Copolymerization of Silyl Vinyl Ethers with Olefins by (α-diimine)PdR+

Abstract

This paper reports that (α-diimine)PdMe⁺ catalyzes the copolymerization of olefins and silyl vinyl ethers. The reactions of (α-diimine)PdMe⁺ (α-diimine = (2,6-ⁱPr₂−C₆H₃)NCMe−CMeN(2,6-ⁱPr₂−C₆H₃)) with excess vinyl ethers CH₂CHOR (1a−d: R = ^tBu (a), SiMe₃ (b), SiPh₃ (c), Ph (d)) in CH₂Cl₂ at 20 °C afford polymers for 1a (rapidly) and 1b (slowly) but not for 1c or 1d. The structures of poly(1a,b) indicate a cationic polymerization mechanism. The reaction of (α-diimine)PdMe⁺ with 1−2 equiv of 1a−d proceeds by sequential CC π-complexation to form (α-diimine)PdMe(CH₂CHOR)⁺ (2a−d), 1,2 insertion to form (α-diimine)Pd(CH₂CHMeOR)⁺ (3a−d), reversible isomerization to (α-diimine)Pd(CMe₂OR)⁺ (4a−d), β-OR elimination to generate (α-diimine)Pd(OR)(CH₂CHMe)⁺ (not observed), and allylic C−H activation to yield (α-diimine)Pd(η³-C₃H₅)⁺ (5) and ROH. The reaction of (α-diimine)PdMe⁺ with 1-hexene/1b and 1-hexene/1c mixtures in CH₂Cl₂ at 20 °C affords copolymers containing up to 20 mol % silyl vinyl ether. The copolymers were purified to be free of any −[CH₂CHOSiR₃]_n− homopolymer. The copolymer structures are similar to that of homopoly(1-hexene) generated under the same conditions. The major comonomer units are CH₃CH(OSiR₃)CH₂−, CH₂(OSiR₃)CH₂− and −CH₂CH(OSiR₃)CH₂−. The 1-hexene/CH₂CHOSiR₃ copolymers can be desilylated to give 1-hexene/CH₂CHOH copolymers. The results of control experiments argue against cationic and radical mechanisms for the copolymerization, and an insertion/chain-walking mechanism is proposed.