Yan-Guo Li

Synthesis of Polyethylene Containing Allene Groups: A Simple and Efficient Route to Functional Polyethylene

Allene groups are first employed as the reactive moiety in the simple and efficient synthesis of well-defined functional polyethylene. By copolymerization of ethylene with allene group substituted norbornene, the allene group is successfully introduced into the polyethylene with a high content. The retained allene groups are demonstrated to be highly reactive in following photoinduced functionalized reactions and can be efficiently converted into the functional groups without the multi-step, time consuming processes that have generally been required in previous reports, providing the side group-functionalized polyethylene with a wide range of functional group content.

Efficient synthesis of diverse well-defined functional polypropylenes with high molecular weights and high functional group contents via thiol–halogen click chemistry

The thiol–halogen click chemistry between halogenated isotactic polypropylenes (iPPs) and thiols was systematically investigated for the first time. The order of copolymer reactivity can be summarized as iodinated ≫ brominated > chlorinated iPP, while for different thiols, the reactivity depended on not only the acidity of the sulfhydryl but also the competition of the polar group in the thiols with –SH to participate in the nucleophilic substitution reaction. In this case, various polar groups including hydroxyl, ester, aryl, thiazolyl and amino have been successfully introduced into iPP in a quantitative way, by employing iodinated iPP as the highly reactive intermediate. The content of polar groups in the high molecular weight functional iPPs (Mw > 100 kg mol−1) could be tuned in a wide range of 0–11 mol%. More notably, this new strategy offers an effective route to prepare the well-defined graft copolymer iPP-g-PCL from the halogen functionalized iPP and poly(e-caprolactone)s with highly reactive –SH end (PCL-PhSH). Initiated by the resulting hydroxyl-containing iPP, ring-opening polymerization of L-lactide (LLA) provided another kind of graft copolymer, iPP-g-PLLA.

Living syndiospecific polymerization of propylene with sterically encumbered titanium complexes activated by MMAO

A series of sterically encumbered (salicylaldiminato)(β-enaminoketonato)titanium complexes [3-tBu-2-OC6H3CHN(C6F5)][RNC(CF3)CHC(tBu)O]TiCl2 [1a: R = Ph, 1b: R = C6H4F(p), 1c: R = C6H4Cl(p), 1d: R = C6H4Br(p), 1e: R = C6H4Br(o)] were synthesized and tested to be efficient catalysts for syndiospecific polymerization of propylene in the presence of modified methylaluminoxane at room temperature. The introduction of a bulky bromine atom ortho to the imine nitrogen of the β-enaminoketonato ligand not only successfully improved the pentad syndiotacticity (rrrr) of the resulting polypropylenes from 88.5% to 97.2%, but also provided better protection of the active site from attack of free AlR3 or monomers and thus contributed to the living polymerization nature, while keeping high catalytic activity. More importantly, compared with the famous pentafluorinated FI-Ti/MAO catalyst system, the sterically congested complex 1e with the bromine atom ortho to a N-aryl group displayed almost two times higher catalytic activity (14.5 vs. 28.0 kg mol−1 h−1), producing polypropylenes with even higher pentad syndiotacticity (rrrr = 97.2% vs. 96.0%) and similar narrow molecular weight distributions (Mw/Mn = 1.12–1.26). In addition, the polymerization proceeded with a different monomer insertion mode of 1,2-insertion and a similar chain-end control mechanism. Quantitative 13C NMR spectra revealed that polymers with various stereo structures ranging from highly syndiotactic and regioregular to atactic and regio-irregular polymers at different reaction temperatures were achieved, and the probable formation routes were proposed. The percentage of regio-irregularities of the monomer sequence arising from 2,1-insertion and 3,1-enchainment increased with the rise of reaction temperature.

Ethylene polymerization and ethylene/hexene copolymerizaion with vandium catalysts bearing thiophenolphosphine ligands

Vanadium(III) complexes bearing thiophenol-phosphine ligands (2a–2b) (2-R-6-PPh2-C6H2S) VCl2(THF)2 (2a: R = H; 2b: R = Me3Si) were prepared from VCl3(THF)3 by treating with 1.0 equiv of the ligand in tetrahydrofuran in the presence of excess triethylamine. The two complexes were characterized by FTIR and mass spectra as well as elemental analyses. On activation with Et2AlCl, these complexes exhibited high catalytic activities (up to 22.1 kg PE/(mmolV·h·bar)) even at high temperature (70 °C), and produced high molecular weight polymers with unimodal molecular weight distributions, indicating the polymerization took place in a single-site nature. This result may be attributed to benefits of introduction of second-row donor atoms for adjusting charge density of the vanadium centers. In addition, these complexes also exhibited high catalytic activities for ethylene/1-hexene copolymerization. Catalytic activity, comonomer incorporation and polymer molecular weight can be controlled in a wide range by the variation of catalyst structure and the reaction parameters such as Al/V molar ratio, comonomer feed concentration and polymerization reaction temperature.

