Chinese Journal of Polymer Science

Precise, efficient copolymerizations of ethylene with cyclic olefins [norbornene (NBE), cyclopentene (CPE)] using nonbridged half-titanocenes of type, Cp'TiCl₂(L) (Cp' = cyclopentadienyl group, L = aryloxo, ketimide)-MAO catalyst systems have been summarized. CpTiCl₂(N = C^tBu₂) exhibited both remarkable catalytic activity and efficient NBE incorporation for ethylene/NBE copolymerization: The NBE incorporation by Cp'TiCl₂(X)X = N = C^tBu₂(X)(X = N = C^tBu₂, O-2,6-ⁱPr₂C₆H₃; Cp'=Cp, C₅Me₅, indenyl was related to the calculated coordination energy after ethylene insertion. (^tBuC₅H₄)TiCl₂(N=C^tBu) exhibited significant catalytic activities with efficient CPE incorporations in ethylene/CPE copolymerization in the presence of MAO. The polymerization proceeded with exclusive 1,2-CPE incorporation, affording high molecular weight (alternating) copolymers with uniform distributions. Therefore, the cyclopentadienyl fragment (Cp') plays an essential role in terms of both the activity and the efficient cyclic olefin incorporation.

Norbornene derivatives exo,endo-2-[2-(3,5-di-tert-butyl-4-hydroxyphenoxy)-acetoxy]methyl-5-norbornene (M1) and 3,3,5,5- etramethyl-4-piperidinyl 5-norbornene-exo,endo-2-carboxylate (M2) were synthesized and polymerized by RuCl₂ (=CHPh)(PCy₃)₂ to prepare a novel kind of bi-functional polymer bearing sterically hindered phenol (SHP) and hindered amine (HLAS) groups via ring-opening metathesis polymerization (ROMP). The resulting copolymers were characterized by gel permeation chromatography (GPC), ¹H-NMR and differential scanning calorimetry (DSC). The number average molecular weight (M_n) and functional unit content of the resulting copolymer could be regulated by varying the concentration of catalyst and monomer feed.

The methyl methacrylate (MMA)/1-propylmethacrylate-polyhedral oligomeric silsesquioxane (PM-POSS) copolymers were synthesized via atom transfer radical polymerization with CuBr as catalyst. The unreacted PM-POSS monomer could be removed completely by washing the copolymerization product with n-hexane. The copolymers were characterized with ¹H-NMR, X-ray diffraction, differential scanning calorimetry, thermogravimetric analysis and gel permeation chromatography. With increasing PM-POSS feed ratio, the total conversion increased while the glass transition temperatures of copolymer decreased. The thermogravimetric analysis demonstrated that the thermal stability of copolymer improved slightly with PM-POSS addition. The molecular weight of copolymers increased with incorporation of PM-POSS.

The N,N'-(phenyl-2-pyridinylmethylene)-3,3',5,5'-tetramethylbenzidine and its dimeric Co(II) and Ni(II) complexes were synthesized. The organic compound was characterized by elemental analyses, IR and NMR spectra, while the bimetal complexes were determined by elemental analyses, IR spectra as well as the single-crystal X-ray diffraction. The nickel complex showed high activity for ethylene polymerization and its cobalt analogue showed negligible active in ethylene activation.

The effects of poisoning materials on catalytic activity and isospecificity of the supported Ziegler-Natta catalyst were investigated. A minor amount of simple structure of Lewis base, i.e., methanol, acetone, ethyl acetate, was introduced into the catalyst slurry for partial poisoning catalytic active centers. It was found that the variations in deactivation power were in the order of methanol > acetone > ethyl acetate. The kinetic investigation via stopped-flow polymerization showed that poisoning compounds caused a decrease in activity through the reduction of the number of active sites whereas no effect on the degree of isotacticity was observed.

