摘要:

AbstractThe paper recalls some aspects of the fascinating history of the discovery by Giulio Natta and his coworkers of the stereospecific polymerization of α‐olefins and of the determination of the structure of synthetic stereoregular polymers.The most relevant recent advances in the comprehension of the mechanisms regulating their synthesis with heterogeneous supported catalysts and with the newly discovered homogeneous metallocene‐based catalysts are briefly outl

ISSN:1022-1360    

DOI:10.1002/masy.19950890104

出版商:Hüthig&Wepf Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractThe interest for the stereospecific polymerization of olefins discovered by prof. Natta in 1954 was so high that only a few years elapsed between the discovery and the commercialization of polypropylene. The polypropylene discovery generated a tremendous scientific and technological effort for the development of the catalyst, the process, the products. The fundamental achievements in the polypropylene technology had a significant impact over the polyolefin technology in general. Thanks to it, polyolefinic polymers, copolymers and rubbers, polyefinic based materials and alloys via heterogeneous Ziegler‐Natta catalyst polymerization are today largerly the most important family of plastic materials.The ability to vary melt flow and the broad properties range of these materials, the excellent thermal and physical‐mechanical properties of isotactic polyolefins together with the favorable economics and their full and easy recyclability widen the concept of monomaterial application fuelling their dynamic expansion since the early sixt

ISSN:1022-1360    

DOI:10.1002/masy.19950890105

出版商:Hüthig&Wepf Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractThe relation between stereoregularity of active centers on a MgCl2/TiCl4catalyst and functions of inside donor (ID) and outside donor (OD) was investigated in the case of ethyl benzoate (EB)/methyl p‐toluate (MPT) as an ID/OD pair. The results indicate that stereregularity depends merely on the amount of MPT supported on the catalyst.On the other hand, regioregularity of active centers was investigated with a MgCl2/TiCl4/dioctyl phthalate(DOP)‐Et3Al/diphenyldimethoxysilane(DPDMS) catalyst system. Regio‐irregular inserted units were detected only in end groups of PP. It indicates that regio‐irregular insertion leads to dormant centers with respect to propene insertion, though such centers are active for hydrogen t

ISSN:1022-1360    

DOI:10.1002/masy.19950890106

出版商:Hüthig&Wepf Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractA precise description of the active site for heterogeneous stereospecific catalysts for propene polymerization has not been achieved, taking into account the effect of all the components of the catalytic system. A new model is proposed based on the following facts: the active Ti is directly bonded to the support MgCl2; it is bridged with an Al atom of the alkylaluminium; the latter is complexed with the external Lewis base; the more efficient Lewis bases are known to be dialkoxy dialkyl (or aryl) silane containing at least one bulky alkyl group; it is suggested that the two alkoxy groups are complexed, one with the aluminium atom, the second with the titanium atom. The structure of such a site has been built using a Chem 3D software and starting from an octahedral Ti and a bipyramidal Al. Both are bridged with the polymer chain. The silane is located so that each oxygen atom is just above each of the two metal atoms; the bulky alkyl group covers the vacancy of the Ti octahedre being in agostic interaction with the Ti. Such a tight geometry imposes the isospecific coordination of the monomers before its insertion in the polymer chain. The model is flexible enough to allow for the transition state, the insertion of the incoming monomer and the rotation of the new bridge, the driving force being the agostic interaction. The successful replacement of the silane with a diether with 3 carbons atoms in between the oxygen atoms is easily explained by the model, as well as the effect of the structure of the alkoxy and of the alkyl groups of the silane.

ISSN:1022-1360    

DOI:10.1002/masy.19950890107

出版商:Hüthig&Wepf Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractExperimental data obtained by means of some high performance techniques allow to identify and register quantitatively the Lewis acidic sites on the surface of highly disperse magnesium chloride. Carbon monoxide adsorption on magnesium chloride has been studied with the IR diffuse reflectance spectroscopy (IRS‐DR). The interaction of some stable nitroxyl radicals with magnesium chloride has been investigated with the ESR techniqu

ISSN:1022-1360    

DOI:10.1002/masy.19950890108

出版商:Hüthig&Wepf Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractThe examination of the reaction between [MgCl2(THF)2], TiCl4(3), AlCl3, AlEt3, AlEt2Cl and the synthesis and isolation of compounds as crystals and resolution of their structure by the X‐ray method were the subject of our study. It was expected that these investigations would help to understand the behaviour of MgCl2towards the transition metal and furnish useful relationships to the structure of catalyst active center and to the polymerization mechanism in TiCl4(3)/MgCl2/AlEt3system. Our studies have revealed that the main difference between the first and higher generations of Ziegler‐Natta catalysts is only the number of active cent

