Synthesis Of Mono- And Bimetallic Aluminum Complexes Stabilized By Bridged Bis(phenolate) Ligands And Their Applications In Polymerization Reactions

A series of monometallic and bimetallic aluminum complexes were prepared and their catalytic behaviors for the polymerization of e-caprolactone(e-CL) and cyclohexene oxide(CHO) were studied and compared. All the bimetallic aluminum complexes exhibited higher activity than corresponding monometallic analogs. It is thus conclusive that cooperation effects exist in bimetallic complexes in the ROP of e-CL and CHO.Through Mannich reactions of phenol, formaldehyde and piperazine or piperidine, a group of piperazidine-bridged bis(phenol)s and piperidylmethylphenol with different steric and electronic properties, which are C4H8N2[1,4-(2-OH-3,5-Me2-C6H2CH2)2](L1H2), C4H8N2[1,4-(2-OH-3-tBu-5-Me-C6H2CH2)2](L2H2), C4H8N2[1,4-(2-OH-3,5-tBu2-C6H2CH2)2](L3H2), 2-(CH2NC5H10)-4,6-Me2-C6H2OH(L4H), 2-(CH2NC5H10)-4-Me,6-tBu-C6H2OH(L5H), 2-(CH2NC5H10)-4,6-tBu2-C6H2OH(L6H), were employed as ligand precursors. Through alkyl elimination reactions with Al Me3 and Al Et3, respectively, the following complexes 1-14 were obtained, which are(Al Me2)2L1(1),(Al Me2)2L2(2),(Al Me2)2L3(3),(Al Et2)2L1(4),(Al Et2)2L2(5),(Al Et2)2L3(6), Al Me2L4(7), Al Me2L5(8), Al Me2L6(9), Al Et2L5(10), Al Et2L6(11), Al Me2(Al Me3)L4(12), Al Me2(Al Me3)L5(13), and Al Me2(Al Me3)L6(14). Also, through Mannich reactions of phenol, formaldehyde and ethanolamine or alkylation reaction of 2-aminophenol with benzyl bromide, a series of ethanolamine and 2-aminophenol bridged bisphenols with different substituents were prepared, which are HOC2H4N(2-OH-3,5-tBu2-C6H2CH2)(L7H3), HOC2H4N(2-OH-3,5-Cl2-C6H2CH2)(L8H3), and HOC6H4N(2-OH-3,5-tBu2-C6H2CH2)(L9H3). By alkyl elimination reactions with trimethyl aluminum, we successfully synthesized three new complexes, respectively labeled as(Al L7)2(15),(Al L8)2(16),(Al L9)2(17). The main conclusions are as follows:1. Treating ligand precursors LnH3(n=1-3) with Al Me3 or Al Et3 in tetrahydrofuran with the molar ratio of 1:5 led to the formation of bimetallic complexes 1-6 in good yields. Meanwhile, monometallic complexes 7-9 were obtained bytreating ligand precursors LnH(n=4-6) with Al Me3 in the molar ratio of 1:2, while monometallic complexes 10-11 were obtained by reacting ligand precursors LnH(n=5-6) with Al Et3 in the molar ratio of 1:1. Complexes 1-11 can initiate the ROP of e-CL, and bimetallic aluminum complexes 1-6 showed higher activities than monometallic analogs 7-11. The activities of the former were 2-8 times of the latter. We also found that alkyl directly connected to aluminum atom and the substituents on ligands could both influence the rate of e-caprolactone polymerization to some extent. The kinetics of polymerization process showed the significant differences in the aspect of activation energy and the change of Gibbs free energy, which provided evidence for the cooperation between two metal centers inbimetallic aluminum complexes. In addition, through adding alcohol to the polymerization reaction to improve controllability of polymerization, the number of initiation groups were discussed in both mono- and bimetallic aluminum complexes as well as the catalytic mechanism.2. Reactions of ligand precursors LnH(n=4-6) with Al Me3 in a 1:2 molar ratio gave the bimetallic aluminum complexes 12-14. Complexes 12-14 and monometallic analog 9 are all active in initiating the ROP of CHO, while bimetallic aluminum complex 14 showed higher efficiency than monometallic analog 9. The MALDI-TOF MS specturm of oligomer sample initiated by complex 14 indicated that there is only a well-de?ned methyl/hydroxyl end-capped chain polymer while MALDI-TOF MS spectra crossponding to complexes 9 and Al Me3 both showed there a cyclic polymerand the methyl/hydroxyl end-capped chain polymer. Through analyzing the relationship between theoretical and experimental molecular weight, it is conclusive that all five methyl groups in bimetallic aluminum complexes participate in the trigger when the ratio of [monomer:catalyst] was in the range from 6000:1 to 8000:1.3. Treating ligand precursors LnH3(n=7-9) with Al Me3 in tetrahydrofuran with the molar ratio of 1:1, bimetallic aluminum complexes 15-17 were obtained in good yields. Complexes 15-17 were characterized by 1H and 13 C NMR analysis. In addition, the solid state structures of complexes 16 and 17 were also determined.