Chain propagation and termination mechanisms for polymerization of conjugated polar alkenes by [Al]-based frustrated Lewis pairs
Chen, Eugene You Xian
KAUST DepartmentChemical Science Program
KAUST Catalysis Center (KCC)
Physical Science and Engineering (PSE) Division
Online Publication Date2014-11-07
Print Publication Date2014-11-25
Permanent link to this recordhttp://hdl.handle.net/10754/563868
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AbstractA combined experimental and theoretical study on mechanistic aspects of polymerization of conjugated polar alkenes by frustrated Lewis pairs (FLPs) based on N-heterocyclic carbene (NHC) and Al(C6F5)3 pairs is reported. This study consists of three key parts: structural characterization of active propagating intermediates, propagation kinetics, and chain-termination pathways. Zwitterionic intermediates that simulate the active propagating species in such polymerization have been generated or isolated from the FLP activation of monomers such as 2-vinylpyridine and 2-isopropenyl-2-oxazoline-one of which, IMes+-CH2C(Me)=(C3H2NO)Al(C6F5)3 - (2), has been structurally characterized. Kinetics performed on the polymerization of 2-vinylpyridine by ItBu/Al(C6F5)3 revealed that the polymerization follows a zero-order dependence on monomer concentration and a first-order dependence on initiator (ItBu) and activator [Al(C6F5)3] concentrations, indicating a bimolecular, activated monomer propagation mechanism. The Lewis pair polymerization of conjugate polar alkenes such as methacrylates is accompanied by competing chain-termination side reactions; between the two possible chain-termination pathways, the one that proceeds via intramolecular backbiting cyclization involving nucleophilic attack of the activated ester group of the growing polymer chain by the O-ester enolate active chain end to generate a six-membered lactone (δ-valerolactone)-terminated polymer chain is kinetically favored, but thermodynamically disfavored, over the pathway leading to the -ketoester-terminated chain, as revealed by computational studies.
SponsorsThis work was supported by the National Science Foundation (NSF-1150792) for the study carried out at Colorado State University. L.C. thanks the HPC team of Enea for using the ENEA-GRID and the HPC facilities CRESCO in Portici, Italy. We thank Boulder Scientific Co. for the research gift of B(C<INF>6</INF>F<INF>5</INF>)<INF>3</INF> and Dr. Brian Newell for assistance on X-ray structural analysis.
PublisherAmerican Chemical Society (ACS)
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