One-Pot Tandem Ring-Opening and Ring-Closing Metathesis Polymerization of Disubstituted Cyclopentenes Featuring a Terminal Alkyne Functionality

Tandem metathesis polymerization of terminal alkynes with a small functional ring-size cycloalkene remains a challenge. A series of 1-propargyl-1′-carboxylate ester monomers M1-M4 derived from 3-cyclopentene initiated by ruthenium-alkylidene complexes have been investigated via tandem polymerization. Detailed DFT calculations and end-group analysis using 1H NMR experiments provided us with a deeper insight into the proposed polymerization mechanism. The energy profile obtained from DFT analysis provides two pathways for the polymerization of monomers having both cycloalkene and alkyne functionalities. The most stable coordination of M was found to be via a π-coordination to the triple bond, which is oriented perpendicularly to the ruthenium-alkylidene bond. Both cycloalkene- and alkyne-initiated polymerizations were found to be feasible pathways. However, the alkyne-initiated polymerization mechanism was found to be the preferred pathway due to the higher stability of the alkyne coordination to the monomer and the slightly lower rate-determining step barrier. In addition, the synthesis of the model MC compound supported our proposed polymerization mechanism as well as the position of the styrene end-group on the polymer chain.