支化结构二氧化碳基六元醇的可控合成

Controllable Synthesis of Branched CO_2-based Hexol

利用Zn-Co双金属氰化物为催化剂,疏水性二季戊四醇(DPE)作为起始剂,实现了二氧化碳(CO_2)和环氧丙烷(PO)的不死共聚合,高效、高选择性地合成了支化结构CO_2基六元醇,且该产物有6个羟基封端.该支化CO_2基六元醇的合成路线具有可控的特点,其分子量可通过PO与DPE的摩尔比准确控制(1500~8000),同时分子量分布很窄(最低至1.08).值得注意的是,降低反应温度可显著改善聚合选择性,例如,当温度为50℃时,产物碳酸酯单元含量高达60%,而反应副产物环状碳酸酯的含量可控制在5.5 wt%以下,但是体系催化活性下降至0.14 kg g-1.该支化结构CO_2基六元醇有望作为高交联密度的多元醇,用于制备高强硬质聚氨酯泡沫材料.

Branched CO2-based oligo（carbonate-ether） hexols were synthesized in high productivity and selectivity by immortal copolymerization of CO2 and propylene oxide（PO） in the presence of hydrophobic dipentaerythritol（DPE） using the zinc-cobalt double metal cyanide（Zn-Co-DMC） catalyst. The structure of the CO2-based hexols was confirmed via FTIR, ^1H-NMR, ^13C-NMR, DEPT-NMR, MALDI-TOF-MS, GPC and DSC. However, because of the overlap of the proton signals assigned to DPE and CO2-based hexols, respectively, it was hard to make sure the fully conversion of the 6 hydroxyl groups in DPE. By using 13C-NMR technique wisely, we confirmed that all the 6 hydroxyl groups participated in the copolymerization due to the chemical shift of characteristic carbon signal of DPE. Moreover, the MALDI-TOF-MS spectrum of the polymer gave a direct visualization of the structure of CO2-based hexols, which contained 6 hydroxyl groups per macromolecule. The number average molecular weight（Mn） of the CO2-based hexol was in good linear relationship to the molar ratio of PO to DPE（PO/DPE）, and hence could be precisely controlled from 1500 to 8000. Besides, the rapid chain transfer reaction in the immortal copolymerization afforded the CO2-based hexol with a narrow polydispersity index（PDI） of 1.08 at Mn of 1600, which was reported as one of the narrowest PDI in heterogeneous catalytic systems. Notably, decreasing reaction temperature could substantially improve the catalytic selectivity, e.g., at copolymerization temperature of 50 ℃, the carbonate unit（CU） content in the CO2-based hexols could reach as high as 60%, while the weight fraction of the unwanted byproduct propylene carbonate（wPC） could be controlled to as low as 5.5 wt%, although the catalytic productivity declined to 0.14 kg g^-1. Unfortunately, the natural hydrophilic polyhydric alcohols, like sucrose and lactose, resulted in uncontrolled reactions and the corresponding CO2-based polyols could not be obtained. These results suggested that the natural polyhydric alcohols could not coordinate to the active center of catalyst.