The effect of chemical composition of highly active supported Ziegler-Natta catalysts with controlled morphology on the MWD of PE has been studied. It was shown the variation of transition metal compound in the MgCl₂-supported catalyst affect of MWD of PE produced in broad range: Vanadium-magnesium catalyst (VMC) produce PE with broad and bimodal MWD (M_w/M_n = 14–21). MWD of PE, produced over titanium-magnesium catalyst (TMC) is narrow or medium depending on Ti content in the catalyst (M_w/M_n = 3.1–4.8). The oxidation state of the initial titanium compounds in TMC has only slight effect on MWD of PE produced. Based on MWD data of PE heterogeneity of active centers of TMC and VMC was studied. The results of resolution of experimental GPC curves into Flory components indicated three Flory components are sufficient to describe MWD curve of PE, produced with TMC; six Flory components are required in the case of VMC. In the case of copolymerization of ethylene with 1-hexene over TMC the addition of 1-hexene leads to decrease of MW and to slight effect on M_w/M_n values. On the contrary the strong effect of 1-hexene on MWD of PE produced over VMC was found: the introduction of 1-hexene results in considerable broadening of MWD due to the shifting of the main MWD peak to low MW region. At that comonomer doesn't affect the position of high molecular weight shoulder. The results indicate that some of active centers of VMC producing high MW polymer are not active in the reaction of chain transfer with comonomer.

The catalytic system of [2-(2-benzimidazolyl)-6-((1-aryliminoethyl)pyridyl)]nickel chloride/MAO (methylaluminoxane) was found to be good active for vinyl polymerization of norbornene and provided polymers with relative narrow molecular distributions. Various reaction parameters, such as the ratios of nickel precursor to MAO or monomer norbornene, and the nature of the ligands in complexes were carefully investigated to realize their effects on the catalytic activities, polymer molecular weight and molecular weight distributions.

A series of new nickel(II) complexes with 2-aminomethylpyridine ligands, (2-PyCH₂NHAr)₂NiBr₂ (Ar = 2,6-dimethylphenyl 2a; 2,6-diisopropylphenyl 2b, 2,6-difluorophenyl 2c), have been synthesized and used as catalyst precursors for ethylene polymerization in the presence of methylaluminoxane (MAO). The catalysts containing ortho-alkyl-substituents afford high molecular weight branched polyethylenes as well as a certain amount of oligomers. Enhancing the steric bulk of the alkyl substituent of the catalyst resulted in higher ratio of solid polymer to oligomer and higher molecular weight of the polymer. Catalyst 2c containing ortho-fluoro-substituents exhibited the highest catalytic activity, but only oligomers in which C₁₂H₂₄ had the maximum content were obtained by the catalyst. The molecular weight, molecular weight distribution, and microstructure of the resulted polymer were characterized by gel permeation chromatography and ¹³C-NMR spectrogram.

[t-BuNSiMe₂(2,7-t-Bu₂Flu)]ZrMe₂ and [t-BuNSiMe₂(3,6-t-Bu₂Flu)]ZrMe₂ were synthesized, and the solid structure of complex [t-BuNSiMe₂(2,7-t-₂Flu)]ZrMe₂ was elucidated by single crystal X-ray analysis. These complexes were applied for propylene polymerization using dried modified methylaluminoxane as a cocatalyst at 0°C and 20°C in toluene. Both systems did not show activity at 0°C, but they conducted the polymerization at 20°C. The introduction of t-butyl substituents to the fluorenyl ligand improved the activity more than six times regardless of the position of the substituents, and both systems gave low molecular weight polypropylenes (PP) with narrow molecular weight distribution. The introduction of t-butyl substituents also improved the syndiospecificity and [t-BuNSiMe₂(3,6-t-Bu₂Flu)]ZrMe₂ gave highly syndiotactic PP with the syndiotactic pentad of 0.91 and the melting point of 145°C.

