Preparation of oligoamide‐ended poly(ethylene glycol) and hydrogen‐bonding‐assisted formation of aggregates and nanoscale fibers

An oligoamide‐ended poly(ethylene glycol) (PEG) with a PEG weight‐average molecular weight of 5000 (PEG‐5000‐oligoamide), with 3,5‐bis‐[2‐(5‐acetylamino‐2‐isobutoxy‐benzoylamino)‐acetylamino]‐benzoyl as the oligoamide, was synthesized. PEG‐5000‐oligoamide aggregated in chloroform or toluene via hydrogen‐bonding interactions among the oligoamide strands as a core and PEG, which was soluble in the solvents, as a shell. When a chloroform solution of PEG‐5000‐oligoamide at a concentration of approximately 0.06 g/L was cast onto a silicon wafer or a mica plate, rapid solvent evaporation induced its self reassembly as nanofibers. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 1119–1128, 2005     

Surface modification of ultrahigh‐molecular‐weight polyethylene fibers

To prevent the loss of fiber strength, ultrahigh‐molecular‐weight polyethylene (UHMWPE) fibers were treated with an ultraviolet radiation technique combined with a corona‐discharge treatment. The physical and chemical changes in the fiber surface were examined with scanning electron microscopy and Fourier transform infrared/attenuated total reflectance. The gel contents of the fibers were measured by a standard device. The mechanical properties of the treated fibers and the interfacial adhesion properties of UHMWPE‐fiber‐reinforced vinyl ester resin composites were investigated with tensile testing. After 20 min or so of ultraviolet radiation based on 6‐kW corona treatment, the T‐peel strength of the treated UHMWPE‐fiber composite was one to two times greater than that of the as‐received UHMWPE‐fiber composite, whereas the tensile strength of the treated UHMWPE fibers was still up to 3.5 GPa. The integrated mechanical properties of the treated UHMWPE fibers were also optimum. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 463–472, 2004     

Synthesis and mechanical properties of para‐aramid nanofibers

Scalable, bottom‐up chemical synthesis and electrospinning of novel Cl‐substituted poly(para‐phenylene terephthalamide) (PPTA) nanofibers are herein reported. To achieve Cl‐PPTA nanofibers, the chemical reaction between the monomers was precisely controlled, and dissolution of the polymer into solvent was tailored to enable anisotropic solution formation and sufficient entanglement molecular weight. Electrospinning processing parameters were studied to understand their effects on fiber formation and mat morphology and then optimized to yield consistently high quality fibers. Importantly, the control of relative humidity during the fiber formation process was found to be critical, likely because water promotes hydrogen bond formation between the PPTA chains. The fiber and mat morphologies resulting from different combinations of chemistry and spinning conditions were observed using scanning electron microscopy, and observations were used as inputs to the optimization process. Tensile properties of single Cl‐PPTA nanofibers were characterized for the first time using a nanomanipulator mounted inside a scanning electron microscope (SEM), and fiber moduli measuring up to 70 GPa, and strengths exceeding 1 GPa were achieved. Given the excellent mechanical properties measured for the nanofibers, this chemical synthesis procedure and electrospinning protocol appear to be a promising route for producing a new class of nanofibers with ultrahigh strength and stiffness. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 563–573     

High‐efficiency poly(phenylenevinylene)‐co‐fluorene copolymers incorporating a triphenylamine as the end group for white‐light‐emitting diode applications

We have synthesized a series of PPV copolymers and investigated the relationships between their chemical structures, thermal stabilities, and optoelectronic properties. Initially, we monitored the effects of the end group and the molecular weight on the behavior of polymer light emitting diodes (PLEDs) prepared from a series of oligo(phenylenevinylene) (OPV)‐fluorene luminescent copolymers. We found that the nature of the end‐capping group was an important factor affecting the luminescence properties of devices, which were improved especially when applying a triphenylamine end group. The molecular weight mainly affected the film quality of the manufactured device; that is, it improved to a certain degree when the molecular weight increased. Based on the luminescence conversion mechanism, we used a simple spin‐coating process to fabricate a high‐efficiency LED displaying white‐light CIE 1931 (coordinates of 0.31, 0.34). The device exhibited a high current‐efficiency of 3.5 cd/A and a brightness of 973 cd/m². © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 4504–4513, 2007     

