Rational Approaches to Regulating Polymer Properties in Ring-Opening Metathesis Polymerization

Abstract

The molecular weight distribution of a living polymerization when chain-transfer agents are intentionally added were computed numerically. Results showed that traditional expressions for number-average degree of polymerization (Xn) and polydispersity index (PDI) of chain-growth polymerization cannot be used. The well-known Mayo equation fails even when the system has achieved steady-state polymerization. Although the behavior of the system is complex, an analytical expression for Xn was derived. Plots based on the analytical expression showed excellent agreement with that from numerical solutions. The implications of the calculations were discussed. The kinetics of living ring-opening metathesis polymerization (ROMP) of norbornene in the presence of neohexene catalyzed by Mo was investigated. The living ROMP of norbornene (n) and bicyclo[3.2.0]hept-2-ene (c) by Cl₂(PPh₃)₂Ru(=CHCHCPh₂) was demonstrated. The molecular weight varied linearly with conversion. Discrete propagating species showed that PPh₃ ligand dissociated during polymerization of c, and that CuCl (abstracts PPh₃) enhanced the rate of polymerization of n. The specific propagation rate constants (knn and kcc, respectively) of homopolymerization of n and c were measured, respectively. Block copolymers were easily prepared. From reactivity ratio studies, the ordering of the specific propagation rate constants are knc >> kknn > kcc > kcn. The effect of styrene as chain-transfer agent on the molecular weight was examined. The hitherto unassigned (and unknown) microstructure of polymers prepared from 7-oxabicyclo[2.2.1]hept-2-ene derivatives have finally been unambiguously assigned. Polymers catalyzed by W(CH-t-Bu)(NAr)(OCMe(CF₃)₂)₂ have all cis double bonds and highly syndiotactic, while those from RuCl₃•3H₂0 and [RuCl(μ-Cl)(μ³:μ³-C₁₀H₁₆)]₂ {C₁₀H₁₆= 2,7-dimethyloctadienediyl} have high trans double bond content and highly isotactic. Studies on the olefin isomerization catalyzed by Ru(H₂O)₆tos₂ revealed that the presence of hydroxyl functionality on the terminal olefin resulted in formation of 1:1 ratio of cis : trans double bonds on the isomerized internal olefin product, in contrast with near exclusive isomerization of double bond to trans when no hydroxyl group was present. A numerical algorithm was developed for the evaluation of a chirality function for triangles on a plane, showing that such algorithm may be easily extended into the case of tetrahedron in 3-dimensional space.