Ambient Temperature Living Radical Copolymerization of Styrene and Methyl Methacrylate with Sodium Hypophosphite as Reducing Agent

A facile, safe and economical reducing agent, sodium hypophosphite (NaH2PO2·H2O), has been successfully employed for ambient temperature living radical copolymerization of styrene (St) and methyl methacrylate (MMA). Such effective reducing agent significantly improved the reactivity of low reactive St monomers during the copolymerization, where the reactivity ratios of St and MMA were determined to be 0.50 and 0.36 by Finemann-Ross method. Thus the copolymerizations proceeded fast and showed typical living/controlled features, as evidenced by pseudo first-order kinetics of polymerization, linear increase in molecular weight versus monomer conversion, and low polydispersity index values. Effects of the concentration of reducing agent and the monomer feed ratio on the copolymerization were investigated in detail. Furthermore, gel permeation chromatography and 1H-NMR analyses as well as chain extension experiments confirmed the high chain-end functionality of the resultant copolymer.

Synthesis and characterization of fluoro-substituted β-enaminoketonato titanium complexes and their catalytic behavior of regioselective ethylene/cyclopentadiene copolymerization

The ethylene/cyclopentadiene (CPD) copolymerization behavior by using fluoro-substituted bis(β-enaminoketonato) titanium complexes [FC6H4NC(CH3)CHCO(CF3)]2TiCl2 (1a–1c) has been investigated in detail. Upon utilizing MMAO as a cocatalyst, complexes 1a–1c exhibit high catalytic activities, affording the copolymers with high molecular weight and unimodal molecular weight distribution. Compared with non-substituted complex [C6H5NC(CH3)CHCO(CF3)]2TiCl2 (1), complexes 1a–1c can produce the copolymers with CPD incorporation adjusted in a wide range due to the enhancement of electrophilicity of metal center caused by introducing electron-withdrawing groups. Especially complex 1c bearing fluorine at the para-position of N-aryl moiety provides the highest CPD incorporation, which is nearly two times (18.5 mol%) higher than the non-substituted complex 1 (8.9 mol%) under the same conditions. The highest CPD incorporation up to 24.6 mol% can be easily achieved using this complex. 1H- and 13C-NMR spectra demonstrate that these fluoro-substituted complexes possess regioselective nature with exclusive 1,2-insertion fashion, and alternating ethylene-CPD sequence can be detected at high CPD incorporation.

Facile, Efficient Copolymerization of Ethylene with Norbornene-Containing Dienes Promoted by Single Site Non-Metallocene Oxovanadium(V) Catalytic System

Non-metallocene oxovanadium(V) complexes bearing either [ONNO]-type amine pyridine bis(phenolate) ligands or [ONN]-type amine pyridine phenolate ligands were employed as efficient catalysts to copolymerize ethylene with several unsymmetrical norbornene-containing dienes, such as 5-vinyl-2-norbornene (VNB), 5-ethylidene-2-norbornene (ENB) or dicyclopentadiene (DCPD), producing copolymers with high comonomer incorporations (VNB: 33.0 mol %; ENB: 30.4 mol %; DCPD: 31.6 mol %, respectively) and high molecular weight (VNB: 86.4 kDa; ENB: 256 kDa; DCPD: 86.4 kDa, respectively). The enchainment of the dienes was proven to be exclusive of vinyl-addition via the C=C double bond of the norbornene ring while the other double bond was retained near the backbone without crosslinking. During the copolymerization of ethylene with ENB, a positive ‘comonomer effect’ was observed. The catalytic activities of the catalysts as well as the molecular weights and comonomer incorporations of the resultant copolymers could be tuned within a wide range by varying the structures of the catalysts and copolymerization conditions. The [ONN]-type oxovanadium(V) complexes showed higher catalytic activities than those of [ONNO]-type oxovanadium(V) complexes, irrespective of the structure of the dienes. In addition, the dominant chain transfer pathway of the non-metallocene oxovanadium(V) catalytic system promoted copolymerization was proven to be transfer to aluminum compounds.