RAFT synthesis of a water‐soluble triblock copolymer of poly(styrenesulfonate)‐b‐poly(ethylene glycol)‐b‐poly(styrenesulfonate) using a macromolecular chain transfer agent in aqueous solution

Triblock copolymers of poly(styrenesulfonate)‐b‐poly(ethylene glycol)‐b‐poly(styrenesulfonate) with narrow molecular weight distribution (M_w/M_n = 1.28–1.40) and well‐defined structure have been synthesized in aqueous solution at 70 °C via reversible addition‐fragmentation chain transfer polymerization. Poly(ethylene glycol) (PEG) capped with 4‐cyanopentanoic acid dithiobenzoate end groups was used as the macro chain transfer agent (PEG macro‐CTA) for sole monomer sodium 4‐styrenesulfonate. The reaction was controllable and displayed living polymerization characteristics and the triblock copolymer had designed molecular weight. The reaction rate depended strongly on the CTA and initiator concentration ratio [CTA]₀/[ACPA]₀: an increase in [CTA]₀/[ACPA]₀ from 1.0 to 5.0 slowed down the polymerization rate and improved the molecular weight distribution with a prolonged induction time. The polymerization proceeded, following first‐order kinetics when [CTA]₀/[ACPA]₀ = 2.5 and 5.0. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 3698–3706, 2007     

Synthesis and opto‐electrical properties of dendron‐containing poly(2,3‐diphenyl‐1,4‐phenylenevinylene) derivatives

A new series of poly(2,3‐diphenyl‐1,4‐phenylenevinylene) derivatives containing dendritic side groups were synthesized. Different generations of dendrons were integrated on the pendant phenyl ring to investigate their effect on optical and electrical properties of final polymers. Homopolymers can not be obtained via the Gilch polymerization because of sterically bulky dendrons. By controlling the feed ratio of different monomers during polymerization, dendron‐containing copolymers with high molecular weights were obtained. The UV–vis absorption and photoluminescent spectra of the thin films are pretty close; however, quantum efficiency is significantly enhanced with increasing the generation of dendrons. The electrochemical analysis reveals that hole‐injection is also improved by increasing dendritic generation. Double‐layer light‐emitting devices with the configuration of ITO/PEDOT:PSS/polymer/Ca/Al were fabricated. High generation dendrons bring benefit of improved device performance. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 3440–3450, 2007     

Dependence of the luminescent properties and chain length of poly[2‐methoxy‐5‐(2′‐ethyl‐hexyloxy)‐1,4‐phenylene vinylene] on the formation of cis‐vinylene bonds during Gilch polymerization

The presence of cis‐vinylene bonds in Gilch‐polymerized poly[2‐methoxy‐5‐(2′‐ethyl‐hexyloxy)‐1,4‐phenylene vinylene] is reported. Through fractionation, species with a weight‐average molecular weight of less than 37,000 exhibited an abnormal blueshift of photoluminescence spectra in toluene solutions, and this was attributed to the presence of cis‐vinylene bonds, as verified by NMR spectroscopy. Surprisingly, the fractionated species (～1 wt %) with a weight‐average molecular weight of 5000 were mostly linked by the cis‐vinylene bonds. The concentration decreased with the molecular weight until a molecular weight of 37,000 was reached; at that point, the polymer chains contained mainly trans‐vinylene bonds. Obviously, the formation of cis‐vinylene bonds strongly inhibited the growth of polymer chains during Gilch polymerization. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 2520–2526, 2005     

Ultrahigh‐molecular‐weight‐polyethylene‐fiber surface treatment by electron‐beam‐irradiation‐induced graft polymerization and its effect on adhesion in a styrene–butadiene rubber matrix

