N-heterocyclic carbene mediated zwitterionic polymerization for cyclic polymers [electronic resource]
Keywords: Chemistry

Zwitterionic polymerization involves a propagating species with both positively and negatively charged groups. Previous investigations on zwitterionic polymerization concern alternating copolymerization of nucleophilic and electrophilic monomers and polymerization of isolated stable zwitterionic monomers. More recently, the ring-opening polymerization of cyclic monomers using nucleophilic initiators have been studied. Specifically, the zwitterionic polymerization of cyclic esters using N-heterocyclic carbenes (NHCs) is the focus of this thesis. The N-heterocyclic carbene mediated zwitterionic polymerization of cyclic monomers provides an expedient route to polymers of various architectures, such as cyclic polymers, cyclic gradient copolymers and linear telechelic polymers. The ring-opening polymerization of lactide initiated by NHCs generates cyclic poly(lactide)s of defined molecular weight and molecular weight distribution. Kinetic studies implicate a mechanism that involves a slow initiation step and a propagation step that is much faster than depropagation and chain termination by cyclization. Stochastic simulations and chain extension experiments showed that only a fraction of the NHC forms the active zwitterion in solution, leading to both chain extension of the zwitterions and re-initiation of the NHC upon addition of the second batch of monomer. These results prompted investigation of a more efficient way to prepare cyclic block copolymers. The difference in reactivity of NHCs towards different monomers was exploited to synthesize cyclic block copolymers of valerolactone (VL) and caprolactone (CL). The faster ring-opening of VL relative to CL resulted in a gradient cyclic copolymer comprised of VL-rich sequences that transition to CL-rich sequences in a cyclic macromolecule, instead of a cyclic diblock copolymer. This work not only provides a simple batch copolymerization protocol to produce cyclic gradient copolymers, but also demonstrates the marked difference in reactivity of the NHCs compared to metal catalysts, which produce random copolymers. Stereocomplexation behavior has been observed in blends of linear poly(L-lactide) and linear poly(D-lactide). The influence of topology on the formation of stereocomplex was investigated using blends of linear and cyclic poly(lactide)s prepared by NHC mediated zwitterionic polymerization. The linear/cyclic and cyclic/cyclic blends all form stereocomplexes when annealed. Analyses of data from various characterization techniques indicate that the cyclic topology imposes constraints on the stereocomplexation formation. The purity of the cyclic polymers is always a concern in the synthesis and physical property studies. Attempts to identify and quantify the linear contamination in cyclic poly(caprolactone) samples are described. Esterification reactions targeting the hydroxyl endgroups of linear contaminants were not successful, but the macroinitiator approach where the linear contaminant in a cyclic polymer sample is used as the macroinitiator to grow polymers to identify and remove the linear contamination shows promise. A cyclic polymer more robust to post-polymerization chemistry may be needed for more thorough purity studies.