Study on chain end structures of polypropylenes prepared with different symmetrical metallocene catalysts

Propylene homopolymerizations were conducted by using three kinds of metallocenes: Cp₂ZrCl₂, En(Ind)₂ZrCl₂ and iPr(Cp)(Flu)ZrCl₂, all of which were activated with methylaluminoxane. Detailed NMR analyses of the chain ends in the resulting polymers were carried out to discuss the chain end structures of the polypropylenes and the mechanism of polymerization. The characteristic of each metallocene for the mechanism of polymerization was also described.     

Propylene polymerisations with novel heterogeneous combination metallocene catalyst systems

Propylene was polymerised with novel combination metallocene catalyst systems prepared by an emulsion-based heterogenisation method in liquid monomer conditions. The catalyst combinations investigated were rac-dimethylsilanylbis(2-methyl-4-phenyl-1-indenyl)zirconium dichloride/rac-[ethylenebis(2-(tert-butyldimethylsiloxy)indenyl)]zirconium dichloride/methylaluminoxane (MAO) (1 + 2) and rac-dimethylsilanylbis(2-methyl-4-phenyl-1-indenyl)zirconium dichloride/rac-dimethylsilanylbis(2-isopropyl-4-[3,5-dimethylphenyl]indenyl)zirconium dichloride/MAO (1 + 3). The effects of polymerisation temperature and hydrogen on catalyst performance and polymer properties, as well as copolymerisation with hexene and ethylene were investigated. Depending on the polymerisation conditions, M_w of polypropylene varied from 144 to 286 kg/mol for 1 + 2 and from 200 to 390 kg/mol for 1 + 3. Combination 1 + 2 produced broader molecular weight distribution (MWD) than 1 + 3, and a bimodal MWD with clearly separated low- and high-M_w polymer fractions was observed with 1 + 2. The two catalyst systems showed similar hydrogen and hexene responses. Each metallocene precursor showed individual response towards the polymerisation conditions, especially polymerisation temperature, suggesting that interaction between the catalyst active sites was negligible in the studied systems.     

Copolymerisation of 1,9-decadiene and propylene with binary and isolated metallocene systems

1,9-Decadiene/propylene copolymers were obtained with isolated metallocenes and with a binary metallocene catalyst system activated by methylaluminoxane. The metallocenes under investigation were syndiospecific diphenylmethyl(cyclopentadienyl)(9-fluorenyl)zirconium dichloride (1) and isospecific rac-dimethylsilylbis(4-tert-butyl-2-methyl-cyclopentadienyl)zirconium dichloride (2). A copolymer structure, in which 1,9-decadiene linked isotactic and syndiotactic polymer chains, was obtained when copolymerisation was started with catalyst 2 at 80 °C followed by injection of catalyst 1 and instantaneous lowering of polymerisation temperature to 40 °C after 15 min of polymerisation. The copolymer was also shown to work as a compatibiliser in a blend of syndiotactic and isotactic polypropylene. We propose that catalyst 2 incorporates 1,9-decadiene into the isotactic main chain without any significant crosslinking within the first 15 min of polymerisation at 80 °C and the produced isotactic macromonomers are further incorporated at 40 °C into the syndiotactic main chain in polymerisation with catalyst 1.     

Comparative study of propylene polymerization using Me2Si(RInd)2ZrCl2/SiO2-SMAO/AlR3 and Me2Si(RInd)2ZrCl2/MAO (R = Me, H)

A mechanistic analysis of propylene polymerization was performed, in which the catalyst system was Me₂Si(R¹Ind)₂ZrCl₂/SMAO/AlR₃² (in situ supported catalyst onto MAO-modified silica) or Me₂Si(R¹Ind)₂ZrCl₂/MAO (homogeneous), where R¹ = H or CH₃, cocatalyzed by AlR₃² = TEA (triethylaluminum), IPRA (isoprenylaluminum), or TIBA (triisobutylaluminum). The catalyst activity of the homogeneous system Me₂Si(2-Me-Ind)₂ZrCl₂/MAO was almost 8 times higher than that observed for Me₂Si(Ind)₂ZrCl₂/MAO (38 vs 4.6 kg PP/g cat h), while the polypropylene molar mass was 3 times higher (M_w: 93 vs 34 kg/mol). Conversely, the in situ supported systems Me₂Si(Ind)₂ZrCl₂/SMAO/AlR₃ and Me₂Si(2-Me-Ind)₂ZrCl₂/SMAO/AlR₃ showed similar activities, ranging from 0.2 to 1.5 kg PP/g cat h. The molar mass of the resulting polymers prepared using the in situ procedure was dependent on the AlR₃ nature and on the Al/Zr ratio. Generally, the heterogeneous catalysts produced PP with higher molecular weights than that obtained with homogeneous ones. The influence of the alkylaluminum, used as the cocatalyst, on the chain-transfer termination reaction to the alkyl compound was evident from the activity and the molecular weight of the produced polymers.     

