Polymerization Catalyzed By Metal Complexes Containing Bi(Phenoxy-imine) Ligand And Structure-properties Relationship Of The Polymers

This dissertation focuses on the living polymerization of ethylene/propylene catalyzed by titanium complexes having fluorine-containing phenoxy-imine chelate ligands employing methyl aluminoxane (MAO) as cocatalyst and the ring-opening polymerization of ε-caprolactone catalyzed by salicylaldimine-aluminum complex in the presence of benzyl alcohol (BnOH) or monomethoxy poly(ethylene glycol). Characterization of ethylene/propylene random copolymers and poly(ε-caprolactone)-b-poly(ethylene glycol) block copolymers were investigated, respectively, and structure-properties relationship of synthesized polymers was studied.Firstly, living copolymerization of ethylene and propylene was catalyzed by a fluorine-containing bis(phenoxy-imine) titanium catalyst. A series of ethylene/propylene copolymers with different propylene contents were prepared and the microstructure was characterized by ¹³C-NMR, GPC and DSC. It shows that ethylene/propylene random copolymers have following characteristics: (1) The molecular weight distribution is narrow; (2) There exist only isolated propylene units distributed along the polymer chains even at propylene content as high as 14.93 mol%; (3) The lengths of ethylene sequence are not homogeneous; (4) The insertion of propylene in propagation of the copolymer chains becomes 1,2-insertion from 2,1-insertion in homopolymerization when the preceding unit is ethylene. Non-isothermal and isothermal crystallization kinetics experiments were carried out. The ethylene-propylene copolymers underwent different crystallization conditions: quenching in ice water, cooling slowly in air and stepwise crystallization, respectively,and then were studied using WAXD and IR. It is found that as propylene content increases, the crystallinity decreases, length of α-axis of the PE crystal lattice increased and length of b-axis almost exhibited no change. In contrast, for the ethylene-propylene copolymers prepared by Ziegler-Natta catalyst, with propylene content of 8.00 mol%, there are successive propylene units (PP dyad). As propylene content increased, the crystallinity decreased slowly, lengths of α-axis and b-axis of the PE crystal lattice almost exhibited no change. It is concluded that the isolated propylene units spaced by longer ethylene sequences is easy to be included into the PE crystal lattice, but the isolated propylene units spaced by shorter ethylene sequences or successive propylene units is difficult to be included into the PE crystal lattice.In the second part of this thesis, ring-opening polymerization of ε-caprolactone (ε-CL) was catalyzed by salicylaldimine-aluminum in the presence of benzyl alcohol (BnOH). The effects of salicylaldimine ligand and BnOH were investigated. It is indicated that ε-CL can inserts into both Al-Et and Al-OBn, but insertion of CL monomer into Al-OBn is far faster than into Al-Et. Moreover, the free BnOH can exchange with the propagating chains, leading to a narrow molecular weight distribution, but smaller molecular weight and lower conversion.A series of poly(ε-caprolactone) homopolymers and two series of poly(ε-caprolactone)-b-poly(ethylene glycol) (PCL-b-PEG) block copolymers with different molecular weights but narrow polydispersity were synthesized with a salicylaldimine-aluminum complex in the presence of benzyl alcohol and monomethoxy poly(ethylene glycol), respectively. The block copolymers were characterized by ¹H-NMR, GPC, WAXD and DSC. The ¹H-NMR and GPC results verify the block structure and narrow molecular weight distribution of the block copolymers. WAXD and DSC results show that the crystallization behavior of the block copolymers varies with the composition. When the PCL block is extremely short, only the PEG block is crystallizable. With further increase in the length of the PCL block, both blocks can crystallize. The PCL crystallizes prior to the PEG block and has a strong suppression effect on crystallization of the PEG block, while the PEGblock only exerts a relatively weak adverse effect on crystallization of the PCL block. The pre-crystallized block has nucleation effect on the second block to some extent.The spherulitic growth rates of the PCL homopolymers and PCL-b-PEG block copolymers were studied. The result shows that for both PCL homopolymers and PCL-b-PEG block copolymers, the spherulitic linear growth rate first increases with molecular weight and reaches a maximum, then decreases as molecular weight increases. Crystallization temperature has greater influence on the spherulitic growth rate of polymers with higher molecular weight. Hoffman-Lauritzen theory was used to analyze spherulitic growth kinetics and the free energy of the folding surface (σ_e) was derived. It is found that the low the values of σ_e decrease with molecular weight at low molecular weight level and become constant for high molecular weight polymers. The chemically linked PEG block doesn't change the σ_e of PCL significantly.The self-assembly of PCL_nPEG₄₄ and PCL_nPEG₁₁₃ was studied in the aqueous medium. PCL_nPEG₁₁₃ diblock copolymers with different molecular weights of PCL block form spherical micelles observed by transmission electron microscopy (TEM) and atomic force microscopy (AFM). PCL_nPEG₄₄ diblock copolymers form micelles of various morphologies including sphere, cylinder, wormlike and lamellae, depending on the length of PCL block. It is correlated to the reduced tethering density, which is defined as the ratio of the area of the free PEG in solutionR_g is the gyration radius of PEG) to the average area occupied byeach PEG chain tethered on the surface of PCL crystals in PCL-b-PEG micelles (S). When S_o/S is below 3, the lamellae are easy to form. When S_o/S exceeds 10, the sphere forms. When S_o/S is within range of 3-10, cylinder and wormlike are formed. On the other hand, PCL-b-PEG diblock copolymers form micelles of various morphologies under different crystallization temperatures. There are two dominating reasons: (1) The lamellar thickness of PCL increases at higher crystallization temperature, so the number of folding of the chains of PCL decreases and the value of S_o/S increases accordingly; (2) The crystallinity of PCL increases at highercrystallization temperature, so the contribution of crystallization of PCL to the total free energy is larger and crystallization becomes dominant in micellar morphology. The final morphology of micelles is determined by the competition between the reduced tethering density and the energy of the crystalline core. Since the length of the PEG block in PCL_nPEG₄₄ is shorter than that in PCL_nPEG₁₁₃, so the reduced tethering density has a greater effect on the morphology of PCL_nPEG₁₁₃ diblock copolymers, while the energy of the crystalline core is dominant in the formation of micelles of PCL_nPEG₄₄ diblock copolymers.