C1 polymerisation and related C-C bond forming 'carbene insertion' reactions

In this critical review we summarise the currently available 'C1 polymerisation' techniques as valuable alternatives for 'C2 polymerisation' in the preparation of saturated main-chain carbon-based polymers. C1 polymerisation involves the growth of polymers from monomers delivering only one functionalised carbon unit (C1 monomers; typically 'carbene precursors') in each chain-growth step, which contrasts with common polymerisation of C=C bond containing substrates (C2 monomers). In the general introduction (section 1) we comment on the availability of C1 monomers and the most important differences between C1 and C2 polymerisation techniques, highlighting the opportunities provided by C1 polymerisation to prepare new polymer structures. In section 2 we describe several Lewis acid mediated C1 polymerisation reactions based on diazocompounds and sulfur ylides as C1 monomers. Some of these are 'living polymerisation' reactions which allow the synthesis of functional telechelic block-copolymers and polymethylenic homo-polymers with a wide variety of different functional end-groups at both polymer chain-ends in a controlled fashion. Miscellaneous related reactions of other C1 monomers are described in section 3. Transition metal mediated C1 polymerisation methods in section 4 allow the polymerisation of polar functionalised C1 monomers. Several homogeneous and heterogeneous catalysts are capable of producing polymers from a variety of diazo compounds, some of which even allow the formation of stereoregular polar functionalised polymers (which is a major challenge in traditional olefin polymerisation). An overview of the current state-of-the art, challenges and opportunities, as well as an overview of the available mechanistic information (sections 4 and 5) is provided. Some related C-C bond forming reactions proceeding via (migratory) carbene insertion (section 6) are also discussed (106 references).