Unprecedented Steric and Positioning Effects of Comonomer Substituents on α-Diimine Palladium-Catalyzed Vinyl Arene/CO Copolymerization

To better understand the substituent effects of vinyl arene, a series of substituted styrenes with different groups and locations (n-X-St, n = 2, 3, 4; X = H, Me, tBu, MeO, tBuO, F, Cl, and Br) were used as comonomers for palladium-catalyzed vinyl arene/CO copolymerization. Dibenzobarrelene-based α-diimine palladium catalyst Pd1 was capable of catalyzing alternating copolymerizations of substituted styrene comonomers and CO in a living fashion, which excluded the effect of the palladium catalyst. Electronic effects of comonomer substituents were quantitatively examined by Hammet constants (σ) of substituents and highest occupied molecular orbital (HOMO) energies of comonomers. Experimental results clearly showed that the turnover number (TON) of copolymerization, copolymer molecular weight, and stereoregularity were greatly affected by the inserted substituent. The steric effect of 4-substituents was presented and clearly proved in addition to a widely acceptable electronic effect. Strikingly, the unprecedented positioning effects of comonomer substituents were initially discovered. Whatever substituents were located on 3-position, vinyl arene/CO alternating copolymerization was greatly promoted because of the neglectable steric effect of 3-substituents.