Syndiospecific polymerization of styrene with multi-nuclear titanium complexes

Two multi-nuclear titanium complexes [Ti(η5–Cp*)Cl(μ–O)]3(1) and [(η5–Cp*TiCl)(μ–O)2(η5–Cp*Ti)2(μ–O)(μ–O)2]2Ti (Cp* = C5Me5)(2) have been investigated as the precatalysts for syndiospecific polymerization of styrene. In the presence of modified methylaluminoxane (MMAO) as a cocatalyst, complexes 1 and 2 display much higher catalytic activities towards styrene polymerization, and produce the higher molecular weight polystyrenes with higher syndiotacticities and melting temperatures (Tm) than the mother complex Cp*TiCl3 does when the polymerization temperature is above 70°C and the Al/Ti molar ratio is in the low range especially.

Improved Catalytic Capabilities of Vanadium Complexes bearing Tridentate Constrained Cyclic β-Enaminoketonato Ligands towards Ethylene (Co)polymerization

A series of vanadium complexes bearing tridentate constrained cyclic β-enaminoketonato ligands, [RC6H3-(O)C=C(CH2)nCH=NC6H4SPh]VCl2(THF) (2a, n =1, R = H; 2b, n =1, R = C6H5; 2c, n = 2, R = C6H5; 2d, n = 3, R = C6H5), were synthesized and characterized, and the molecular structures of complexes 2b and 2c were further confirmed by X-ray crystal analysis. In the presence of ethyl trichloroacetate as reactivating agent, Et2AlCl activated vanadium complexes produced polyethylenes with unimodal distribution (PDI = 1.4-1.8) in high activities (107-108 gPE/molV•h). Complex 2b bearing phenyl at R position showed higher catalytic activity than complex 2a without bulky R group, and the catalytic properties of these catalysts could be easily tuned by varying the ligand structure and polymerization conditions. These tridentate vanadium complexes showed promising thermal stabilities and did not deactivate in 30 minutes. The copolymerization of ethylene with norbornene (NBE) and exo-1,4,4a,9,9a,10-hexahy-dro-9,10(1′,2′)-benzeno-1,4-methanoanthracene (HBM) were also efficiently catalyzed by these vanadium catalysts, and copolymers possessing high comonomer incorporations (NBE: 45.2 mol%; HBM: 25.2 mol%) and high glass transition temperature (NBE: 113°C; HBM: 191 °C) were obtained.

Introduction of constrained cyclic skeleton into β-enaminoketonato vanadium complexes: A strategy for stabilization of active centre of vanadium catalyst for ethylene polymerization

Several novel mono(β-enaminoketonato) vanadium complexes bearing constrained cyclic skeleton, including [(C6H5)C6H3C(O)=C(CH2)nCH=N-Ar]VCl2(THF)2 (V3a: n = 1, Ar = C6H5; V4a: n = 2, Ar = C6H5; V4b: n = 2, Ar = C6F5; V4c: n = 2, Ar = (C3H7)2C6H3; V5a: n = 3, Ar = C6H5), were synthesized and their structure and properties were characterized. The structures of V4c and V5a in solid-state were further confirmed by X-ray crystallographic analysis. Density functional theory (DFT) results indicated that these complexes showed enhanced steric hindrance around the metal center as compared with the acyclic analogues. Upon activation with Et2AlCl and in the presence of ethyl trichloroacetate as a reactivator, all of the complexes exhibited high catalytic activities (107 gPE/(molV·h)) toward ethylene polymerization, and the obtained polymers exhibited unimodal distributions (Mw/Mn = 2.0–2.3) even produced at elevated temperatures (70–100 °C) and prolonged reaction time. When MAO was employed as a cocatalyst, they only showed moderate catalytic activities (105 gPE/(molV•h)), but the resulting polymers had higher molecular weights (168–241 kg/mol). These vanadium complexes with cyclic skeleton also showed high catalytic activities toward ethylene/norbornene copolymerization. The produced copolymers displayed approximate alternating structure at high in-feed concentration of norbornene. The catalytic capabilities of these complexes could be tuned conveniently by varying ligand structure. Furthermore, the cyclic voltammetry results also proved that these complexes exhibited better redox stabilities than the complexes bearing linear skeleton.