Copolymerization Of Epoxide And CS₂ Catalyzed By The Series Of Asymmetrical Schiff-base Zinc （Ⅱ） Complexes

Poly(thiocarbonate), as one of important polymers, possessing some excellent features, such as chemical resistance, conductivity, magnetism, optical property and heavy-metal recognition, is currently of special interest due to its broad applications in seal materials, plastic lenses, fiber optic manufacture and treatment of heavy-metal-containing wastewater. Nevertheless, the high cost, the high toxicity and low selectivity for the commonly used catalysts and unclear catalytic mechanism restrict its promotion. Herein, the series of asymmetrical Schiff-base transition metal catalysts were developed and used to efficiently catalyze the ring-opening polymerization of epoxide or biepoxide and CS2.Firstly, complexes [Co(L1)(OAc)] (1))、([Mn(L1)OAc)] (2))、([Zn(L1)] (3)) based on the Salalen-type asymmetrical Schiff-base ligand H2L1 were synthesized and characterized by EA, FT-IR, NMR (1H and 13C), XRD, PXRD and TGA. All the three complexes (1-3) as the catalysts were shown to effectively catalyze the copolymerization of CHO and CS2, giving rise to linear poly(thiocarbonate) as the main product and a few cyclic(thiocarbonate)s as the by-products due to the inevitable oxygen/sulfur exchange. The suitable cocatalyst was bis(triphenylphosphine)iminium chloride (PPNCl), and the catalyst system 1/PPNCl exhibited the CHO conversion of 45%, the number average molecular weight (Mn) of 2982 g/mol and molecular weight distribution index (PDI) of 1.51 for the obtained poly(thiocarbonate) under the optimum reaction condition of the molar ratio of CHO, CS2, catalyst 1 and PPNCl of 1000:1000:1:1, the reaction time of 8 h and the polymerization temperature of 80 ℃. Additionally, through the catalytic behavior comparison, catalyst 3 with Zn2+-containing active species gave the relatively higher catalytic activity, where the CHO conversion of 61%, the Mn of 5230 g/mol and the PDI of 1.41 for the obtained polymer were observed.Secondly, the series of [Zn(Ln)](n= 2-5,4-7) catalysts based on the Phenyl-modified Salphen-type asymmetrical Schiff-base ligands were developed, in which [Zn3(L2)2(OAc)2] (4) with the trinuclear framework was shown to effectively catalyze the copolymerization of CHO and CS2 (56% of CHO conversion), and the main product linear poly(thiocarbonate) (Mn= 5983 g/mol and PDI= 1.42) and a series of cyclic byproducts were also produced under the reaction condition of the molar ratio of CHO, CS2, catalyst 4 and PPNCl of 1000:1000:1:1, the reaction time of 8 h and the polymerization temperature of 80 ℃. Additionally, the catalytic behaviors of catalysts 5-7 with similar mononuclear units in solution showed that catalyst 7 endowed the better catalytic activity, followed by 5 and 6, giving the CHO conversion up to 81%, the Mn up to 9987 g/mol and the PDI down to 1.23 under the optimum condition of the molar ratio of CHO, CS2, catalyst 7 and PPNCl of 1000:1000:1:1, the reaction time of 8 h and the reaction temperature of 80 ℃. Moreover, the characterization results of the obtained copolymers showed that-S(CS)S-,-O(CS)S-,-S(CO)S-,-O(CO)S-and-O(CO)O-containing structure segments were grafted, and the polymeric Tg values decreased with the increase of sulfur contents while increased with the increase of the Mn values especially at the same sulfur content. Meanwhile, besides the Tg values up to 112 ℃, two cyclic thiocarbonate byproducts of ditiocarbonate as the primary part and trithiocarbonate were observed.Thirdly, the series of [Zn(Ln)] (n=6-10; 8-12) based on the typical Salphen-type Schiff-base ligands containing different electronic and steric effects were used for the copolymerization of CHO and CS2. The results showed that catalyst 12 with the involvement of 3,5-dibromo groups showed the better catalytic activity, followed by 9,11,8 and 10, where The CHO conversion up to 70%, the Mn of 10179 g/mol and the PDI of 1.21 for the linear polymer under the optimum condition of the molar ratio of CHO, CS2, catalyst 12 and PPNCl of 1000:1000:1:1, the reaction time of 8 h and the polymerization reaction temperature of 80 ℃. Similarly, the O/S exchange also gave the-S(CS)S-,-O(CS)S-,-S(CO)S-,-O(CO)S-and-O(CO)O-grafting poly(thiocarbonate) with relatively larger Tg values up to 117 ℃ and trithiocarbonate as the main cyclic byproduct.Finally, from the viewpoint of the improved thermal properties for the copolymer, catalysts 8-12 were tried to catalyze the copolymerization of biepoxide and CS2. The catalysis results showed all the complexes 8-12 could effectively catalyze the copolymerization of the representative biepoxide ECHE (4-epoxycyclohexylmethyl-3,4-epoxycyclohexanecarboxylate) and CS2 with the catalytic activity order of 12> 9> 11> 8> 10. Especially for the catalytic system 12/PPNCl, under the optimum condition of the molar ratio of ECHE, CS2, catalyst 12 and PPNCl of 1000:2000:1:1, the reaction time of 16 h and the polymerization reaction temperature of 80 ℃, the ECHE conversion was up to 89%, the Mn of obtained copolymer was increased to 26217 g/mol. In addition, th characterization results on the polymerization products showed that three kinds of-S(CS)S-,-O(CO)O-and-O(CO)S-containing structure segments were grafted in net-moding poly(thiocarbonate), and their Tg values decreased with the increase of sulfur content of copo-lymer, and Tg values increased up to 132 ℃ with the increase of the Mn of the obtained copolymer. Meanwhile, there are four cycle byproducts determined by LC-MS.In conclusion, the series of asymmetrical zinc(Ⅱ) complexes based on Salalen and Salphen-type Schiff-base ligands were developed and shown to successfully realize the copolymerization of CHO (or ECHE) and CS2, giving rise to poly(thiocarbonate) with irregular segments as the main product and a series of cyclic thiocarbonates as the byproducts due to oxygen/sulfur exchange. Moreover, introduction of 3,5-dibromo for the catalysts was shown to do favor to the improved catalytic activity, and the involved steric effect of 5-position functional group was beneficial to polymeric chain extension. In the presence of co-catalyst of PPNCl, the net-moding poly(thiocarbonate) from ECHE-CS2 could exhibit better thermal stability than those from CHO-CS2.