Functional polyolefins (i.e., polyethene or polypropene bearing functional groups) are highly desired materials, due to their beneficial surface properties. Many different pathways exist for the synthesis of these materials, each with its own advantages and drawbacks. This review focuses on those synthetic pathways that build up a polymer chain from ethene/propene and functionalised polar vinyl monomers. Despite many recent advances in the various fields of olefin polymerisation, it still remains a challenge to synthesise high molecular-weight copolymers with tuneable amounts of functional groups, preferably with consecutive insertions of polar monomers occurring in a stereoselective way. To overcome some of these challenges, polymerisation of alternative functionalised monomers is explored as well.

In this perspective we highlight the applicability of migratory carbene insertion reactions into TM-C bonds as a new tool for catalytic C-C bond formation. In Section 1 we introduce the reaction, wherein we also discuss the applicability of transition metal carbene formation from reactive carbene precursors. In Section 2 we summarise the available mechanistic information about this elementary step derived from stoichiometric model reactions. In Section 3 we review the available catalytic examples, with a focus on new developments in palladium mediated cross-coupling reactions (thus expanding the substrate scope with carbene precursors) and carbene polymerisation (allowing the synthesis of highly functionalised stereoregular polymers that are difficult to prepare otherwise). Recent developments in these fields in combination with the close analogy of carbene insertion reactions with CO (and alkene) insertions open up new possibilities for the development of interesting new reactions based on carbene insertions.

Copolymerisation of olefins and carbene monomers was attempted with several well-defined Pd catalysts active in both olefin polymerisation and carbene polymerisation. In none of the attempts copolymer formation or even formation of the homopolymers was observed. This indicates that olefin polymerisation and carbene polymerisation are incompatible, despite the fact that the proposed transition states for these processes are very similar. Detailed investigations of Pd catalysed homopolymerisation of carbenes using both Pd II and Pd 0 complexes revealed that the active species in these reactions are most likely low-valent Pd species rather than Pd II -alkyl species generally assumed to mediate carbene polymerisation. Well-defined Pd II -alkyl species showed only a few insertions of carbene monomers, while longer oligomers ($20 carbene units) are formed from Pd 0 salts. In agreement with previous investigations, Pd 0 -NHC complexes allow formation of higher-M w materials. Activation of the catalyst by NaBPh 4 is required. Mechanistic studies revealed that involvement of Pd II species in this process is highly unlikely, but the exact nature of the low-valent active species (Pd nanoparticles, molecular Pd 0 or Pd I species) is not clear. However, involvement of free radical species can be ruled out. Since olefin polymerisation requires Pd II as the active species, the likely involvement of lower-valent Pd species in carbene polymerisation explains the incompatibility of both processes. The absence of formation of olefin homopolymers by well-known Pd-based olefin polymerisation catalysts in the presence of EDA can be explained by in situ reduction of the Pd II species by EDA.