Well-controlled Ring-opening Polymerization Of ε-CL Initiated By Dialkylaluminum β-diketiminates

Polycaprolactone (PCL) is an important polymer in biomedical and pharmaceutical applications due to their biodegradable, biocompatible and permeable properties. As an efficient method to synthesize the polymer, metal catalysts can initiate the ring-opening polymerization (ROP) of caprolactone by selecting suitable metals and ligands. Although many catalysts have been investigated, the catalytic systems which have high activity and can well control molecular weight well are still rare. Therefore, chemists are endeavoring to explore novel catalysts which possess the characteristics of good biocompatibility and high catalytic activity.This thesis reports the preparation of alkylaluminum catalysts by using the abundant aluminum metal and N-alkyl β-diketiminate ligand, and investigate their catalytic activities in the ROP of ε-CL. The details are as follows:A series of alkylaluminum complexes nacnacRAlMe2(R=CH3,2a;iPr,2b;tBu,2c; Bn,2d; CH2CH2Ph,2e; CHPh2,2f), nacnactBuAlEt2(2g) and nacnactBuAliBu2(2h) bearing aliphatic N-substituted β-diketiminate ligands (nacnacRH=N,N-dialkyl-2-amino-4-iminopent-2-enes) were obtained from the reaction of trialkyl aluminum and the corresponding (3-diketimines. All these aluminum complexes were characterized by NMR, elemental analysis and HRMS spectroscopy. The molecular structures of complexes2a,2c and2h were confirmed by single-crystal X-ray diffraction. These aluminum alkyl complexes show notable activities towards the ring-opening polymerization of ε-caprolactone in the absence and presence of alcohol, and the resulting polymers have narrow molecular weight distributions. The effect of the ligand substituents in complexes2a-2h on the catalytic activities for ROP of ε-CL was investigated, and complex2c showed the highest catalytic activity with99%conversion after30min at80℃and the obtained polymer had a narrow distribution (PDI=1.27) among the studied aluminum alkyls. In the presence of BnOH, complex2c still showed the good catalytic activity with99%conversion after30min at80℃and the resulting polymer had a more narrow distribution (PDI=1.07). The influence of the reaction parameters on the ROP in detail was investigated using complex2c as the initiator. The higher the temperature was. the faster the polymerization proceeded. The polymerization failed in CH2Cl2compared with toluene. It is found that even when the ε-CL/Al molar ratio was increased to800, complex2c still showed good catalytic activity by93%conversion within2h at80℃.