Pd-mediated carbenepolymerisation: activity of palladium(ii) versus low-valent palladium

Abstract

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.

Polymeric palladium-mediated carbene polymerization

“…22 Besides, the observation that the stereoregular PCEC show higher polymerization degree than atactic PCEC and they have different spectroscopic patterns point to the presence of different active species, differing in their chain-propagation behavior. 9 The increased steric hindrance of the polyimidazole ligands on the palladium centre could affect the carbene Scheme 2 Proposed chain-initiation, chain-transfer and chaintermination mechanisms for poly(imidazole-Pd)-mediated carbene polymerization.…”

“…While Ihara and coworkers proposed the in situ generated Pd II -alkyl species as the active species, 3 de Bruin and coworkers supposed the involvement of low-valent Pd species (Pd 0 or Pd I ). 9 Even though the NMR spectra of the atactic polycarbenes suggests a radical polymerization mechanism, the involvement of free radical species was excluded. 9 Thus, the exact nature of the active intermediate is still unclear and so far, the mechanisms of the chain initiation and termination processes have been poorly understood, which require more investigations.…”

“…9 Even though the NMR spectra of the atactic polycarbenes suggests a radical polymerization mechanism, the involvement of free radical species was excluded. 9 Thus, the exact nature of the active intermediate is still unclear and so far, the mechanisms of the chain initiation and termination processes have been poorly understood, which require more investigations. Metalloenzyme-inspired polymeric metal catalysts, which catalyze high active and selective organic transformations with high efficiency, stability and reusability for sustainable and green chemistry, have attracted growing interest of the chemist community.…”

A different route to functional polyolefins: olefin–carbene copolymerisation

“…Previous attempts to achieve these copolymerisations with Pd catalysts proved to be unsuccessful, due to clear differences in the nature of the active species responsible for both homopolymerisation reactions. 42 We decided to study these copolymerisations in more detail in the presence of Rh as a catalyst due to the activity of Rh towards both the polymerisation of functionalised carbenes (vide supra) as well as ethene polymerisation. [54][55][56] We anticipated that these two processes are potentially compatible, since ethene polymerisation is catalysed by Rh III complexes 55,56 and recently we demonstrated that the active species for carbene polymerisation is based on Rh III as well.…”

“…C1 monomers) as 'functional-group carriers' in coordinationinsertion polymerisation has proven to be successful, especially if densely functionalised, highly stereoregular polymers are desired. [29][30][31][32][33][34][35][36][37][38][39][40][41][42] Highly stereospecific (co)polymerisation of diazoesters (N 2 CHCOOR) can be achieved in good yields and with high M w in a Rh catalysed process. 30,[43][44][45][46][47][48][49][50][51] This leads to the formation of syndiotactic polymers bearing an ester functionality at every single carbon atom of the polymer main chain.…”