The dependence of the steric microstructure of cationically polymerized poly(N-vinylcarbazole) (PVK) upon catalyst, polymerization temperature, and polymerization solvent has been investigated. The effect of polymerization temperature variation was found to be small, whereas the choice of catalyst and polymerization solvent was found to have a strong influence upon the PVK steric microstructure. A correlation was found between the syndiotacticities Xs and the π* solvent polarities of the polymerization solvents for a given catalyst. A decrease in Xs with increasing π* solvent polarity was observed using BF3OEt2 and AlEt2Cl catalysts and has been interpreted in terms of propagation via contact ion-pair ring structures reversibly formed between the active end group and a preceding repeating unit. The increase in Xs with increasing π* solvent polarity observed with several of the catalysts investigated has been interpreted in terms of chain ion pairs whose separation increases with increasing π* solvent polarity. The influence of the various Lewis acid catalysts upon the steric microstructures of cationically polymerized PVK allowed the following order of nucleophilicity to be established: 相似文献
This paper discusses the nature of the living growing species in cationic polymerization from the viewpoint of the steric structure of poly(isobutyl vinyl ether) [poly(IBVE)]. At −78 °C, living polymerization was induced with the HCl-IBVE adduct (1)/ZnCl2 system in a EtNO2/CH2Cl2 mixture, whereas similar systems with EtAlCl2 led to conventional cationic polymerization. In this polar medium, both systems gave polymers with very similar and low isotacticity (meso ≈ 56%), indicating that the propagating reaction is mediated by free ions. Thus, regardless of solvent polarity, or involvement of free ions or ion pairs, living cationic polymerization requires a suitably nucleophilic counteranion. As model reactions of the growing species, 1/ZnCl2 and 1/EtAlCl2 were directly analyzed by 1H NMR spectroscopy. 相似文献
Summary: Propylene polymerization was conducted at 0 °C in heptane or chlorobenzene (CB) by an ansa‐fluorenylamidodimethyltitanium complex combined with dried, modified methylaluminoxane that had been treated by dilution with heptane and successively repeatedly vacuum dried to remove any free trialkylaluminiums. The post‐polymerizations testified that the living polymerization proceeded irrespective of the solvent used. The heptane system gave the syndiotactic poly(propylene)s (PPs), whereas the CB system gave almost atactic PPs. 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 narrow molecular‐weight distributions.
Structure of the ansa‐fluorenylamidodimethyltitanium complex and its application to the synthesis of syndiotactic‐atactic stereoblock poly(propylene)s reported here. 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
Inorganic siliceous porous materials such as MFI type zeolite, mesoporous silica MCM‐41 and silica gel with different average pore diameters were applied to the adsorptive separation of methylaluminoxane (MAO) used as a cocatalyst in α‐olefin polymerizations. The separated MAOs combined with rac‐ethylene‐(bisindenyl)zirconium dichloride (rac‐Et(Ind)2ZrCl2) were introduced to propylene polymerization, and their influences on the polymerization activity and stereoregularity of the resulting polymers were investigated. The polymerization activity and isotactic [mmmm] pentad of the produced propylene were markedly dependent upon the pore size of the porous material used for adsorptive separation. From the results obtained from solvent extraction of the produced polymers, it was suggested that there are at least two kinds of active species with different stereospecificity in the rac‐Et(Ind)2ZrCl2/MAO catalyst system. 相似文献
The influence of stereoregular poly(methyl methacrylate) (PMMA) as a polymer matrix on the initial rate of radical polymerization of methyl methacrylate (MMA) has been measured between ?11 and +60°C using a dilatometric technique. Under proper conditions an increase in the relative initial rate of template polymerization with respect to a blank polymerization was observed. Viscometric studies showed that the observed effect could be related to the extent of complex formation between the polymer matrix and the growing chain radical. The initial rate was dependent on tacticity and molecular weight of the matrix polymer, solvent type and polymerization temperature. The accelerating effect was most pronounced (a fivefold increase in rate) at the lowest polymerization temperature with the highest molecular weight isotactic PMMA as a matrix in a solvent like dimethylformamide (DMF), which is known to be a good medium for complex formation between isotactic and syndiotactic PMMA. The acceleration of the polymerization below 25°C appeared to be accompanied by a large decrease in the overall energy and entropy of activation. It is suggested that the observed template effects are mainly due to the stereoselection in the propagation step (lower activation entropy Δ Sp?) and the hindrance of segmental diffusion in the termination step (higher activation energy Δ Et?) of complexed growing chain radicals. 相似文献
A novel catalyst precursor, (η5‐pentamethylcyclopentadienyl)titanium triallyloxide (Cp*Ti(OCH2—CH=CH2)3), was prepared and employed in a study of propylene polymerization in the presence of methylaluminoxane (MAO). This work has revealed that the half‐titanocene catalyst is desirable for the production of elastomeric poly(propylene) with high molecular weight (Mw = 8–69×104) as well as in good yields under typical polymerization conditions. 相似文献
A dual catalytic setup based on N‐heterocyclic olefins (NHOs) and magnesium bis(hexamethyldisilazide) (Mg(HMDS)2) was used to prepare poly(propylene oxide) with a molar mass (Mn) >500 000 g mol?1, in some cases even >106 g mol?1, as determined by GPC/light scattering. This is achieved by combining the rapid polymerization characteristics of a zwitterionic, Lewis pair type mechanism with the efficient epoxide activation by the MgII species. Transfer‐to‐monomer, traditionally frustrating attempts at synthesizing polyethers with a high degree of polymerization, is practically removed as a limiting factor by this approach. NMR and MALDI‐ToF MS experiments reveal key aspects of the proposed mechanism, whereby the polymerization is initiated via nucleophilic attack by the NHO on the activated monomer, generating a zwitterionic species. This strategy can also be extended to other epoxides, including functionalized monomers. 相似文献
Iron(II) complexes were synthesized with bisiminepyridine ligands of low steric demand. Activation with modified-methylaluminoxane (25 mol.% isobutyl groups) generated very active catalysts for propylene oligomerization. The oligomerizations were carried out in liquid propylene in a heat flow calorimeter. The oligomers were separated by preparative gas chromatography and the dimers and trimers analyzed using analytical gas chromatography, 1H-NMR- and 13C-NMR-spectroscopy. With knowledge of the dimer and trimer structure, we were able to establish a mechanistic pathway for propylene insertion and obtained knowledge about the iron alkyl species involved. Analysis of the various dimers formed allowed us to determine the percentage of 1,2 versus 2,1 propylene insertions. Considering the same iron alkyl species with ligands of different steric demand, a change in the probabilities for 1,2 versus 2,1 propylene insertions can be observed. With this knowledge, the catalyst behavior for ligands of varying steric demand can be predicted. 相似文献
Prevailingly isotactic poly(propylene) samples were prepared with a homogeneous catalytic system based on a bis(imino)pyridyl Fe(II) derivative and methylaluminoxane. The polymer microstructure is in agreement with Bernoullian statistics of dyad formation, implicating a “chain-end” mechanism of steric control. The latter is operative even at polymerization temperatures as high as +50°C. NMR analysis of polymer end groups indicates that chain growth proceeds via 2,1 monomer insertion. The last two findings are unprecedented for isotactic-specific polymerization of propene and are reasonably related to each other. 相似文献