Zwitterionic ring-opening polymerization for the synthesis of cyclic polymers [electronic resource]
Keywords: Chemistry.

Cyclic polymers are the simplest topological isomer of the ubiquitous linear analogues. Yet despite overwhelming chemical similarities, cyclic polymers exhibit wholly different physical properties from their linear counterparts, though the specifics of these differences are not entirely understood. The difficulty of synthesizing appropriately pure and high molecular weight cyclic samples has hindered experimental studies. Ring-closure methods, while versatile, are inherently limited in the range of molecular weights that can be achieved. Ring-expansion methods such as zwitterionic polymerization are a much more promising strategy towards obtaining high molecular weight cyclic polymers. Experimental rate studies of the N-heterocyclic carbene (NHC)-catalyzed zwitterionic ring-opening polymerization (ZROP) of valerolactone (VL) in THF revealed that a slow initiation step is most likely responsible for the observed molecular weights. Further computational investigations suggest that the barrier to ring-opening of the initial VL unit by NHC catalyst is quite high, due to ring torsion and partial charges formed in the intermediate. Such phenomena are not present in subsequent monomer addition steps, and the slow initiation step is likely the culprit for the experimentally observed deviations from predicted molecular weights. The addition of LiCl was found to significantly modulate the behavior of the NHC-catalyzed ZROP of VL in THF. Rate acceleration was observed along with good control of molecular weights, suggesting that initiation is rapid and efficient in the presence of LiCl. Furthermore, the topology of the polymer synthesized in high concentrations of LiCl was found to be linear, suggesting that the coulombic attraction between chain ends is no longer strong enough to effect cyclization under these conditions. The ZROP of ε-caprolactone (CL), 4-methylcaprolactone (mCL), and 4-methylcaprolactone-2,2,6,6-d4 (d4-mCL) catalyzed by several different NHCs was investigated. The rate of polymerization of all three monomers were comparable and Mn values of the resulting polymers ranged from 40 ~ 160 kDa. A series of stochastic simulations were carried out on the ZROP of CL using several sets of rate and molecular weight data. A set of elementary reactions were identified comprising of intitiation, propagation, chain transfer, and backbiting steps. Through the use of χcarbene, the ratio of active carbene present in solution, the model could be tuned to accommodate rate differences between different batches of CL and NHC. In an effort to develop more hydrolytically robust polymers, the ZROP of 2,2,5,5-tetramethyl-2,5-disila-1-oxacyclopentane (TMOSC) was carried out. Remarkably, the p(TMOSC) samples obtained from polymerizations using very active alkyl-substituted NHCs ranged from Mn = 480 ~ 940 kDa, demonstrating the feasibility of utilizing ZROP towards the synthesis of ultra-high MW polymers. The physical characteristics and hydrolytic stability of these polymers were analyzed through degradation studies. The scope of ZROP was broadened to poly(carbonate)s as well. A series of 8-membered N-substituted cyclic carbonates were subject to NHC catalysts in the absence of alcohol initiators. It was found that the N-benzyl cyclic carbonate (8CCBn) undergoes ZROP to yield cyclic polymers of Mn = 14 -- 96 kDa. The N-phenyl cyclic carbonate (8CCPh) was found to dimerize in the presence of NHC, which was confirmed by X-ray crystallography. It is hypothesized that the dimerization of 8CCPh is a kinetically driven phenomenon due to the conformation of the dimeric 8CCPh zwitterion. 8CCPh was found to co-polymerize with VL to yield copolymers of Mn = 33 -- 50 kDa.