Aiming to develop a high‐performance fiber‐reinforced rubber from styrene–butadiene rubber (SBR), we applied a special technique using electron‐beam (EB)‐irradiation‐induced graft polymerization to ultrahigh‐molecular‐weight‐polyethylene (UHMWPE) fibers. The molecular interaction between the grafted UHMWPE fibers and an SBR matrix was studied through the evaluation of the adhesive behavior of the fibers in the SBR matrix. Although UHMWPE was chemically inert, two monomers, styrene and N‐vinyl formamide (NVF), were examined for graft polymerization onto the UHMWPE fiber surface. Styrene was not effective, but NVF was graft‐polymerized onto the UHMWPE fibers with this special method. A methanol/water mixture and dioxane were used as solvents for NVF, and the effects of the solvents on the grafting percentage of NVF were also examined. The methanol/water mixture was more effective. A grafting percentage of 16.4% was the highest obtained. This improved the adhesive force threefold with respect to that of untreated UHMWPE fibers. These results demonstrated that EB irradiation enabled graft polymerization to occur even on the inert surface of UHMWPE fibers. However, the mechanical properties of the fibers could be compromised according to the dose of EB irradiation. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2595–2603, 2004     

Volume‐exclusion effects in polyethylene blends filled with carbon black,graphite, or carbon fiber

Conductive polymer composites possessing a low percolation‐threshold concentration as a result of double percolation of a conductive filler and its host phase in an immiscible polymer blend afford a desirable alternative to conventional composites. In this work, blends of high‐density polyethylene (HDPE) and ultrahigh molecular weight polyethylene (UHMWPE) were used to produce ternary composites containing either carbon black (CB), graphite (G), or carbon fiber (CF). Blend composition had a synergistic effect on electrical conductivity, with pronounced conductivity maxima observed at about 70–80 wt % UHMWPE in the CB and G composites. A much broader maximum occurred at about 25 wt % UHMWPE in composites prepared with CF. Optical and electron microscopies were used to ascertain the extent to which the polymers, and hence filler particles, are segregated. Differential scanning calorimetry of the composites confirmed that the constituent polymers are indistinguishable in terms of their thermal signatures and virtually unaffected by the presence of any of the fillers examined here. Dynamic mechanical analysis revealed that CF imparts the greatest stiffness and thermal stability to the composites. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1013–1023, 2002     

A kinetic study of metallocene‐catalyzed ethylene polymerization using different aluminoxane cocatalysts

The kinetics of ethylene polymerization using homogeneous Cp₂ZrCl₂/aluminoxane catalysts in toluene has been investigated at 70 °C with an ethylene pressure of 30 psi. Four aluminoxanes were used: methylaluminoxane, modified methylaluminoxanes with a fraction of methyl groups substituted with isobutyl (MMAO‐4) or octyl (MMAO‐12) groups, and polymethylaluminoxane (PMAO‐IP). The cocatalyst‐to‐catalyst ratio, [Al]/[Zr], varied from 1000 to 10,000. The experimental results obtained using the four cocatalysts were compared and a model was proposed to fit the rate of polymerization as a function of polymerization time and [Al]/[Zr] ratio. Molecular weight distributions with polydispersities between three and four indicate the presence of more than one active site type. We proposed a model that explained these broad molecular weight distributions using an unstable active complex that is formed in the early stages of the reaction and is transformed over time to a more stable active complex via an intermediate. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 1677–1690, 2007     

High‐molecular‐weight AB‐type benzoxazines as new precursors for high‐performance thermosets

Novel high‐molecular‐weight polybenzoxazine precursors, namely AB‐type benzoxazine precursors, were synthesized from aminophenols and formaldehyde. Both ¹H NMR and IR confirmed the structure of the precursors, indicating the presence of a cyclic benzoxazine structure in the backbone of the precursors. The weight‐average molecular weight was estimated by size exclusion chromatography to be to in the range of 1300–4500. The precursors gave self‐standing thin films when their solutions were cast in dioxane over glass plates and dried, and upon a gradual thermal cure up to 250 °C, they afforded polybenzoxazine films. The viscoelastic analyses showed that the glass transition temperatures of the polybenzoxazine films obtained from these novel precursors were as high as 260–300 °C. Thermogravimetric analysis results indicated that the onset of decomposition and the char yield of the thermosets derived from these AB‐type precursors were higher than those of traditional polybenzoxazine. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 1878–1888, 2007.     

