Propylene polymerizations were conducted by [t-BuNSiMe2(3,6-t-Bu2Flu)]TiMe2 using dried modified methyaluminoxane as a cocatalyst in heptane, chlorobenzene (CB), and a mixture of heptane/CB (1:1 in
volume) at 0°C. Postpolymerizations testified that the propylene polymerization proceeded in a living manner regardless of
the solvent used. The heptane system gave highly syndiotactic crystalline polypropylene (PP), whereas the CB and heptane/CB
mixture systems gave amorphous PP. After the first polymerization in heptane had been completed, the same amounts of propylene
and CB were added for the second polymerization. This procedure gave the syndiotactic-atactic stereoblock PP with a melting
point of 119°C.
The text was submitted by the authors in English. 相似文献
Highly syndiotactic “living” poly(propylene)s were synthesized at 25°C using a bis[N‐(3‐tert‐butylsalicylidene)‐2,3,4,5,6‐pentafluoroanilinato]titanium (IV) dichloride/MAO catalyst system, and microstructures of the polymer were analyzed by means of 13C NMR spectroscopy. The syndiotactic poly(propylene) contains isobutyl, isopentyl and propyl end groups, suggesting that the living polymerization of propylene was initiated via 1,2‐insertion, followed by 2,1‐insertion as the principal mode of polymerization. Pentad distribution analysis revealed that the syndiospecific polymerization proceeds under chain‐end control. 相似文献
Forty years after Natta's discoveries of stereospecific olefin polymerization with Ziegler catalysts, new catalysts are causing a renaissance in stereospecific olefin polymerization. Metallocene Ziegler-Natta catalysts are unprecedented in their ability to polymerize α-olefins to a variety of polymer microstructures.1 Isotactic, syndiotactic, atactic and stereoblock poly(α-olefins) have been produced using catalysts derived from group 4 metallocene catalyst precursors. 相似文献
The living polymerization of propene was conducted at 0°C using a chelating diamide complex of titanium combined with dried modified methylaluminoxane from which free trialkylaluminium compounds were removed as much as possible before use. The number‐average molecular weight of the statistically atactic poly(propylene) produced increased almost linearly with increasing polymerization time accompanied by a narrowing of the molecular weight distribution from 1.34 to 1.16. 相似文献
The catalytic behavior of three bis(phenoxy‐imine) group‐4 transition‐metal complexes (M = Ti, Zr, Hf), with iBu3Al/Ph3CB(C6F5)4 cocatalyst systems towards propylene polymerization was investigated under atmospheric pressure at 25 °C. The Ti complex produced ultrahigh‐molecular‐weight atactic poly(propylene), whereas Zr and Hf complexes formed high‐molecular‐weight isotactic poly(propylene)s via a site‐control mechanism. The isotactic poly(propylene) obtained with the Hf complex displayed a high melting temperature of 123.8 °C.
Catalytic precursors Ti(IV) and Zr(IV) complexes bearing cyclopentadienyl and substituted cyclopentadienyl anionic ligands, bonded to phenyl or substituted phenyl through an isopropylidene bridge have been utilized in the polymerization of propene and styrene and in ethylene‐styrene copolymerization. In the presence of trichloro[(1,2,3,4,5‐η)‐1‐(1‐methyl‐1‐phenylethyl)‐2,4‐cyclopentadien‐1‐yl]titanium (LTiCl3) we have obtained either partially isotactic (chain‐end type) or atactic poly‐(propylene), and either atactic or syndiotactic polystyrene depending on the reaction temperature. [1‐Methyl‐1‐naphthylethyl‐2‐inden‐1‐yl]titanium(IV) behaves like (LTiCl3) in styrene polymerization, while it affords metal‐controlled partially isotactic poly(propylene), as well as the corresponding zirconium compounds. The experimental data are tentatively explained by the temperature dependence of coordination of the bridged aryl group of the ligand. 相似文献
Olefinic thermoplastic elastomers can be prepared by incorporating semi‐crystalline macromonomers (e.g. isotactic or syndiotactic poly(propylene), high‐density polyethylene) onto amorphous backbones (e.g. atactic poly(propylene), ethylene/α‐olefin copolymers). The macromonomer incorporation reaction can be carried out in semi‐batch reactors by adding previously synthesized macromonomers to the reactor (ex situ approach), or by generating and incorporating the macromonomers in a single step (in situ approach). The differences in the microstructure of copolymers synthesized by in situ and ex situ techniques are explored herein through a mathematical model that can predict the concentration of linear and branched chains, their average molecular weights, polydispersity indices, and molecular weight distributions. In both cases linear chains predominate, but the ex situ approach produces a larger amount of branched chains with thermoplastic elastomer properties. Furthermore, for the in situ strategy, a significant amount of branched chains is only formed after the macromonomer concentration reaches a critical value.
