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44542-G7
Development of Transition Metal Catalysts for Radical (Co)Polymerization of Nonpolar Olefins and Polar Monomers

Tomislav Pintauer, Duquesne University

Copolymers of olefins and polar monomers are industrially very attractive materials and have a wide variety of applications in materials that require extreme toughness or superior sealing properties.  Conventionally, olefins are copolymerized with polar monomers in the presence of free radical initiators.  However, the synthesis often requires extreme reaction conditions such as elevated temperatures and pressures and yields copolymers which are enriched in polar monomer due to the large differences in reactivity ratios.  The main objective of our project is to rationally develop catalytic systems that can be used in radical copolymerization of olefins and polar monomers based on changing the reactivity ratios through complexation of the monomers with copper(I) complexes.  This change can be induced by altering the electronic properties of the coordinated double bond by varying the relative contributions of π-backbonding from the metal center and σ-donation from the double bond. The systematic approach to this project involves (a) fundamental investigation of the parameters that affect copper(I)-π-olefin(polar monomer) bond strength, (b) synthesis and activity studies of model compounds in free radical cyclization reactions and (c) homo- and copolymerization of nonpolar olefins and polar monomers in the presence of copper(I) complexes. During the past year, our research group has concentrated on the synthesis and characterization of copper(I) complexes with bidentate nitrogen based ligands containing polar and nonpolar monomers.  As a result, we were able to synthesize and characterize novel copper(I)/2,2’-bipyridine complexes [Cu^I(bpy)(π-CH₂CHCOOCH₃)][A] (A = CF₃SO₃^-, ClO₄^- and PF₆^-).  These complexes are used as catalysts in copper(I) mediated cyclopropanation and aziridination reactions of methyl acrylate and represent the first class of trigonal pyramidal copper(I) complexes with electron poor olefins.  The  coordination of CF₃SO₃^- (2.388(4) Å) and PF₆^- (2.609(2) Å) anions to the copper(I) center was observed.  In the case of noncoordinating ClO^₄- counterion, the complex was dimeric in the solid state with the oxygen atoms of the carbonyl moieties in methyl acrylate bridging two copper(I) centers.  The synthesized complexes are very stable in the solid state even in the presence of air.  However, they disproportionate in CD₃OD, (CD₃)₂CO and CD₂Cl₂ within 10 minutes at room temperature, unless excess monomer is present.  ¹H NMR spectra of [Cu^I(bpy)(π-CH₂CHCOOCH₃)][A] complexes in CD₂Cl₂ indicated 1:1 ratio between 2,2’-bipyridine and methyl acrylate.  However, only four resonances for bpy ligand were observed for all compounds at temperatures as low at -80 ^oC, which is not consistent with solid state structures.  This is most likely induced by the rapid exchange on the NMR time scale between free and complexed methyl acrylate or rotation about the alkene-copper(I) bond.  Very strong shielding of vinyl protons was observed which indicated π-backbonding donation from Cu^I, although with different magnitudes.  The shielding effect was the weakest with α-carbon, which is also further away from Cu^I than the β-carbon.  The π-nature of C=C of methyl acrylate was further supported by a decrease in the IR stretching frequency of C=C by approximately 30 cm^-1 upon coordination.   In conjunction with copper(I)/2,2’-bipyridine complexes with methyl acrylate, we were recently able to isolate identical complexes with nonpolar styrene.  Similar structural features were observed, with the exception that in the case of ClO₄^- counterion no dimerization was observed.  Low temperature NMR competition experiments have indicated that that the ratio of binding constants of methyl acrylate and styrene was 10±1, which was consistent with previous results.  [Cu^I(bpy)(π-styrene)][A] complexes showed much weaker shielding effect due to the electron donating nature of the phenyl ring.  We are presently conducting simple DFT calculations to access the relative amounts of π-backbonding and σ-donation in these complexes.  Furthermore, the reactivity of coordinated methyl acrylate and styrene in these complexes is currently being investigated in free radical polymerization reactions.

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