γ‐Irradiation of ultrahigh‐molecular‐weight polyethylene: Electron paramagnetic resonance and nuclear magnetic resonance spectroscopy and imaging studies of the mechanism of subsurface oxidation

The shelf aging of irradiated ultrahigh‐molecular‐weight polyethylene (UHMWPE) causes subsurface oxidation, which leads to failure in UHMWPE orthopedic components, yet the mechanisms causing subsurface oxidation remain unclear. The shelf aging of γ‐irradiated UHMWPE bars has been studied with electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) imaging and with microtoming and Fourier transform infrared microscopy. The bars initially contained only allyl radicals, and upon air exposure, a surface layer of peroxyl radicals formed through the reaction of allyl radicals with oxygen. Importantly, a band of low radical intensity just beneath the peroxyl layer became apparent. NMR imaging showed a zone of altered proton relaxation in this zone. With increasing time, surface peroxyl radicals persisted in comparison with the interior allyl radicals, although oxygen did not appear to penetrate any more deeply into the bar. The area of maximal oxidation and mechanical disruption, measured after 3 years, was at the interface between the zone of exterior peroxyl radicals and the zone of low radical intensity. We present a mechanism involving the intermediacy of sterically strained reactive dialkyl peroxides at this interface to explain subsurface oxidation. We also demonstrate that EPR and NMR imaging provides information that could potentially be used to identify subsurface oxidized UHMWPE components before failure. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5929–5941, 2004     

Nonisothermal crystallization and melting behavior of a luminescent conjugated polymer,poly(9,9‐dihexylfluorene‐alt‐co‐2,5‐didecyloxy‐1,4‐phenylene)

The nonisothermal crystallization kinetics of a luminescent conjugated polymer, poly(9,9‐dihexylfluorene‐alt‐co‐2,5‐didecyloxy‐1,4‐phenylene) (PF6OC10) with three different molecular weights was investigated by differential scanning calorimetry under different cooling rates from the melt. With increasing molecular weight of PF6OC10, the temperature range of crystallization peak steadily became narrower and shifted to higher temperature region and the crystallization rate increased. It was found that the Ozawa method failed to describe the nonisothermal crystallization behavior of PF6OC10. Although the Avrami method did not effectively describe the nonisothermal crystallization kinetics of PF6OC10 for overall process, it was valid for describing the early stage of crystallization with an Avrami exponent n of about 3. The combined method proposed in our previous report was able to satisfactorily describe the nonisothermal crystallization behavior of PF6OC10. The crystallization activation energies determined by Kissinger, Takhor, and Augis‐Bennett models were comparable. The melting temperature of PF6OC10 increased with increasing molecular weight. For low‐molecular‐weight sample, PF6OC10 showed the characteristic of double melting phenomenon. The interval between the two melting peaks decreased with increasing molecular weight, and only one melting peak was observed for the high‐molecular‐weight sample. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 976–987, 2007     

Characteristics of several kinds of polyethylene gel estimated by small‐angle light scattering under cross polarization

Unusual phenomenon was confirmed in the gelation of polymer solution when branched low molecular weight polyethylene (B‐LMWPE) and ultrahigh molecular weight polyethylene (UHMWPE) solutions were quenched at their gelation temperature. That is, polarized light scattering (Hv scattering) from B‐LMWPE gels containing 95% solvent yielded a four‐leaf clover type as like the scattering from a perfect spherulite under Hv scattering but the corresponding polarized optical microscopy (POM) showed dark image showing no spherulite with Maltese cross color indicating considerable orientation fluctuation between the optical axes with respect to the radial axis of the spherulite. Hv scattering from pristine UHMWPE gels containing more than 99% solvent had an X‐type pattern, which became clearer with time. The corresponding POM images change from being dark, indicating no superstructure, to being slightly brighter, indicating the presence of indistinct superstructures. To analyze this unusual phenomenon of Hv scattering from B‐LMWPE and UHMWPE gels, new models were proposed using a statistical approach and optically anisotropic elements in three‐dimensional space. The theoretical patterns were in agreement with observation. Thus, it came to a conclusion that Hv scattering from the gels is attributed to strong distance correlation between polar and rotational angles of two optically anisotropic elements in the polymer‐rich phase. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2010     

