Efficient Amine End-Functionalization of Living Ring-Opening Metathesis Polymers

An efficient strategy for the synthesis of monoamine end-functionalized living polymers using ringopening metathesis polymerization with ruthenium initiators is reported. A new end-capping agent for this purpose was synthesized, and its efficiency for end-functionalization was evaluated using two common ruthenium-based initiators. Finally, terminal cross-metathesis was also explored as another alternative toward the synthesis of amine end-functionalized polymers, and the comparison between the two techniques is presented. ■ INTRODUCTION Ring-opening metathesis polymerization (ROMP) has very quickly become one of the methods of choice of chemists to synthesize highly functional low dispersity polymers with control over molecular weight and architecture. The air and moisture sensitive molybdenum carbene catalysts typically used for olefin metathesis are sensitive to many protic and polar functional groups such as for example aldehydes. Polymers prepared with molybdenum and tungsten carbene initiators can therefore be end-functionalized in a relatively straightforward manner using suitably substituted aldehydes. However, due to the restricted functional group tolerance of these initiators, many functional groups cannot be present in the polymer structure. In contrast, the Grubbs type ruthenium carbene catalysts (Figure 1) are stable toward many functional groups. Polymers prepared with these ruthenium initiators can thus tolerate the presence of many functional groups, but at the same time methods for a functional termination are scarce for the same reason. Ethyl vinyl ether termination is one of the most widely used nonfunctional termination methods for ROMP transferring a methylene unit onto the polymer chain end. Many strategies have been developed for the synthesis of monotelechelic functional polymers by ROMP. Prefunctionalized initiators have been used to synthesize such polymers, but the synthesis of a new catalyst for every new functionality is difficult and the yields are often low. End-capping agents based on custom terminating agents, acrylates, vinyl carbonates, and lactones have also been used to synthesize mono-endfunctionalized polymers from ring-opening metathesis polymerization. Recently, Hilf et al. reported the “sacrificial synthesis” strategy for the synthesis of mono-end-functionalized living ring-opening metathesis polymers. The strategy involves the polymerization of a new monomer onto the end of a first polymer block, thus leading to a diblock copolymer. This new second block is then chemically cleaved to yield the desired functionality at the end of the polymer chain. Via this method a variety of mono-end-functional polymers carrying alcohol, thiol, or carboxylic acid end groups are readily accessible. Endfunctionalization strategies for a number of different functional groups for living ROMP were recently reviewed. Amine end-functionalization is more difficult due to the ability of the free amino groups to coordinate to the ruthenium catalyst. Protected amine chain transfer agents have been used to overcome this difficulty. However, very long reaction times are sometimes required which could potentially give rise to chain transfer to the growing polymer chain itself depending on the monomer structure polymerized. A new cyclic olefin, 2-phenoxy-2,3,4,7-tetrahydro-1H-1,3,2diazaphosphepine 2-oxide (1), was synthesized to overcome this difficulty. In this compound, the phosphorus bridge plays two roles: it forms a closed ring, which allows its use as a sacrificial monomer in ROMP, and simultaneously acts as a protecting agent for the amino groups. Single crystal X-ray Figure 1. Ruthenium initiators used for ring-opening metathesis polymerization. Published in which should be cited to refer to this work. ht tp :// do c. re ro .c h diffraction confirmed the synthesis of the new compound