Haiyang Gao

A thermally robust amine–imine nickel catalyst precursor for living polymerization of ethylene above room temperature

A bulky amine-imine nickel complex containing two 2,6-diisopropyl substituents after activation with MMAO or Et(2)AlCl can polymerize ethylene in a living fashion over a period of 120 minutes at room temperature or above.

Synthesis of hyperbranched polyethylene amphiphiles by chain walking polymerization in tandem with RAFT polymerization and supramolecular self-assembly vesicles.

A novel polymerization methodology for efficient synthesis of hyperbranched polyethylene amphiphiles by chain walking polymerization (CWP) followed by RAFT polymerization has been developed. Hyperbranched polyethylene with hydroxyl ends (HBPE-OHs) is first synthesized via chain walking copolymerization of ethylene with 2-hydroxyethyl acrylate with Pd-α-diimine catalyst. The hydroxyl groups of hyperbranched polyethylene are then converted into thiocarbonyl thio moieties by an esterification reaction with trithiocarbonate 3-benzylsulfanylthiocarbonyl sulfanylpropionic acid (BSPA). The hyperbranched polyethylene with thiocarbonyl thio moiety ends (HBPE-BSPAs) is used as a macro-RAFT agent for the synthesis of hyperbranched polyethylene amphiphiles, HBPE-PDMAEMAs, by RAFT polymerization of N,N-dimethylaminoethyl methacrylate (DMAEMA). The resultant HBPE-PDMAEMAs can self-assemble to form supramolecular polymer vesicles in aqueous solution. A preliminary investigation on thermo- and pH-responsive behaviors of the polymer is also reported.

Thermo- and pH-sensitive polyethylene-based diblock and triblock copolymers: synthesis and self-assembly in aqueous solution

Well-defined polyethylene-block-poly(N-isopropylacrylamide) (PE-b-PNIPAM) and polyethylene-block-poly(N-isopropylacrylamide)-block-poly(2-vinylpyridine) (PE-b-PNIPAM-b-P2VP) were successfully synthesized by a combination of coordination chain transfer polymerization (CCTP) with reversible addition-fragmentation chain transfer (RAFT) polymerization in a “living”/controlled manner. Hydroxyl-terminated polyethylene (PE–OH) was firstly prepared by in situoxidation of polymer produced by CCTP with bis(imino)pyridine iron/MAO/ZnEt2 catalytic system. After an esterification of PE–OH with S-1-dodecyl-S′-(α,α′-dimethyl-α′′-acetate) trithiocarbonate, trithiocarbonate-terminated polyethylene (PE–trithiocarbonate) was obtained in high yield and used as a macromolecular chain transfer agent (macro-CTA) for RAFT polymerizations of NIPAM and 2VP. The results confirm that the tandem polymerization is an effective approach for synthesizing polyolefin-based amphiphiles. The amphiphilic block copolymers in aqueous solution can self-assemble into a nanodisk-like micelle containing a thin crystalline PE lamella domain between layers of the hydrophilic blocks. Dynamic light scattering (DLS) analyses demonstrated the thermo-responsive property of diblock copolymer PE-b-PNIPAM and double thermo- and pH-responsive properties of triblock copolymer PE-b-PNIPAM-b-P2VP.

Design of thermally stable amine-imine nickel catalyst precursors for living polymerization of ethylene: effect of ligand substituents on catalytic behavior and polymer properties.

Nickel complexes bearing amine-imine ligands with various backbone substituents were synthesized and employed as ethylene polymerization catalysts on activation with Et2 AlCl. The substituent on the backbone carbon atom of the amine moiety is decisive for the living nature of ethylene polymerization. A bulky amine-imine nickel precursor with a tert-butyl group on the carbon atom of the amine group can polymerize ethylene in a living fashion at an elevated temperature of 65 °C, which is the highest temperature of living polymerization of ethylene with late transition-metal catalysts. The wide applicable temperature range for living polymerization and sensitivity of the branch structure of the polyethylene to temperature enable precise synthesis of di- and triblock polyethylenes featuring different branched segments by sequential tuning of the polymerization temperature.