Schematic representation of the polymerization mechanism. 相似文献
A one‐pot method for the preparation of a new family of PLA materials is reported that combines heterotactic (soft) and isotactic stereoblocks (hard). The ring‐opening polymerization of rac‐lactide with a salan–rare‐earth‐metal–alkyl complex in the presence of excess triethanolamine was performed in an immortal mode to give three‐armed heterotactic poly(lactide) (soft) with excellent end‐hydroxy fidelity. The in situ addition of a salen–aluminum–alkyl precursor to the above polymerization system under any monomer‐conversion conditions activated the “dormant” hydroxy‐ended PLA chains to propagate through the incorporation of the remaining rac‐lactide monomer, but with isospecific selectivity (hard). The resultant PLA had a three‐armed architecture with controlled molecular weight and extremely narrow molecular‐weight distribution (PDI<1.08). More strikingly, each side‐arm simultaneously possessed highly heterotactic (soft) and highly isotactic (hard) segments and the ratio of these two stereoregular sequences could be swiftly adjusted by tuning the addition time of the salen–aluminum–alkyl precursor to the polymerization system. Therefore, star‐shaped hard–soft stereoblock poly(lactide)s with various Pm values and crystallinity were achieved in a single reactor for the first time. This strategy should be applicable to the synthesis of a series of new types of stereoblock polyesters by using an immortal‐polymerization process and a proper choice of specific, selective metal‐based catalysts. 相似文献
Unnatural‐type syndiotactic and atactic poly[(R,S)‐3‐hydroxybutanoate]s [P(3HB)s] were enzymatically transformed into a reactive cyclic 3HB oligomer of molecular weight ca. 500 in an organic solvent, such as toluene, using immobilized lipase from Candida antarctica at 40°C for 24 h. It was confirmed that similar results were obtained for both syndiotactic and atactic P(3HB)s. On the other hand, the acidic degradation of these polymers using a protonic acid, such as p‐toluenesulfonic acid, exclusively produced the linear 3HB oligomer instead of the cyclic oligomer. The formation of the cyclic oligomer was regarded as the characteristic feature of the lipase‐catalyzed degradation in organic media. The cyclic oligomer obtained readily reacted with alcohol as a nucleophile, and using lipase, to produce the alkyl ester of the 3HB oligomer. 相似文献
Syndiospecific silyloxy‐functionalized polystyrene with high molecular weight was prepared using a (pentamethylcyclopentadienyl)titanatrane/MMAO catalyst system. The resulting polymer is soluble in polar organic solvents such as THF and shows good thermal stability. In addition, the compound Ni(acac)2 was used as a catalyst in preparing authentic atactic polymer of 4‐tert‐butyldimethylsilyloxystyrene under the same conditions. The chemical transformation of syndiospecific poly(4‐tert‐butyldimethylsilyloxystyrene) also gave a new polar polymer, namely syndiotactic poly(4‐hydroxystyrene) which is unattainable by traditional synthetic methods. 相似文献
Syndiotactic poly(propylene)s (s‐PPs) were freeze‐dried from toluene solutions of various concentrations. FT‐IR spectroscopy and wide‐angle X‐ray diffraction were used to characterize the s‐PPs prepared. The isothermal crystallization behavior was also investigated by means of differential scanning calorimetry. The disentangled particles are of high crystallinity and show rapid crystallization rates. A trans‐planar conformation is developed when s‐PP was freeze‐dried from more dilute solution. 相似文献
Acid hydrolysis of a stereoblock poly(methyl methacrylate) sample leads to a mixture of isotactic and syndiotactic poly(methacrylic acid) which can be separated by electrophoresis. The experiment confirms the stereochemical identity between the so-called “stereoblock” poly(methyl methacrylate) and the stereocomplex which syndiotactic and isotactic poly(methyl methacrylate) form in the ratio 2:1. A possible mechanism of replica polymerization is suggested to account for this effect. 相似文献