As one of the most important catalysts in polyethylene industry, Phillips catalyst (CrO_x/SiO₂) was quite unique for its activation by ethylene monomer without using any activator like alkyl-aluminium or MAO. In this work, the density functional theory (DFT) calculation combined with paired interacting orbitals (PIO) method was applied for the theoretical studies on coordination reaction mechanism between ethylene monomer and two model catalysts namely Cr(II)(OH)₂ (M1) and silsesquioxane-supported Cr(II) (M2) as surface Cr(II) active site precursors on Phillips catalyst at the early stage of ethylene polymerization. Unexpected multiplicity of the coordination states of ethylene monomer on both M1 and M2 model catalysts had been first reported on a molecular level. In general, increasing the coordination numbers of ethylene, the corresponding binding energy per ethylene for all the complexes was decreased. The supporting effect of chromium oxide onto silica gel surface was found to be destabilizing the corresponding complexes and decreasing the multiplicity of the coordination states as well due to both electronic and steric effect. Moreover, tri- and tetra- or higher ethylene coordination states could not be possibly formed on the supported catalyst as on the Cr(II)(OH)₂. The optimized complex geometries were adopted for determining the intermolecular orbital interactions. In-phase overlap orbital interaction for all the molecular complexes indicated favorable coordination between ethylene and Cr(II) sites. The molecular orbital origin of the π-bonded Cr(II), and mono- and di-C₂H₄ M1 complexes had been elucidated by PIO method showing high possibility of the formation of metallacyclopropane or metallacyclopentane active sites in the subsequent initiation of polymerization stage.

In this paper, crystallization kinetics of a series of ethylene-propylene copolymers prepared by living polymerization coordination catalyzed by a fluorinated bis(phenoxyimine) Ti catalyst (FI-EP copolymers) was studied, and was compared with that of ethylene-propylene copolymers prepared by a conventional Ziegler-Natta catalyst (ZN-EP copolymers). It is found that, the Avrami exponent and the crystallization rate constant of the FI-EP and ZN-EP copolymer show similar dependence on crystallization temperature, but the FI-EP copolymers exhibit a larger Avrami exponent than corresponding ZN-EP copolymers at high crystallization temperature and at low propylene content level. The crystallization temperature, equilibrium melting temperature and crystallinity of the FI-EP copolymers decrease more rapidly with propylene content than those of the ZN-EP copolymers. This can be attributed to their different comonomer distributions and partially to the different molecular weight distributions at low propylene content level.

i-PP/m-EPR reactor alloy were prepared through ethylene/propylene slurry copolymerization catalyzed by metallocene (rac-Et(Ind)₂ZrCl₂) supported on porous iPP particles. Polar monomer (dihydromyrcene alcohol) treated with triethyaluminum was added in the preparation of porous iPP particles to introduce hydroxyl groups and thus enhance the ability for chemically supporting the metallocene catalyst. The effects of MAO/Zr ratio and monomer composition in feed on the reaction activity and property of polymer were investigated. DSC analysis results imply that EPR prepared by metallocene supported PP particles showed much more obvious features of random copolymer compared with that prepared with conventional Z-N catalyst. Under appropriate reaction conditions, well separated spherical particles of iPP/m-EPR reactor alloy containing up to about 40 wt% m-EPR were obtained. SEM analysis results showed that m-EPR microphases smaller than 2 μm in size were uniformly distributed in PP matrix after blending the iPP/m-EPR reactor alloy.

Distribution of active centers (ACD) of ethylene or 1-hexene homopolymerization and ethylene-1-hexene copolymerization with a MgCl₂/TiCl₄ type Z-N catalyst were studied by deconvolution of the polymer molecular weight distribution into multiple Flory components. Each Flory component is thought to be formed by a certain type of active center. ACD of ethylene-1-hexene copolymer with very low 1-hexene incorporation was compared with that of ethylene homopolymer to see the effect of introducing α-olefin on ethylene polymerization. On the other hand, ACD of ethylene-1-hexene copolymer with very low ethylene incorporation was compared with that of 1-hexene homopolymer. Adding small amount of 1-hexene in ethylene polymerization caused marked activation of all the Flory components of the polymer, in which the low molecular weight components are activated more than the high molecular weight components. In 1-hexene polymerization system, the activity can also be greatly enhanced by introducing small amount of ethylene, but the different Flory components (or active centers) are activated with similar extent, except a newly emerged active center producing polymer with the lowest molecular weight. The total number of active centers is markedly increased by adding small amount of ethylene in 1-hexene polymerization, but the average catalysis efficiency of the active centers decreased. The broad composition distribution of the ethylene-1-hexene copolymer can be well understood from the ACD of catalyst and its dependence on the monomer.