Living/controlled polymerization of 4-methyl-1-pentene with α-diimine nickel-diethylaluminium chloride: effect of alkylaluminium cocatalysts

4-Methyl-1-pentene (4MP) was polymerized with a classical α-diimine nickel complex [(2,6-(iPr)2C6H3)NC(acenaphthene)CN(2,6-(iPr)2C6H3))NiBr21] in the presence of various alkylaluminium compounds. Influences of cocatalysts on 4MP polymerization behavior were evaluated in detail. The different effects of trialkylaluminium cocatalysts between ethylene polymerization and 4-methyl-1-pentene polymerization were observed. Inexpensive diethylaluminium chloride (DEAC) compound could replace methylaluminoxane (MAO) as a more active cocatalyst for 4MP polymerization, and the influences of polymerization parameters including temperature and [Al]/[Ni] mole ratio were examined. At 0 °C, living/controlled polymerization of 4-methyl-1-pentene (4MP) was also achieved using inexpensive DEAC as cocatalyst, and trialkylaluminium compounds as chain transfer agents were closely relevant to achieve living/controlled polymerization. The obtained poly(4-methyl-1-pentene)s are amorphous elastomers with low glass transition temperature (Tg). Nuclear magnetic resonance (NMR) analyses showed that various branches such as methyl, isobutyl, long 2-methylalkyl branches are present in the polymer.

Synthesis and self-assembly of isotactic polystyrene-block-poly(ethylene glycol)

Isotactic polystyrene-block-poly(ethylene glycol) (iPS-b-PEG) was synthesized via a thiol–ene click coupling reaction of vinyl-terminated isotactic polystyrene (iPS–) with thiol-terminated poly(ethylene glycol) (PEG-SH). iPS– was prepared by the extremely highly isospecific polymerization of styrene with 1,4-dithiabutandiyl-2,2′-bis(6-tertbutyl-4-methylphenoxy) titanium dichloride and methylaluminoxane (MAO) in the presence of 1,7-octadiene as a chain transfer agent. PEG-SH was synthesized by the direct esterification of hydroxyl-terminated PEG (PEG-OH) with 3-mercaptopropionic acid using HfCl4·2THF as a catalyst. The behavior and micelle morphology of the crystallization-driven self-assembly of iPS-b-PEG in N,N-dimethylformamide (DMF) were investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), dynamic light scattering (DLS), differential scanning calorimetry (DSC) and X-ray diffraction (XRD). The results revealed that the crystallinity of the iPS blocks in the micelle cores was increased with increasing the storage time of the micelle solution at room temperature, and the increase of crystallinity led to the transition from spherical to petal-like micelles. With time these petal-like micelles could self-associate into flower-like aggregates. Moreover, when the newly prepared micelle DMF solution of iPS-b-PEG was dialyzed against deionized water, bowl-like micelles were formed due to the diffusion of DMF from the iPS cores into the aqueous phase.

Glass transition of polystyrene nanospheres under different confined environments in aqueous dispersions

Two aqueous dispersions of anionic and nonionic polystyrene (PS) latex nanospheres were prepared via activators generated by electron transfer (AGET) atom transfer radical polymerization (ATRP) and reverse ATRP in microemulsions using sodium dodecyl benzene sulfonate (SDBS) and polyoxyethylene (20) oleyl ether (Brij 98) as surfactant, respectively. Moreover, the third aqueous dispersion of surfactant-free PS nanospheres was obtained by the dialysis of a PS microemulsion against deionized water. The molecular weight distributions (Mw/Mn) of the resultant PS with controlled-molecular weight were less than 1.7. We investigated the glass transition of these PS nanospheres in different aqueous dispersions by nano-DSC. We observed an apparent size-dependent glass transition on the surfactant-free PS nanospheres and nonionic PS latex nanospheres, and that the glass transition temperature (Tg) decreases with the reduction of size for PS nanospheres. However, the anionic PS latex nanospheres show an unambiguous glass transition near to the bulk PS. These results suggest that the effects of size and interface on glass transition may become significant as the diameter of the polymer nanospheres is decreased to less than 100 nm.