Plywood-like structure of injection-moulded polypropylene

Injection-moulded products having unique structure, in which the direction of molecular orientation in the skin layer is perpendicular to that in the core layer, are developed employing isotactic polypropylene with a nucleating agent. The extraordinary three-layered structure with β trigonal crystal form in the core layer, which shows higher impact strength than the conventional α monoclinic form, leads to high level of toughness. Moreover, an injection-moulded product having five-layered structure is also demonstrated in this paper. Because of the complicated crack propagation nature due to the abrupt change of molecular orientation, which avoids fractured pieces with sharp-edge, the products with plywood-like structure will be employed in various applications to improve the safety.     

聚丙烯催化剂研究进展

综述了Ziegler-Natta聚丙烯催化剂技术的发展历史与研究动态,介绍了茂金属催化剂和后过渡金属催化剂的研究进展,展望了未来聚丙烯催化科技术的发展前景．提出了我国发展聚丙烯催化剂的建议。     

聚丙烯新产品研发近况

聚丙烯的研究与开发近期一直快速向前发展,显示出前所未有的活力,其结构及改进的可能性几乎可说是无穷无尽。目前正在开发的催化剂和生产技术的不断改进,将能生产出基于聚丙烯的、但却更加复杂的产品。本文综述了这方面的研发近况。     

In-situ analysis of fiber structure development for isotactic polypropylene

Structural development of isotactic polypropylene (iPP) fibers was analyzed in real time through in-situ WAXD/SAXS and fiber temperature measurements during CO₂ laser-heated drawing because the CO₂ laser irradiation can nearly fix the necking position on the running fiber. The in-situ WAXD/SAXS measurements were carried out with a high time-resolution of 0.4 ms. The as-spun iPP fibers of two different initial structures were laser-heat-drawn to a draw ratio of 6.5. For the drawing of PP fiber containing mesophase structure, diffraction from the oriented mesophase remained until an elapsed time of 1.0 ms, when oriented α-phase crystal started to form. Meanwhile, for the drawing of PP fiber containing both an α-phase and a mesophase structure, fragmented microcrystals were reorganized by orientation-induced crystallization before an elapsed time of 1.0 ms. The long period increased drastically with fragmentation, and decreased with reorganization. The long period was about 16 nm for both drawn fibers.     

The structure and physical properties of polypropylene and thermoplastic olefin nanocomposites containing nanosilica

The morphology and physical properties of thermoplastic olefin blend (TPO) based nanocomposites containing nanosilica are reported. Addition of maleated PP resulted in improved filler dispersion within the PP matrix, where the filler resided exclusively. This separated morphology resulted in selective reinforcement of the PP matrix without compromising ductility, as demonstrated by mechanical property characterization. The tensile moduli, impact and flexural properties of TPO/nanosilica composites showed improvements at low loadings of nanosilica, indicating a good balance of stiffness and toughness. The addition of nanosilica into the TPOs decreased the size of the dispersed elastomer phase, which was a factor in the observed improvements of impact strength. Silane-modified nanosilica dispersed more efficiently in the polymer matrix, giving rise to improved impact properties of the TPO composites, compared to the unmodified filler.     

Al-MCM-41 catalyzed decomposition of polypropylene and hybrid genetic algorithm for kinetics analysis

Mesoporous catalysts (Al-MCM-41) are synthesized by sol–gel and hydrothermal methods to study their effects on the catalytic decomposition of polypropylene (PP) sample. The catalysts are characterized by X-ray diffraction (XRD) analysis and nitrogen adsorption study. Since sol–gel Al-MCM-41 catalyst shows better catalytic activity, further experimental studies were conducted to find its reusability and its activity at five different heating rates. The constant pattern behaviour of the TG curves for different catalyst percentages possibly suggests existence of similar reaction mechanism where large polymer fragments are cracked on the external surface of the catalyst and then enters into the mesopores for further cracking. Thus, presence of catalyst surfaces not only converts the polymer into comparatively smaller fractions, but also makes the decomposition of PP energy effective. Kinetics parameters are estimated based on 15 different decomposition models and the multi-heating rate experimental data both for catalytic and noncatalytic decomposition of PP using hybrid genetic algorithm (HGA). Suitability of the model is tested using corrected Akaike's Information Criteria (AIC_c). Results show that Nucleation and Growth model better predicted the experimental TGA data. However, nth order model also shows good AIC_c score and well predicted the experimental TGA data. Thus, though apparently it seems that Nucleation and Growth model controls the decomposition of PP sample, further investigation in detail including infrared or mass spectroscopy, morphology study using SEM or TEM during such decomposition is very much essential to conclude upon the actual reaction mechanism that controls decomposition of PP sample.     

