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

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- i Pr2−C6H3)NCMe−CMeN(2,6- i Pr2−C6H3)) with excess vinyl ethers CH2CHOR (1a−d: R = t Bu (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 CC π-complexation to form (α-diimine)PdMe(CH2CHOR)+ (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)(CH2CHMe)+ (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/CH2CHOSiR3 copolymers can be desilylated to give 1-hexene/CH2CHOH copolymers. The results of control experiments argue against cationic and radical mechanisms for the copolymerization, and an insertion/chain-walking mechanism is proposed.