Polymerization of isobutyl vinyl ether (IBVE) has been studied with mononuclear half-titanocene, CpTiCl₃ [1] and dinuclear half-titanocenes, [(C₅H₄)₂(CH)₂)_n][(TiCl₃)₂][2 (n = 3), 3 (n = 6)], and [(C₅H₄)₂(CH₂)_n][(TiCl₂OR)₂] (R = 2,6-diisopropylphenoxyl) [4 (n = 3), 5 (n = 6)], in the presence of methyl aluminoxanes (MAO) as cocatalyst in methylene chloride. The influences of the length of polymethylene brigde and the substitution of aryloxy group at the metal center have been investigated at three polymerization temperature (25°C, -10°C, and -30°C). It was observed that the structure of catalyst displayed significant effects on not only the polymerization behavior but also the properties of the produced polymers. At -30°C it was noted that polymerization catalyzed by dinuclear titanocenes indicated a living character where a linear relation between molecular weight of the polymers and monomer conversion. The catalytic activity decreased in the order of 1 > 2 > 4 > 3 > 5 whereas the stereoregularity of PIBVE, calculated by racemic dyad value, r, increased in the order of 1 > 2 > 3 > 4 > 5. The collected polymers have the high molecular weight (1 × 10⁵–3 × 10⁵) and narrow molecular weight distribution (PDI ≤ 2.3). Very interestingly the dinuclear half-titanocenes containing a different bridge and substituent generated the polymers containing a different level of syndiotacticity from 47% to 56% according to the employed catalyst, which clearly demonstrated the property of aryloxy substituent at the metal center played a key role to adjust polymer stereoregularity.

The ternary hybrid films consisting of chitosan (CS), polyethylene glycol (PEG) and nano-sized silica which was surface-modified by amino groups (RNSA) were prepared. The structures of the blend membranes were characterized by attenuation total reflection-infrared spectroscopy (ATR-IR), X-ray diffraction (XRD), optical microscopy (OM) and differential scanning calorimetry (DSC). The results showed that the addition of silica affected not only the distribution and crystallinity of PEG on the sample surface, but also the phase coarseness and the crystalline structure of chitosan in the blend system. Moreover, PEG changed the crystalline structure of chitosan. Upon annealing (at 100°C for 1 h), the blends would show the altered crystalline structure of chitosan, the reinforced phase coarseness, as well as the decreased miscibility and interaction between chitosan and PEG.

Environmentally friendly thermosetting composites were successfully prepared by conventional blending wheat gluten as matrix, glycerol as plasticizer and silica as filler followed by thermo-molding of the mixture at 120°C. The strong interfacial interaction between silica particles and gluten proteins leaded to an increase in storage modulus and a decrease in loss factor as revealed by dynamic mechanical analysis. The moisture absorption and elongation at break decrease while Young's modulus and tensile strength increase with increasing silica content from 0 to 10 wt%. However, the moisture absorption and mechanical properties show discontinuous changes at a silica content of 6 wt%. The glycerol content also has a marked influence on the moisture absorption and mechanical properties of the composites with a constant gluten-to-silica ratio.

The rheological behavior of poly(vinylidene fluoride) (PVDF) samples of different molecular weights was investigated by means of high pressure capillary rheometer and rotational rheometer. Information on the rheological properties of such materials above melt temperatures is of interest as this can lead to an improved understanding of polymer behavior in processing and fabrication technologies. Shift factors derived from time-temperature superposition showed good fit to the Arrhenius equation with a flow activation energy E of almost 74 kJ/mol, which was essentially independent of polymer molecular weight suggesting the unit of flow was a collective motion of chain segments rather than the whole chain itself.

A series of spherical activated carbons (SACs) with different pore structures were prepared from chloromethylated polydivinylbenzene by ZnCl₂ activation. The effects of activation temperature and retention time on the yield and textural properties of the resulting SACs were studied. All the SACs are generated with high yield of above 65% and exhibit relatively high mesopore fraction (me%) of 35.7%–43.6% compared with conventional activated carbons. The sample zlc28 prepared at 800°C for 2 h has the largest BET surface area of 891 m² g^-1 and pore volume of 0.489 cm³ g^-1. SEM and XRD analyses of zlc28 verify the presence of developed porous structure composed of disordered micrographite stacking with large amounts of interspaces in the order of nanometers.