Rapid additive‐free TEMPO‐mediated stable free radical polymerizations of styrene

The stable free radical polymerization (SFRP) of styrene, initiated with benzoyl peroxide in the presence of TEMPO, under bulk conditions, is demonstrated to proceed rapidly without the need for any rate enhancing additives such as camphorsulfonic acid, 2‐fluoro‐1‐methyl pyridinium p‐toluenesulfonate, or acetic anhydride. Monomer conversions as high as 70% can be achieved in 5 h or less while maintaining polydispersity indexes of 1.15. These results stand in stark contrast to earlier reactions that required 70 h to achieve similar conversions. This study demonstrates that the single largest factor governing the rates of polymerization is the molar concentration of excess TEMPO remaining in solution after initiation. A reduction in the TEMPO to BPO ratio is required when large amounts of BPO are used to target low molecular weight polystyrenes. However, when a lower molar amount of BPO is used to obtain high molecular weight polystyrenes, a higher TEMPO to BPO ratio is required. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5487–5493, 2007     

End group effect on the thermo‐sensitive properties of well‐defined water‐soluble polystyrenics with short pendant oligo(ethylene glycol) groups synthesized by nitroxide‐mediated radical polymerization

The effects of hydrophobic chain end groups on the cloud points of thermo‐sensitive water‐soluble polystyrenics were investigated. Well‐defined poly (4‐vinylbenzyl methoxytris(oxyethylene) ether) (PTEGSt) and poly(α‐hydro‐ω‐(4‐vinylbenzyl)tetrakis(oxyethylene)) (PHTrEGSt) were prepared by nitroxide‐mediated radical polymerization using α‐hydrido alkoxyamine initiators including two monomer‐based initiators. The polymers were reduced with (n‐Bu)₃SnH to replace the alkoxyamine end group with hydrogen. In the studied molecular weight range (M_n,GPC = 3000 to 28,000 g/mol), we found that the hydrophobic end groups decreased the cloud point by 1–20 °C depending on the molecular weight and the largest depression was observed at the lowest molar mass. The cloud points of PTEGSt and PHTrEGSt with two hydrophobic end groups, phenylethyl and alkoxyamine, exhibited a monotonic increase with the increase of molecular weight. For polymers with only one hydrophobic end group, either phenylethyl or alkoxyamine, the cloud point initially increased with the increase of molecular weight but leveled off/decreased slightly with further increasing molar mass. For polymers with essentially no end groups, the cloud point decreased with the increase of chain length, which represents the “true” molecular weight dependence of the cloud point. The observed molecular weight dependences of the cloud points of polystyrenics with hydrophobic end group(s) are believed to result from the combined end group effect and “true” molecular weight effect. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 3707–3721, 2007     

Round‐robin experiment in high‐temperature gel permeation chromatography

The results of an interlaboratory or round‐robin experiment in high‐temperature gel permeation chromatography (HT‐GPC) analysis are presented. The intention was to determine and raise awareness of interlaboratory reproducibility of HT‐GPC techniques. Fifteen laboratories performed analyses of five polyethylene samples and standards SRM 1475 and 1476. Reproducibility, as measured by the interlaboratory standard deviation (s_LAB), was greatly influenced by the breadth of the molecular weight distribution (MWD) and branching. The s_LAB values for the weight‐average molecular weight (M_w) of linear polyethylenes of narrow and broad MWDs were 4 and 14%, respectively. For branched polymers, GPC viscometry methods are shown to measure significantly higher molecular weights than the noncoupled GPC method, with higher variance. For branched polyethylenes measured with GPC viscometry, the reproducibility of M_w was characterized by s_LAB = 18%. Reproducibility of the SRM 1475 standard was better than for unknowns. The results for branched standard SRM 1476 emphasize the important role of the detection method in GPC but call into question the use of this material as a molecular weight standard. For single‐site polyethylene, only a handful of labs measured an MWD that closely matched the Flory distribution. Qualitatively, the responses indicate that many variations in instrument and analytical methods exist among laboratories; this is partly a reflection of the development and refinements that this technique must yet undergo before a truly standard method is widely accepted and practiced. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 905–921, 2002     

