The general characteristics of the active center of the catalysts (including zinc-cobalt(Ⅲ) double metal cyanide complex [Zn-Co(Ⅲ) DMCC]) for the copolymerization reaction of carbon dioxide (CO₂) with epoxide are summarized. By comparing the active center, catalytic performance of the Zn-Co(Ⅲ) DMCC (and other catalysts) with HCAII enzyme in the organism for activating CO₂ (COS and CS₂), we proposed that the metal-hydroxide bond (M-OH), which is the real catalytic center of human carbonic anhydride II (HCAII), is also the catalytic (initiating) center for the copolymerization. It accelerates the copolymerization and forms a closed catalytic cycle through the chain transfer reaction to water (and thus strictly meets the definition of the catalyst). In addition, the metal-hydroxide bond catalysis could well explain the oxygen/sulfur exchange reaction (O/S ER) in metal (Zn, Cr)-catalyzed copolymerization of COS (and CS₂) with epoxides. Therefore, it is very promising to learn from HCAII enzyme to develop biomimetic catalyst for highly active CO₂/epoxide copolymerization in a well-controlled manner under mild conditions.

Two types of bifunctional bis(salicylaldimine) ligands (syn-L and anti-L) were designed and synthesized to support bimetallic aluminum complexes. Owing to the rigid anthracene skeleton, syn-L and anti-L successfully locked two Al centers in close proximity (syn-Al₂) and far apart (anti-Al₂), respectively. The distance between two Al centers in syn-Al₂ was defined by X-ray diffraction as 6.665 Å, which is far shorter than that in anti-Al₂. In the presence of stoichiometrical BnOH, syn-Al₂ and anti-Al₂ were both efficient for ring-opening polymerization (ROP) of rac-LA with the former being more active. In the presence of excess BnOH, syn-Al₂ showed an efficient and immortal feature, consistent with high conversions, matched M_ns, narrow molecular weight distributions and end group fidelity, while anti-Al₂ had a much lower activity or even became entirely inactive due to rapid decomposition, indicated by in situ ¹H-NMR experiments of Al complexes with BnOH.

Naphthyl-α-diimine nickel complexes with systematically varied ligand sterics, activated by modified methylaluminoxane (MMAO), were tested in the polymerization of higher α-olefin (1-hexene, 1-decene and 1-hexadecene) under suitable conditions. The polymerization results indicated the possibility of precise microstructure control, depending on catalyst structure, polymerization temperature, monomer concentration and types of monomers, which in turn strongly affects the resultant polymer properties. Naphthyl-α-diimine nickel complex bearing chiral bulky sec-phenethyl groups in the o-naphthyl position showed good catalytic activity, and resulted in branched polymers (42-88/1000C) with high molecular weights (M_n:(4.3-15.2)×10⁴ g·mol^-1) and narrow molecular weight distribution (M_w/M_n=1.13-1.29, RT), which suggested a living polymerization. The increasing steric hindrance of catalyst leads to enhance insertion for 2, 1-insertion of α-olefin and the chain-walking reaction.

Three novel enantiopure phenyl isocyanide monomers with BH₃-protected phosphine functional group were designed and synthesized. Polymerization of these monomers using a alkyne-Pd(Ⅱ) complex as a catalyst led to the formation of respective helical polyisocyanides in high yields with controlled molecular weights (M_ns) and narrow molecular weight distributions (M_w/M_ns). Removing the protecting BH₃ groups afforded helical poly(phenyl isocyanide)s bearing phosphine pendants. Thanks to the chiral induction of monomer, the isolated helical polyisocyanides showed high optical activity, as revealed by circular dichroism (CD) and absorption spectroscopies and polarimetry. The helical structures of these polymers were quite stable in various organic solvents with different polarities and in a wide temperature range. Moreover, these helical polymers could be used as organocatalysts and showed good performance in enantioselective cross Rauhut-Currier reaction. The enantiomeric excess (ee) values of the isolated products of cross Rauhut-Currier reaction could be up to 90%. The polymer organocatalysts could be easily recovered from the reaction mixtures and reused at least five times in the reaction without significant loss of their enantioselectivities and catalytic activities.

Copolymerizations of ethylene and 1-dodecene were conducted with a series of ansa-fluorenylamidodimethyltitanium complexes, [t-BuNSiMe₂Flu]TiMe₂ (1a), [t-BuNSiMe₂(2, 7-^tBu₂Flu)]TiMe₂ (1b), and[(1-adamantyl)NSiMe₂(2, 7-^tBu₂Flu)]TiMe₂ (1c) activated by modified methylaluminoxane. The activity increased by the introduction of the alkyl groups on the fluorenyl and amido ligands, and 1c produced the highest molecular weight copolymers. Complex 1c also promoted copolymerization of ethylene and ⁱBu₃Al protected 10-undecen-1-ol with high activity (~2000 kg·mol^-1·h^-1), affording hydroxy-functionalized ultrahigh molecular weight polyethylene. The hydroxy content of the copolymers obtained was controllable by changing comonomer feed ratio. The introduction of a small amount of hydroxy group can alter the surface properties of polyethylene.

