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

Abstract

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, t Bu, MeO, t BuO, 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.

(Iminopyridine)Pd (II) complexes as versatile catalysts for copolymerization and terpolymerization of vinyl arene, ethylene, and carbon monoxide

“…The mechanism of polyketone formation using vinyl arenes favors the coordination of electron‐rich alkenes, as opposed to electron‐poor alkenes, with palladium ions, which results in higher catalytic activities. [ 39–41 ] By comparison, polymerization reactions conducted with p ‐methoxystyrene also exhibited high catalytic activity and vinyl arene insertion (entries 3 and 6 of Table 3). Other groups have reported that the copolymerization of p ‐methoxystyrene and CO is unfavorable and that homopolymerization of p ‐methoxystyrene occurs in the presence of CO. [ 38,41 ] Based on 1 H NMR analyses of the polymers of entry 3 (Table 3), polymers synthesized from p ‐methoxystyrene and CO are estimated to be a mixture of polyketone (minor component) and homopolymers of p ‐methoxystyrene (major component).…”

“…[ 39–41 ] By comparison, polymerization reactions conducted with p ‐methoxystyrene also exhibited high catalytic activity and vinyl arene insertion (entries 3 and 6 of Table 3). Other groups have reported that the copolymerization of p ‐methoxystyrene and CO is unfavorable and that homopolymerization of p ‐methoxystyrene occurs in the presence of CO. [ 38,41 ] Based on 1 H NMR analyses of the polymers of entry 3 (Table 3), polymers synthesized from p ‐methoxystyrene and CO are estimated to be a mixture of polyketone (minor component) and homopolymers of p ‐methoxystyrene (major component). The terpolymer in entry 6 (Table 3) displays a 31% insertion of p ‐methoxystyrene‐CO units and incorporates small amounts of poly( p ‐methoxystyrene).…”

“…These values closely match those reported in the literature, further supporting the formation of well‐defined alternating copolymers (Table 1, entry 7; Table 3, entries 1 and 2; and Figures S2 and S3). [ 41 ] The thermal properties of terpolymers synthesized from ethylene, vinyl arenes, and CO were subsequently evaluated. The ratio of styrene incorporated into terpolymers was varied, yielding changes in thermal properties.…”

Alternating copolymerization of carbon monoxide and vinyl arenes using [N,N] bidentate palladium catalysts

“…Our group has preciously found that increasing the steric bulk of the α ‐diimine ligand backbone is highly efficient for enhancing stability and living fashion of α ‐diimine nickel and palladium catalysts for (co)polymerization of ethylene and polar monomer 114–122 . This approach is also applied to enhancement on palladium‐catalyzed CO/styrene copolymerization 17,18,114,115 .…”

Improvement of catalytic activity for α‐diimine nickel complex with active sites stabilized by bulky boron counterions at elevated temperature

“…Gao et al synthesized bulkier dinaphthyobarrelene backbone-based α-diimine Ni (II) and Pd (II) catalysts with 3Dconstrainedspace (II in Chart 1), and the 3D-constrained space around metal center enhanced the thermal stability, living fashion, and tolerance toward polar groups in polymerizations of ethylene and polar monomers. [25][26][27][28][29][30][31][32][33] Jian et al prepared a new α-diimine nickel catalyst with a concerted double-layer steric environment (III in Chart 1), this catalyst featured highly thermally robust was ultrahighly active toward ethylene polymerization. [34] Dai et al developed a series of highly sterically demanding α-diimine Ni (II) catalysts with abundant tert-butyl substituents (IV in Chart 1) were efficient for the synthesis of PE thermoplastic elastomer at elevated temperatures (up to 90 C).…”