Isomerization polymerization and copolymerization of glycidyl alkanoates catalyzed by methylaluminum bis(2,6‐di‐t‐butyl‐4‐methylphenoxide)

The isomerization polymerizations of glycidyl propionate (1b), octanoate (1c), and stearate (1d) with methylaluminum bis(2,6‐di‐tert‐butyl‐4‐methylphenoxide) (3) were investigated. The polymerizations selectively gave poly(2‐alkyl‐1,3‐dioxolane‐4,2‐diyloxymethylene)s (2), although the polymer yield as well as the polymer molecular weight significantly decreased as the acyl chain of 1 was lengthened. These polymers readily hydrolyzed to glycerin and the corresponding fatty acids under mild conditions. The copolymerizations of glycidyl acetate (1a) with these monomers were also examined. In any combination, the composition of the obtained copolymer was essentially identical with the feed ratio, while both copolymer yield and molecular weight decreased as the feed of 1a was decreased. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 435–444, 1999 (See graphics.)     

Effects of molecular characteristics and processing conditions on melt‐drawing behavior of ultrahigh molecular weight polyethylene

The effects of molecular characteristics and processing conditions on melt‐drawing behavior of ultrahigh molecular weight polyethylene (UHMW‐PE) are discussed, based on a combination of in situ X‐ray measurement and stress–strain behavior. The sample films of metallocene‐ and Ziegler‐catalyzed UHMW‐PEs with a similar viscosity average MW of ～10⁷ were prepared by compression molding at 180 °C. Stress profiles recorded at 160 °C above the melting temperature of 135 °C exhibited a plateau stress region for both films. The relative change in the intensities of the amorphous scattering recorded on the equator and on the meridian indicated the orientation of amorphous chains along the draw axis with increasing strain. However, there was a substantial difference in the subsequent crystallization into the hexagonal phase, reflecting the molecular characteristics, that is, MW distribution of each sample film. Rapid crystallization into the hexagonal phase occurred at the beginning point of the plateau stress region in melt‐drawing for metallocene‐catalyzed UHMW‐PE film. In contrast, gradual crystallization into the hexagonal phase occurred at the middle point of the plateau stress region for the Ziegler‐catalyzed film, suggesting an ease of chain slippage during drawing. These results demonstrate that the difference in the MW distribution due to the polymerization catalyst system dominates the phase development mechanism during melt‐drawing. The effect of the processing conditions, that is, the including strain rate and drawing temperature, on the melt‐drawing behavior is also discussed. The obtained results indicate that the traditional temperature–strain rate relationship is effective for transient crystallization in to the hexagonal phase during melt‐drawing, as well as for typically oriented crystallization during ultradrawing in the solid state. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2455–2467, 2006     

Synthesis of long‐chain‐branched polyethylene by ethylene homopolymerization with a novel nickel(II) α‐diimine catalyst

Long‐chain‐branched polyethylene with a broad or bimodal molecular weight distribution was synthesized by ethylene homopolymerization via a novel nickel(II) α‐diimine complex of 2,3‐bis(2‐phenylphenyl)butane diimine nickel dibromide ({[2‐C₆H₄(C₆H₅)]? N?C? (CH₃)C(CH₃)?N? [2‐C₆H₄(C₆H₅)]}NiBr₂) that possessed two stereoisomers in the presence of modified methylaluminoxane. The influences of the polymerization conditions, including the temperature and Al/Ni molar ratio, on the catalytic activity, molecular weight and molecular weight distribution, degree of branching, and branch length of polyethylene, were investigated. The resultant products were confirmed by gel permeation chromatography, gas chromatography/mass spectrometry, and ¹³C NMR characterization to be composed of higher molecular weight polyethylene with only isolated long‐branched chains (longer than six carbons) or with methyl pendant groups and oligomers of linear α‐olefins. The long‐chain‐branched polyethylene was formed mainly through the copolymerization of ethylene growing chains and macromonomers of α‐olefins. The presence of methyl pendant groups in the polyethylene main chain implied a 2,1‐insertion of the macromonomers into [Ni]? H active species. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 1325–1330, 2005