新型丙烯聚合催化剂的性能与微观结构分析

采用激光粒度分布仪,扫描电子显微镜及差示扫描量热仪,分别对新型及原丙烯聚合催化剂及其聚合物的物性进行了分析,发现新型催化剂比原催化剂颗粒粒径分布窄且分布均匀,平均粒径小,比表面积大,颗粒形态良好。小试聚合实验结果表明：新型催化剂的催化效率高,氢调敏感,聚合物等规指数容易调节并可以控制,聚合物复制了催化剂的形态,且粒径分布较窄。同时,进行了丙烯－乙烯共聚实验,从聚合物熔点及乙烯含量的测定结果可以说明：新型催化剂共聚性能优良。     

茂金属弹性体在聚丙烯薄膜中的应用

以茂金属弹性体为增韧树脂,对聚丙烯(PP)进行共混增韧改性,用下吹水冷方法生产PP薄膜。测试结果表明,改性后的PP薄膜,在保持其优良挺括性的同时,提高了在低温条件下的耐冲击和跌落性能。     

The role of interlamellar chain entanglement in deformation-induced structure changes during uniaxial stretching of isotactic polypropylene

In-situ small-angle X-ray scattering (SAXS), and wide-angle X-ray diffraction (WAXD) were carried out to investigate the deformation-induced structure changes of isotactic polypropylene (iPP) films during uniaxial stretching at varying temperatures (room temperature, 60 °C and 160 °C). From the WAXD data, mass fractions of amorphous, mesomorphic and crystal phases were estimated. Results indicate that at room temperature, the dominant structure change is the transformation of folded-chain crystal lamellae (monoclinic α-form) to oriented mesomorphic phase; while at high temperatures (>60 °C); the dominant change is the transformation of amorphous phase to oriented folded-chain crystal lamellae. This behavior may be explained by the relative strength between the interlamellar entangled network of amorphous chains, which probably directly influence the tie chain distribution, and the surrounding crystal lamellae. It appears that during stretching at low temperatures, the interlamellar entanglement network is strong and can cause lamellar fragmentation, resulting in the formation of oriented mesomorphic phase. In contrast, during stretching at high temperatures, the chain disentanglement process dominates, resulting in the relaxation of restrained tie chains and the formation of more folded-chain lamellae.     

The pore structure of chrominophosphate catalysts

A series of chrominophosphates (CrPs) with various P/Cr ratios were prepared by the precipitation method. The pore structures of these catalysts were characterized by nitrogen adsorption and mercury-penetration porosimetry. The results indicated that the micro-pores with diameters less than 20 nm were due to the dehydration process and had a slit-shaped geometry. There were two types of large pores (meso- and macro-) with diameters greater than 20 nm. These pores had a cylindrical pore-shape. The meso-pores can be attributed to the packing of particles and the macro-pores are essentially due to the packing of the aggregates of the particles.     

The use of polypropylene in pipeline coatings

Despite the fact that the pipeline coating represents only about 5% of the total cost, the choice of the most effective coating is a key point to guarantee the life of the installed pipelines. Since the eighties, polypropylene copolymer coatings have been used for the protection of the external surface of on-shore and off shore pipelines. Polypropylene is one of the most suitable coatings when high mechanical properties (impact resistance, penetration resistance, etc.) and/or heat resistance are required. Polypropylene has also been used in ordinary pipelines giving advantages compared with the standard coatings, e.g. fusion bonded epoxy resin, coal tar enamel and polyethylene. New applications for polypropylene copolymers such as pipeline coatings have recently been developed, these include an insulating coating obtained with foamed polypropylene and an internal coating obtained by spraying a polypropylene adhesive powder.     