Two neutral five-membered pyridine-imine palladium complexes with the bulky dibenzhydryl (CH(Ph)₂) substituted aniline were synthesized and fully characterized by nuclear magnetic resonance (NMR) and X-ray crystal diffraction. Well-defined cationic palladium complexes were further obtained by treatment of chloromethylpalladium complexes with sodium tetrakis(3, 5-bis(trifluoromethyl)phenyl)borate (NaBArF) in CH₃CN. Cationic palladium complexes were capable of catalyzing ethylene oligomerization without any cocatalysts. The influences of catalyst structure, reaction temperature, and ethylene pressure on ethylene oligomerization were studied in detail. The introduction of bulky benzhydryl (CH(Ph)₂) on the ortho position of the aniline moiety enhanced catalytic activity, thermal stability of the catalyst, and molecular weight of the obtained products. Highly branched oligomers with molecular weights of 600-800 g/mol and narrow polydispersities (1.03-1.12) were produced.

Schiff-base metal complexes as efficient catalysts are widely used in ring-opening polymerization of cycle esters. The salen Fe complexes were formed with their excellent biocompatibility and less toxicity. A series of salen Fe complexes were designed in this work in order to study the activity and control of polymerization of lactide. The salen Fe complexes' activities changed with the ligands configuration and substituent groups.

Binuclear aluminum alkyl complexes 2a-4g supported by linked bis(β-diketiminate) ligands were synthesized via the reaction of AlEt₃ or AlMe₃ and the corresponding proligand in a 2:1 molar ratio with moderate yields. The isolated complexes were well-characterized by ¹H-NMR, ¹³C-NMR and elemental analysis. The binuclear nature of aluminum complex 2b was further confirmed by an X-ray diffraction study. All complexes 2a-4g could efficiently initiate the ring-opening polymerization (ROP) of ε-caprolactone in toluene. The substituents at the aromatic rings and the linker unit in the auxiliary ligands exerted significant influence on the catalytic behavior of the investigated aluminum complexes. Complex 4g (R¹=R²=Cl) containing propylenyl bridging unit exhibited the highest catalytic activity among these complexes, which might be attributed to the increased electrophilicity of the metal center as well as more opened coordination sphere. The molecular weights of obtained poly(ε-caprolactone)s deviating considerably from the theoretical values indicated that the ROP of ε-caprolactone by complexes 2a-4g was not well-controlled, which was also judged from the broad molecular weight distributions (MWD=1.47-2.47) of produced poly(ε-caprolactone)s. These complexes proved to be inactive toward the polymerization of rac-lactide alone. In the presence of alcohol the polymerization occurred, which was actually initiated by the decomposition species of the aluminum complex upon the treatment with isopropanol.

A highly efficient strategy for the synthesis of polylactide with the UV absorption ability was established by employing a Salan-yttrium complex (acting as a fast runing catalyst) combined with large excess hydroxyl functionalized benzophenone, BP'-OH. During polymerization, BP'-OH, acting as the chain transfer agent, attached to the active rare-earth metal catalyst via a rapid-reversible exchange reaction to initiate the polymerization. Thus, more polyester chains appeared to grow from one active metal species, and the UV absorption fragments were incorporated into the polymer chains at specific sites, in situ. A high productivity up to 1000 mol_LA/mol_(Salan-Y) was successfully achieved and 100 BP'-labeled PLA chains grew from each active metal center.

5, 6, 7-Trihydroquinolin-8-one was condensed with the corresponding benzidine to give N, N'-bis(5, 6, 7-trihydroquinolin-8-ylidene)-[1, 1'-biphenyl]-4, 4'-diamine derivatives (L1-L3). The ligands were reacted with two equivalents of NiCl₂·6H₂O in a mixture of EtOH and CH₂Cl₂ to afford the corresponding dinickel(Ⅱ) chloride complexes (Ni1-Ni3). The organic compounds were completely characterized, whilst the bi-metallic complexes were characterized by FTIR spectra and elemental analysis. These nickel complexes exhibited high activities towards ethylene polymerization in the presence of either MAO or Me₂AlCl, maintaining a high activity over a prolonged period. The obtained polyethylenes were confirmed as having low molecular weights by GPC analysis.

Cyclic olefin polymers (COPs) with high glass transition temperature, high transparency (higher than 80%) in the visible light range, excellent thermal stability and outstanding mechanical properties have been synthesized by effective ring opening metathesis polymerization (ROMP) of exo-1, 4, 4a, 9, 9a, 10-hexahydro-9, 10(1', 2')-benzeno-l, 4-methanoanthracene (HBM) and dicyclopentadiene (DCPD) or norbornene (NBE) using WCl₆/i-Bu₃Al/ethanol/1-hexene catalyst system, followed by hydrogenation of double bonds. 1-Hexene acted as a molecular weight controller in the polymerization reaction, tuning the number-average molecular weight (M_n) of P-HBM from 5.8×10⁴ to 41.1×10⁴. The monomer composition and thermal properties of the copolymers were characterized by nuclear magnetic resonance (NMR), differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The saturated polymers exhibited high decomposition temperatures (T_d) around 340℃ and glass transition temperatures (T_g) in the range from 117.5℃ to 219.7℃. What is more, tensile tests indicated that the mechanical properties of the COPs could be effectively tuned in a wide range by introducing varying amount of small cyclic olefin such as DCPD or NBE.

