Catalytic Lewis Pair Polymerization of Renewable Methyl Crotonate to High-Molecular-Weight Polymers

Abstract

Crotonates, unlike their constitutional isomers of methacrylates, are not readily polymerized by conventional radical or anionic polymerization methods. A recent effort in polymerizing biorenewable methyl crotonate (MC) using organic N-heterocyclic carbene (NHC) initiators led to only dimerization. This contribution reports an effective polymerization of MC using Lewis pairs consisting of an NHC or N-heterocyclic olefin (NHO) Lewis base and a group 13 Lewis acid, in particular sterically encumbered methylaluminum bis­(2,6-di-tert-butyl-4-methylphenoxide) (MAD), producing high-molecular-weight poly­(MC) (PMC) with M n up to 161 kg/mol under ambient temperature and solvent free conditions. Depending on the nature of the Lewis pair and reaction conditions, the polymerization proceeds either catalytically producing lower molecular weight PMC or noncatalytically leading to high-molecular-weight PMC. Investigations into initiation mechanisms have revealed both nucleophilic and basic pathways. The observation of the susceptibility of MC to simple deprotonation and subsequent propagation in the presence of MAD and a strongly basic NHCthe basic pathwayhas led to a facile approach to high-molecular-weight vinyl-functionalized PMC (M n = 97.1 kg/mol) using simple KO t Bu and MAD.

Lewis pairs polymerization of polar vinyl monomers

“…Chen and co-workers [63] also found that the combination of B (C 6 F 5 ) 3 with TPT, NHO7 or I t Bu are all effective for polymerization of renewable monomer MC, but furnishing PMC with bimodal distribution and low initiation efficiency (I*% < 50). In sharp contrast, replacing B(C 6 F 5 ) 3 with (BHT) 2 AlMe led to a significantly enhanced activity towards polymerization of MC.…”

“…In 2018, Chen and co-workers employed (BHT) 2 AlMe as the LA to combine with NHO, t BuOK, or NHC to polymerize methyl crotonate (MC), a renewable monomer derived from poly(3hydroxybutyrate) [62], into PMC with M n up to 161 kg/mol under ambient temperature and solvent-free conditions [63]. Without LA, a single addition product was generated from the reaction of MC with TPT or IMes via a proton transfer reaction, while a dimerization product was obtained from the reaction of MC with I t Bu or NHO7.…”

“…(Color online) Proposed mechanism for MC polymerization by (BHT) 2 AlMe/I t Bu, involving basic initiation as well as bimetallic chain propagation and transfer (Not including unimolecular chain transfer). Reprinted with permission from Ref [63]…”

Direct and Tandem Routes for the Copolymerization of Ethylene with Polar Functionalized Internal Olefins

“…[39] Thea bovementioned polymerization can be achieved by either group-transfer polymerization [40] or Lewis-pair polymerization. [41] In contrast, ad ianisyl-substituted phosphine sulfonate palladium catalyst (PO-Pd:{ ( o-MeOC 6 H 4 ) 2 PC 6 H 4 SO 3 }PdMe-(DMSO)) [42] can mediate efficient ethylene copolymerization with trans-methyl crotonate,although the activity and molecular weight were dramatically reduced compared with those during ethylene homopolymerization ( Table 1, entries 1-3). Copolymers with moderate comonomer incorporation ratios (1.2-2.5 %) and molecular weights (M n :5 500-9300) were generated.…”

Unique acrylic resins with aromatic side chains by homopolymerization of cinnamic monomers

“…Group-transfer polymerization (GTP) systems for various alkyl crotonates have been developed using a Lewis acid (HgI 2 or CdI 2 ) as a catalyst and iodotrialkylsilane as a cocatalyst [23][24][25][26][27] . Chen and co-workers reported an efficient catalytic Lewis pair polymerization system for methyl crotonate using N-heterocyclic carbene as a Lewis base and an aluminum or a boron compound as a Lewis acid 28,29 . Additionally, organic-acid-catalyzed GTP systems were developed by Chen and co-workers and Kakuchi et al for the polymerization of (meth)acrylates [30][31][32] .…”