Globules of microparacrystals in nascent isotactic polypropylene

SAXS and TEM measurements were used in studying the morphology of nascent isotactic polypropylene (PP) polymerized with a high-yield heterogeneous Ziegler-Natta MgCl₂ supported catalyst. In contrast to the melt- or solution- crystallized isotactic PP cases, the SAXS datam obtained here using Kratky-camera and TEM diagrams, show the characteristic scattering of globules (～ 1100Å in diameter) and a remarkably small polydispersity of 0.14. TEM observations also show sometimes the presence of smaller globular particles of about 300Åin diameter whose amount is below the resolving power of SAXS. Due to the lack of interparticle interference, lamellae do not exist and therefore no mention of an amorphous phase is required. WAXS can be analysed as a series of overlapping reflections with paracrystalline (liquid-like) distortions at g = 5%. These microparacrystals (mPC's) have in accordance with the α*-law, a mean size of (α*/g)²d¯=60Å and constitute the globules. From line profile analysis one finds that their polydispersity g_N is 0.4 and this agrees well with a relation between N and g_N which is deduced theoretically for microparacrystal-ensembles in the equilibrium state. A new relation between the polydispersity g^N of microparacrystals and the size of globules is derived from TEM observations. These observations sometimes show clearly separated spherical particles whose size distribution is in the range of the SAXS results, but varies from micrograph to micrograph. The morphology of the nascent PP is greatly influenced by the distribution of the active centres on the catalyst's carrier and this excludes the regular chain-folding in a crystalline phase and the existence of larger disordered chain segments in an amorphous phase. The whole volume is therefore filled with microparacrystals.     

Carbon nanotube dispersion and exfoliation in polypropylene and structure and properties of the resulting composites

Nitric acid treated single and multi wall carbon nanotubes (SWNT and MWNT) have been dispersed in polypropylene using maleic anhydride grafted polypropylene (MA-g-PP) and butanol/xylene solvent mixture. SWNT exfoliation was characterized by Raman and UV-vis-NIR spectroscopies. Evidence for hydrogen bonding between maleic anhydride grafted polypropylene and nitric acid treated nanotubes was obtained using infrared spectroscopy. Polypropylene/carbon nanotube composites were melt-spun into fibers. Dynamic mechanical studies show that for fibers containing 0.1 wt% SWNT, storage modulus increased by 5 GPa at −140 °C and by about 1 GPa at 100 °C, suggesting temperature dependent interfacial strength. The crystallization behavior has been monitored using differential scanning calorimetry and optical microscopy. Control fibers exhibited 27% shrinkage at 160 °C, while the shrinkage in the composite fibers was less than 5%. Fibers heat-treated to 170 °C show very narrow polypropylene melting peak (peak width about 1 °C).     

The influence of stem conformation on the crystallization of isotactic polypropylene

The influence of stem conformation on the crystallization of i-PP is studied by growing α-form lamellae in melts of β-form lamellae at different temperatures. The melting of β-form lamellae and the crystallization of α-form lamellae is observed in situ at the interface of α- and β-form spherulites by AFM. The growth rate of α-form lamellae in the melt of β-form lamellae is much lower than that in the isotropic melt due to the stem conformation barrier, which originates from the difference in the α and β unit cell packing models.     

Tacticity distribution of isotactic polypropylene prepared with heterogeneous Ziegler-Natta catalyst. 2. Application and analysis of SSA data for polypropylene

Accurate characterisation of the tacticity distribution in isotactic polypropylene is important for obtaining better structure property correlations and for the indirect investigation of the active sites in heterogeneous Ziegler-Natta catalysts. In this work, successive self-nucleation and annealing (SSA) DSC measurement is applied for the study of tacticity distribution in polypropylene fractions with varying isotacticity. The results are compared with ¹³C NMR and temperature rising elution fractionation (TREF). Analysis of the lamellar thicknesses and average meso sequence lengths (MSL) revealed that the fractions could be classified into three main groups according to the lamellar structure generated in SSA. The fractions with relatively low isotacticity crystallise mainly by the longest crystallisable sequence of the chain. In more isotactic fractions, the lamellar thicknesses are close to the average crystallisable sequence lengths of the chains. For the highest isotacticity fractions correlation was not found. Because of these differences the correlation between the melting temperature and isotacticity (meso diad-%) was non-linear. Good correspondence between the SSA melting curves and the TREF fractograms was observed.     

Morphology and mechanical properties in the binary blends of isotactic polypropylene and novel propylene-co-olefin random copolymers with isotactic propylene sequence 1. Ethylene-propylene copolymers

The additive effects of the novel ethylene-propylene random (EP) copolymers with high isotacticity in propylene sequence on the morphology and mechanical properties of isotactic polypropylene (iPP) were investigated using polarized optical microscopy, transmission electron microscopy, dynamic mechanical analysis and tensile behavior. According to these results, the EP copolymers with a propylene content of more than 84 mol% were miscible with iPP, in which the crystallizable PP sequences in these EP copolymers were incorporated in crystal lattice of iPP and the other portions in the EP chains were excluded to the amorphous phases. Consequently, they act as tie molecules linking between adjacent lamellae, leading to enhancement of yield toughness of iPP. On the other hand, the EP copolymers with a propylene-unit content of less than 77 mol% were incompatible with iPP. The iPP/EP blends showed the phase-separated morphology.