The syndiotactic polymerization of styrene (St) and the copolymerization of St with ethylene (E) were carried out by using a series of chiral half-sandwich rare-earth metal dialkyl complexes (Cp^{x*) as the catalysts. The complexes are Ln(CH₂SiMe₃)₂(THF) (1-4:Ln=Sc (1), Ln=Lu (2), Ln=Y (3), Ln=Dy (4)) bearing chiral cyclopentadienyl ligand containing bulky cylcohexane derivatives in the presence of activator and AliBu₃. For the St polymerization, a high activity up to 3.1×106 g of polymer molLn-1·h-1 and a high syndiotactic selectivity more than 99% were achieved. The resulting syndiotactic polystyrenes (sPSs) have the molecular weights (M_n) ranging from 3700 g·mol-1 to 6400 g·mol-1 and the molecular weight distributions (M_w/M_n) from 1.40 to 5.03. As for the copolymerization of St and E, the activity was up to 2.4×106 g of copolymer mol_Sc-1·h-1·MPa-1, giving random St-E copolymers containing syndiotactic polystyrene sequences with different St content in the range of 15 mol%-58 mol%. These results demonstrate that the bulky cyclopentadienyl ligands of the chiral half-sandwich rare-earth metal complexes effectively inhibit the continued insertion of St monomers into the (co)polymer chain to some extent in comparison with the known half-sandwich rare-earth metal complexes.}

Despite the extraordinary success has been achieved in metal catalyst-promoted stereoselective ring-opening polymerization (ROP) of rac-lactide (rac-LA), well-controlled stereoselective rac-LA ROP by organic catalyst still remains a scientific challenge. Here we report our investigations into organocatalytic stereoselective ROP of rac-LA by utilizing novel bulky chiral and achiral N-heterocyclic carbenes (NHC), 1, 3-bis-(1'-naphthylethyl)imidazolin-2-ylidene. The effect of polymerization conditions (e.g. solvent, temperature, alcohol initiator) on ROP behavior by these bulky NHCs has been fully studied, leading to the formation of isotactic-rich stereoblock polylactide (P_i=0.81) under optimized conditions with high activity (Conv.=98% in 30 min) and narrow molecular weight dispersity (Đ=1.05).

Bergman cyclization has shown great promise in constructing conjugated polymers. However, the application of this reaction in polymer science is still limited due to the harsh reaction condition and ill-defined structure of the achieved polymers. To this end, the cycloaromatization polymerization of enediynes catalyzed by a series of transition metal catalysts is investigated in this work, by taking advantage of the coordination chemistry of the enediyne with the transition metal complexes. According to the nuclear magnetic resonance (NMR), Fourier transform infrared (FTIR), ultraviolet-visble (UV-Vis) spectroscopies and matrix-assisted laser desorption/ionization time-offlight mass spectrometry (MALDI-TOF MS) analysis, the cycloaromatization polymerization of enediynes proceeds under milder conditions and in a more controlled manner in the presence of palladium(Ⅱ) complexes, giving structurally regulated conjugated polymers in high yields.

A series of binuclear nickel phenoxyiminato catalysts with different linkers and fluorine substituents were efficiently synthesized. Binuclear nickel catalysts with rigid linkers showed higher catalytic activity and thermal stability in ethylene polymerization and produced polymers with higher molecular weight possibly due to the larger steric hindrance and metal-metal synergistic effect. The introduction of fluorine atoms on the N-terphenyl moity also enhanced polymerization activity and molecular weight of polymer due to the electronic effect of fluorine atoms.

Aluminum porphyrin complexes are heavy-metal-free and soil-tolerant green catalysts for the copolymerization of CO₂ and propylene oxide (PO), but they suffer from relatively poor poly(propylene carbonate) (PPC) selectivity. Herein, steric hindrance porphyrin ligand was used to enhance the PPC selectivity. Typically, a bulky anthracene-like group was incorporated into the porphyrin ring to form 5, 10, 15, 20-tetra(1, 2, 3, 4, 5, 6, 7, 8-octahydro-1, 4:5, 8-dimethanoanthracen-9-yl) porphyrin, the aluminum porphyrin complex with this ligand, in combination with bis(triphenylphosphine) iminium chloride as a co-catalyst, produced completely alternate PPC. Additionally, the obtained PPC showed high regioselectivity, with a head-to-tail linkage content (HT) of 92%. Therefore, we demonstrated that introduction of bulky steric ligand into the porphyrin ring could reduce the propylene oxide homopolymerization activity leading to excellent PPC selectivity, and improve regioselectivity for the PO ring-opening during the copolymerization.