The relationship between the degree of branching and glass transition temperature of branched polyethylene: experiment and simulation

Polyethylene (PE) samples with similar number-averaged molecular weight but various degrees of branching (DBs) were synthesized by means of ethylene coordination polymerization catalyzed by an amine–imine nickel catalyst or α-diimine palladium catalyst, respectively. Different from the conventional DB of branched PE, here we calculated the DB according to the equation proposed by Hawker et al., i.e., DB is the ratio of the number of dendritic units (–CHCH2–) and terminal units (–CH2CH3) to the number of all units (dendritic, terminal and linear units (–CH2CH2–)) in the PE backbone and branches. The glass transition temperature (Tg) of the prepared PE samples was studied by dynamic mechanical analysis (DMA), differential scanning calorimetry (DSC) and ultra-fast differential scanning calorimetry (UF-DSC). The DSC data show that the crystallization and the glass transition processes of branched PE samples are correlated with each other, and both the melting temperature and the glass transition temperature decreased with increasing DB. The UF-DSC results show that the crystallization of the PE samples with high DB can be prevented when the cooling rate in calorimetry is high enough. For example, the crystallization peak of branched PE with DB = 0.310 totally disappears when the cooling rate in calorimetry reaches more than 5000 K s−1 and only a glass transition takes place at −51.3 °C. Furthermore, correlation between the glass transition temperature and the DB of branched PE has been well established by atomistic molecular dynamics (MD) simulations. The values of MD-determined Tg are in good agreement with experimental results. The difference of free volume in PE systems with different DBs, which is reflected by the calculated radial distribution function, is the reason for the observed change of Tg on DB.

Synthesis of well-defined amphiphilic branched polyethylene-graft-poly (N-isopropylacrylamide) copolymers by coordination copolymerization in tandem with RAFT polymerization and their self-assembled vesicles

A tandem synthetic strategy combining the metal catalyzed copolymerization of ethylene with trimethylsilyl-protected 10-undecen-1-ol and RAFT polymerization was successfully used to prepare well-defined branched polyethylene-graft-poly(N-isopropylacrylamide) copolymers (BPE-g-PNIPAM). The branched PEs containing multiple hydroxyls (BPE–(OH)n) were firstly synthesized by the copolymerization of ethylene with trimethylsilyl-protected 10-undecen-1-ol using a pyridine–amine nickel catalyst in a controlled fashion (Mw/Mn ∼ 1.2). Macro-chain transfer agents (macro-CTAs) for a subsequent polymerization of N-isopropylacrylamide were quantitatively obtained by an esterification reaction of BPE–(OH)n with S-1-dodecyl-S′-(α,α′-dimethyl-α′′-acetate) trithiocarbonate. Chain extensions by a RAFT polymerization were successfully achieved to afford BPE-g-PNIPAM graft copolymers with narrow polydispersities of ∼1.2. Investigations of the self-assembly behavior of the obtained BPE-g-PNIPAM graft copolymers in water by means of TEM, AFM and laser light scattering confirm that the amphiphilic graft copolymers form supramolecular vesicles with average diameters of 170–190 nm in aqueous solution. Dynamic light scattering (DLS) measurements demonstrate that the polymer vesicles are thermo-sensitive and show a distinctive two-stage response from 20 to 32 °C.

Synthesis of amphiphilic copolymers with a dendritic polyethylene core and poly(ethylene oxide) arms and their self-assembled nanostructures

A tandem synthetic strategy combining chain walking polymerization and azide–alkyne click chemistry was successfully used to prepare amphiphilic copolymers with a dendritic polyethylene core and poly(ethylene oxide) (PEO) arms. Dendritic polyethylene (DPE) tethered with multi-reactive hydroxyl groups (DPE-(OH)11) was synthesized by copolymerization of ethylene and trimethylsilyl-protected 2-hydroxyethyl acrylate using an α-diimine palladium catalyst under low ethylene pressure of 0.1 atm. Alkynyl-terminated DPE (DPE-()11) was quantitatively obtained by esterification reaction of DPE-(OH)11 with pentynoic acid. Using a “grafting to” technique, PEO precursors with a terminated azido group were introduced to the DPE core with high efficiency by azide–alkyne click coupling reaction. The self-assembly behaviors of the obtained core–shell amphiphiles in selective solvents including water and hexane were studied by TEM, AFM and fluorescence. Supramolecular vesicles, unimolecular micelles, and multi-molecular micelles were observed, which is dependent on selective solvent as well as number and chain length of PEO arm.