ISSN:1022-1360    

DOI:10.1002/masy.19950890109

出版商:Hüthig&Wepf Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractThe role of Lewis bases in MgCl2supported catalysts for olefin polymerization is a subject of continuous interest and discussion in order to obtain more and more active and stereospecific catalysts and to explain their stereoregulating mechanism.Through molecular calculation and conformational analysis it was possible to identify chelating diethers that have the correct oxygen‐oxygen distance necessary to tightly coordinate with the Mg ions of the support, even in the presence of other strong Lewis acids, and unable to give secondary reactions with TiCl4, AlR3, Ti‐C and Ti‐H bonds. The use of these donors has allowed the synthesis of catalytic systems that are both highly active and stereospecific even in the absence of external donors. Kinetic data of propylene polymerization with these catalyst systems are reported.The importance of the distance between the donor atoms in bifunctional Lewis bases has been proved also in the case of new classes of internal donors.Molecular modelling studies have enabled us to formulate models of active sites, located on some corners of MgCl2crystallites, whose chirality is induced by the presence of a donor molecule in their environment. These models could explain, at least in part, the exceptional increase of isotactic polymer productivity observed for stereospecific catalyst systems, containing only the internal donor, with respect to catalysts lacking the Lewis base and could account for the influence of the donor on the molecular properties of the obtained pol

ISSN:1022-1360    

DOI:10.1002/masy.19950890110

出版商:Hüthig&Wepf Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractThe results of a study on a new generation of MgCl2supported Ziegler‐Natta catalysts that contain dialkyl propane diethers as internal donors and produce highly stereospecific polypropene even without any external donor are presented. The results concerning three different catalysts containing 2,2‐diisobutyl‐1,3‐dimethoxy propane (DIBDMP), 2,2‐dicyclopentyl‐1,3‐dimethoxy propane (DCPDMP) and 2‐ethyl,2‐butyl‐1,3‐dimethoxy propane (EBDMP) as internal donors are compared. The same donors have also been studied by using them as external donors with a catalyst contaning diisobutyl phthalate as internal donor. The present results, along with previously obtained findings on traditional supported Ziegler‐Natta catalysts, permit a general comprehension of the role of inter

ISSN:1022-1360    

DOI:10.1002/masy.19950890111

出版商:Hüthig&Wepf Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractNew models for steric environment of Ti isospecific polymerization sites on MgCl2microcrystals are presented. They directly involve the presence of a donor molecule in order to obtain chiral activable Ti atoms otherwise belonging to isolated adsorbed TiCl4molecules or Ti2Cl8dimers which are lacking the required symmetry. The most important steric features of donor molecules have been obtained through structure‐activity relationships and molecular comparisons, while their adsorption on MgCl2faces lateral to (001) has been studied through a conformational analysis approac

ISSN:1022-1360    

DOI:10.1002/masy.19950890112

出版商:Hüthig&Wepf Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractThe kinetics of the ethylene/1‐hexene copolymerization reaction with a Ti‐based Ziegler‐Natta catalyst has been studied. Kinetic analysis established the existence of several populations of active centers in the catalyst. The centers differ in two aspects: their ability to incorporate α‐olefin units into copolymer chains (i.e., their reactivity ratios) and the average molecular weights of the polymer chains they produce. The centers of different populations are formed at different rates and have different kinetic stabilities. As a consequence, both the molecular weight distributions of the copolymers and their compositional distributions are relatively broad and change with in time. Two kinds of catalyst poisons were found. The poisons of the first type, arylalkoxysilanes, preferentially deactivate the centers which have the highest ability to copolymerize α‐olefins with ethylene. These poisons decrease the average α‐olefin content in the copolymers and the fraction of their olefin‐rich components. The poisons of the second type, conjugated dienes, preferentially deactivate the centers which have the lowest ability to copolymerize α‐olefins with ethylene. These poisons significantly increase the content of the olefin‐rich compone

ISSN:1022-1360    

DOI:10.1002/masy.19950890113

出版商:Hüthig&Wepf Verlag    

年代:1995

数据来源: WILEY