A Simple and Efficient Protocol for the Catalytic Insertion Polymerization of Functional Norbornenes

Abstract

Norbornene can be polymerized by a variety of mechanisms, including insertion polymerization whereby the double bond is polymerized and the bicyclic nature of the monomer is conserved. The resulting polymer, polynorbornene, has a very high glass transition temperature, T g , and interesting optical and electrical properties. However, the polymerization of functional norbornenes by this mechanism is complicated by the fact that the endo substituted norbornene monomer has, in general, a very low reactivity. Furthermore, the separation of the endo substituted monomer from the exo monomer is a tedious task. Here, we present a simple protocol for the polymerization of substituted norbornenes (endo:exo ca. 80:20) bearing either a carboxylic acid or a pendant double bond. The process does not require that both isomers be separated, and proceeds with low catalyst loadings (0.01 to 0.02 mol%). The polymer bearing pendant double bonds can be further transformed in high yield, to afford a polymer bearing pendant epoxy groups. These simple procedures can be applied to prepare polynorbornenes with a variety of functional groups, such as esters, alcohols, imides, double bonds, carboxylic acids, bromo-alkyls, aldehydes and anhydrides. Video LinkThe video component of this article can be found at https://www.jove.com/video/54552/ temperature (T g ) polymer whereby the bicyclic backbone of NBE is conserved. A variety of catalysts such as metallocene catalysts and late transition metal catalysts can be used to promote the polymerization of NBE. 4,5,6,7 However, due to its low solubility and due to difficulties associated with the processing of a very high T g polymer, the PNBE homopolymer has, to our knowledge, never found any use.Functional polynorbornenes (PNBEs) have been the object of considerable scrutiny for the last 20 years, because they combine the high T g imparted by the bicyclic rigid repeat unit as well as desirable properties endowed by their functionalities. 8,9,10 NBE monomers are obtained from rather simple and inexpensive feedstocks, using a one-step Diels-Alder reaction between cyclopentadiene and a functionalized dienophile. However, the Diels-Alder reaction leads to two stereoisomers, endo and exo, which have very different reactivities. 11,12 In fact, the endo stereoisomer is less reactive than exo form and deactivates the catalyst. 11,12 Thus, in the past, the preparation of functional polynorbornenes usually required the separation of the endo and exo stereoisomers, and only the exo stereoisomer was used. Such a separation procedure was time-consuming, and led to the accumulation of unreacted endo stereoisomers as undesirable waste.Recently we have shown that the polymerization of functionalized NBEs containing both stereoisomers is in fact feasible. 13 We have thus been able to prepare a variety of substituted PNBEs, containing functional groups such as esters, anhydrides, aldehydes, imides, alcohols and double bonds. Due to their high T g and functionality, these polymers show desirable properties. We des...