Copolymerization of Polar and Nonpolar Vinyl Monomers: Mechanistic Insight and Free Radical Polymerization

Open Access
Author:
Nagel, Megan Lee
Graduate Program:
Chemistry
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
July 14, 2006
Committee Members:
  • Ayusman Sen, Committee Chair
  • John V Badding, Committee Member
  • Alan James Benesi, Committee Member
  • Mike Chung, Committee Member
Keywords:
  • Copolymerization
  • Acrylates
  • Palladium complexes
  • Scandium(III)Triflate
Abstract:
[1,2-bis(4,4-dimethyl-2-oxazolin-2-yl)ethane]copper(II)dichloride, in the presence of MAO, is an effective catalyst for the homopolymerization of methyl methacrylate (MMA) and methyl acrylate (MA), and for the copolymerization of the latter with ethene and propene. The latter copolymers were acrylate rich. Several other metal salts also catalyze the homopolymerization of MMA in the presence of MAO. The addition of galvinoxyl had no effect on the polymerization ability of these systems. EPR experiments suggest that the commonly employed radical traps, galvinoxyl, DPPH, and TEMPO, are destroyed by methyl aluminoxane (MAO), thereby enabling radical polymerizations to proceed despite the addition of these traps to the reaction mixture. Thus, probing for a radical polymerization mechanism through the use of stable radical traps may not be valid for such systems employing MAO. The addition of the Lewis acid, Sc(OTf)3, to 2,2’-azobis(2-methylpropionitrile) (AIBN) initiated copolymerizations of both methyl acrylate (MA) and methyl methacrylate (MMA) with 1-alkenes results in increased reaction rate and increased incorporation of the latter monomer into the polymer backbone. As little as 4 mol% of the Lewis acid is effective in forming a nearly alternating copolymer of MA and ethene at 67% MA conversion. This procedure allows for the control of copolymer composition independent of the starting monomer feed ratio. Several palladium-phosphine complexes were synthesized and subsequently treated with methyl 2-bromobutyrate to form a complex analogous to the product formed from the single insertion of methyl acrylate into a palladium-methyl bond. For all of the ligands used, the complexes were shown to decompose by two distinct pathways, β-hydrogen elimination and homolytic cleavage of the palladium-alkyl bond, which forms active radicals. The preferred decomposition pathway can be changed dramatically depending on the properties of the ligand employed. One of the few late-transition metal catalysts reported to successfully insert methyl acrylate to form a stable complex is a palladium diimine system that forms a stable six-membered chelate. Upon disruption of this chelate with PPh4Br, the complex undergoes a rearrangement by “chain-walking”. The palladium-alkyl of both the opened chelate and the final rearrangement product are shown to homolytically cleave. This demonstrates the propensity of these compounds to decompose by radical methods not only when the ester group is  to the palladium, but also when it is removed from the metal center.