The polymerization of dicyclopentadiene: an investigation of mechanism

This contribution presents further information about the mechanism of the cross-linking reaction which occurs during the polymerization of dicyclopentadiene with the classical catalyst, WCl6/Et2AlCl (1), and the well-defined preformed alkylidene, Mo(N-2,6-C6H3-i-Pr)(CHC(CH3)(2)Ph)(OCCH3(CF3)(2))(2) (2). When the classical system 1 was used as a catalyst, insoluble polymer was formed in all cases. However, when molybdenum catalyst 2 is employed, solution concentration determines whether soluble or insoluble polymer will form. The formation of insoluble material is attributed to an olefin addition process catalyzed by the heat released upon ring-opening metathesis polymerization of the norbornene subunit of the monomer. If the heat is removed from the polymerization system through dilution or by cooling the solution, soluble linear polymer is formed. These results suggest that an olefin addition process is at least partly responsible for the cross-linking reaction that occurs during the polymerization of dicyclopentadiene. All attempts to cross-link oligomers of linear polydicyclopentadiene with the well-defined molybdenum alkylidene 2 resulted in only the recovery of soluble polymer. With the classical catalyst system 1, insoluble material was obtained, which was assumed to be cross-linked through olefin addition. These results disprove the idea that metathesis cross-linking can be induced by a critical chain length or concentration of polydicyclopentadiene. On the contrary, no indication of metathesis cross-linking was observed whatsoever for these polymerization systems. (C) 1998 Elsevier Science B.V. All rights reserved.