structure and synthetic accessibility. In contrast, cyclic polymers remain poorly understood due to the difficulty in generating sufficient quantities of pure samples; in particular, high molecular weight samples where phenomena related to entanglement could be observed have eluded polymer chemists until recently. Among the several strategies employed for the synthesis of high molecular weight cyclic polymers, recent reports have highlighted the utility of zwitterionic ring-opening polymerization (ZROP). Efforts to understand the N-heterocyclic carbene (NHC) mediated ring-opening of strained lactones have led to the development of several mechanistic models. Kinetic studies on the NHC-mediated polymerization of [epsilon]-caprolactone (CL) and [delta]-valerolactone (VL) were conducted to account for the notably high molecular weight cyclic polyerms generated. While the two monomer systems are closely related and polymerize with the same NHC initiators, a difference in the dependence of the reaction rate on monomer is observed. A general mechanism is proposed for the polymerization of VL. Exploiting the utility of ZROP to generate high molecular weight poly([epsilon]-caprolactone ) allowed for a comparative study on the bulk crystallization of cyclic PCL and linear PCL across a range of molecular weights. WAXS and SAXS studies show that linear and cyclic PCL exhibit the same global and local crystalline structure but that cyclic PCL crystallizes more rapidly than linear PCL. Isothermal DSC crystallization kinetic studies support this observation and indicate that the difference in crystallization rate increases with increasing molecular weight. Approximating the equilibrium melting temperature of these samples using the Hoffmann-Weeks method indicates that there is no significant difference in Tm0 between cyclic and linear PCL. In an attempt to broaden the scope of initiators for the ZROP of strained lactones, the use of amidines for the polymerization of lactide was explored. 1,8-Diazabicyclo[5.4.0] undec-7-ene (DBU) and 1,5-Diazabicyclo[4.3.0]non-5-ene (DBN) were shown to mediate the ZROP of lactide to give predominantly cyclic poly(lactide) (PLA). A notable solvent effect with no polymerization occurring in neat tetrahydrofuran (THF), fast polymerization with formation of significant amounts of linear PLA in dichloromethane (DCM), and moderate rate of polymerization with minimal formation of linear PLA in THF:DCM blends. DFT computations suggest that the formation of a neutral tricyclic species, a zwitterionic species, and a ketene-aminal are energetically feasible. This ketene-aminal is thought to be the source of the linear PLA observed. Our efforts to develop a more hydrolytically stable family of cyclic polymers led to the investigation of the ZROP of strained cyclic carbosiloxanes. NHC-mediated polymerization of 2,2,5,5-tetramethyl-2,5-disila-1-oxacyclopentane occurs rapidly to give polymers with molecular weights exceeding 106 g/mol. While these polymerizations are difficult to control, the cyclic topology of the resulting polymers could be confirmed by comparative dilute solution viscosity studies and MALDI-TOF MS. As a complimentary strategy to the use of neutral nucleophiles to initiate anionic-like zwitterionic polymerization, neutral Lewis acids were tested for the electrophilic zwitterionic ring opening polymerization (EZROP) of strained heterocycles. Polymerization of 2-ethyl-2-oxaline proceeds slowly at elevated temperature with boron-based Lewis acids to give HF-capped linear chains, determined by MALDI-TOF MS. In contrast, the polymerization of 3,3-dimethyloxetane proceeds very rapidly at room temperature with B(C6F5)3 to give a mixture of polymer products likely resulting from methyl scrambling between chains.