Group Transfer Polymerization of Acrylates Catalyzed by N-Heterocyclic Carbenes

Abstract

Group transfer polymerization (GTP) is an effective method for the synthesis of poly(methyl methacrylate) (pMMA) with controlled molecular weights and narrow polydispersities. Silyl ketene acetals initiate the polymerization of methyl methacrylate in the presence of either nucleophilic or Lewis acid catalysts. We report the use of N-heterocyclic carbenes (NHCs) as neutral nucleophilic catalysts for GTP of methyl methacrylate (MMA) and tert-butyl acrylate (TBA). For MMA, polymer molecular weights increase linearly with conversion and are predictable over a range of [M] 0 /[I] 0 ratios. Polydispersities trend downward with conversion and approach 1.2 at >95% conversion. Significantly, NHCs were also shown to be effective for the controlled GTP of TBA, generating living polymers with predictable molecular weights, narrow molecular weight distributions (M w /M n e 1.2), and living chain ends, as illustrated by chain-extension experiments.

N‐Heterocyclic carbenes for metal‐free polymerization catalysis: an update

“…NHC‐mediated GTP of MMA via two different pathways, resulting in the same poly(methyl methacrylate) (PMMA) polymer structure …”

From the N‐Heterocyclic Carbene‐Catalyzed Conjugate Addition of Alcohols to the Controlled Polymerization of (Meth)acrylates

“…However, despite the different ways to activate α,β‐unsaturated esters such as (meth)acrylates, their catalytic and direct transformation into poly(meth)acrylates has never been considered. In fact, only the so‐called group transfer polymerization (GTP), which is based on the repetition of the Mukaiyama–Michael reaction and requires the specific use of air‐sensitive silyl ketene acetal (SKA) initiators, has been implemented to prepare well‐defined poly(meth)acrylates 38–40. Here, we demonstrate, for the first time, that a particular NHC, namely, 1,3‐bis( tert ‐butyl)imidazol‐2‐ylidene (NHC t Bu ), can be used as an organocatalyst to enable the selective oxa‐Michael addition of simple alcohols onto MMA or MA, in the absence of any other additives.…”

Sustainable Polymers from Biomass‐Derived α‐Methylene‐γ‐Butyrolactones

“…NHC-catalyzed stepgrowth polymerization has been reported as well (168)(169)(170). Polymerization of α,β-unsaturated esters, such as MMA, has also been recently realized through the classic GTP initiated by SKAs (35,144,145), using NHCs as alternative nucleophilic catalysts to activate the acetal initiator (171,172). Using the Arduengo carbenes, 1,3-di-tert-butylimidazolin-2-ylidene (I t Bu) and 1,3-di-mesitylbutyl-imidazolin-2-ylidene (IMes) (173,174) alone, no MMA conversion was observed in either toluene (52) or THF (172).…”

“…Polymerization of α,β-unsaturated esters, such as MMA, has also been recently realized through the classic GTP initiated by SKAs (35,144,145), using NHCs as alternative nucleophilic catalysts to activate the acetal initiator (171,172). Using the Arduengo carbenes, 1,3-di-tert-butylimidazolin-2-ylidene (I t Bu) and 1,3-di-mesitylbutyl-imidazolin-2-ylidene (IMes) (173,174) alone, no MMA conversion was observed in either toluene (52) or THF (172). On the other hand, the Enders triazolylidene carbene TPT (1,3,4-triphenyl-4,5-dihydro-1H-1,2,4-triazol-5-ylidene) (175,176), which was estimated to be 10 3 times less nucleophilic than the imidazolylidene carbene (IMes) (177), catalyzes tail-to-tail dimerization (umpolung) of MMA and other methacrylate substrates, whereas the common imidazolylidene carbenes are ineffective (178,179).…”