posted on 2021-01-12, 16:35authored byZefan Xiao, Liu Zhong, Cheng Du, Wenbo Du, Handou Zheng, Chi Shing Cheung, Lingzhi Wang, Haiyang Gao
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.