Copolymer stereospecific

Syndiospecific catalytic systems based on metallocenes are highly regioreg-ular. As a consequence, their stereoselectivity in possible regioirregular insertions has been experimentally investigated for propene copolymers only.78,79 However, an analysis of the stereoselectivity of possible secondary propene insertions on syndiospecific catalytic models based on -symmetric metallocenes is reported here, also due to its relevance to the rationalization of the dependence of regiospecificity on the type of stereospecificity (see Section 3.1.4.1).80... 

Campbell (22) refers that copolymers of 4-methyl- 1-pentene (4MP) with M-pentene-1 and with w-hexene-1, synthesized with the aid of stereospecific catalysts, were crystalline for M-hexene contents up to 25%, probably because of a partial cocrystallization. Analogous conclusions were attained by Hewett and Weir (23) for the same copolymer systems studied by Campbell on the basis of dynamic measurements correlated with DTA observations. 

Shen et al.120,121) found that the compounds of lanthanoid metals (from La to Lu) were active for the stereospecific polymerization of butadiene in the presence of alkylaluminum. Recently, Ouyangetal.122) reported that a NdCl3/C2H5OH/Al(C2Hs)3 catalyst exhibited a living character for the polymerization of diene and ethylene at temperatures below —30 °C. Diblock or triblock copolymers of diene and ethylene were obtained upon further addition of a diene monomer to a living polydiene or polyethylene. 

The same conclusion as in the case of propylene homopolymerisation has been drawn considering IR [396] and NMR [389,395] spectra of ethylene/propylene copolymers obtained with vanadium-based syndiospecific catalysts. The type of propylene insertion depends on the kind of last inserted monomer unit secondary insertion [scheme (40)] occurs more frequently when the last monomeric unit of the growing chain is propylene, while primary propylene insertion [scheme (39)] is more frequent when the last monomeric unit of the growing chain is ethylene [2]. The above explains the microstructure of ethylene/propylene copolymers obtained with vanadium-based Ziegler-Natta catalysts. These copolymers contain both m and r diads when the sequence of propylene units is interrupted by isolated ethylene units i.e. a propylene insertion after an ethylene insertion is substantially non-stereospecific [327,390,397], The existence of a steric interaction between the incoming monomer molecule and the last added monomer unit is also confirmed by the fact that the propagation rate for the secondary insertion of propylene in syndiospecific polymerisation is lower than for primary insertion in non-stereospecific polymerisation [398],... 

Palladium-based catalysts bearing chiral ligands have also been found to be capable of the stereospecific copolymerisation of allylbenzene [492] and its derivatives [493] with carbon monoxide the formed copolymers appeared to be of an alternating, isotactic structure. 

As early as in 1985, supported catalysts were described for the use in a solution process [399]. Bergbreiter et al. used catalyst supports on the basis of divinylbenzene-styrene-copolymers as well as on polyethylene. These authors found that the use of supported catalysts has no influence on the stereospecificity of diene polymerization. 

For instance, in the field of elastomers, alkyllithium catalyst systems are used commercially for producing butadiene homopolymers and copolymers and, to a somewhat lesser extent, polyisoprene. Another class of important, industrial polymerization systems consists of those catalyzed by alkylaluminum compounds and various compounds of transition metals used as cocatalysts. The symposium papers reported several variations of these polymerization systems in which cocatalysts are titanium halides for isoprene or propylene and cobalt salts for butadiene. The stereospecificity and mechanism of polymerization with these monomers were compared using the above cocatalysts as well as vanadium trichloride. Also included is the application of Ziegler-Natta catalysts to the rather novel polymerization of 1,3-pentadiene to polymeric cis-1,4 stereoisomers which have potential interest as elastomers. 

During recent years an increasing evidence has accumulated that discernible types of centers exist in Z—N catalysts, particularly in their heterogeneous versions. The centers may differ in their kp values, monomer coordination abilities, stereospecificities and reactivities in copolymerization. This concept can explain — at least qualitatively — wide MWD of polyolefins, composition heterogeneity of copolymers and specific responses of the catalyst performance to electron-donor additives. The origin of the differently behaving centers should be seen in a diversity of chemical processes... 

Radical reactions can often be rationalized on the basis of frontier orbital considerations for intermediate radical species, the reactivity and stereochemistry of which can certainly be regulated with Lewis acid additives [21-23]. The first appearance of Lewis acids in radical reactions was in polymerization reactions resulting in alternation of copolymers different from that obtained without Lewis acids [24-26]. This concept, Lewis acid-directed radical reactions, has been applied to reductions and alkylations of organic halides or olefins, and has resulted in highly stereospecific processes. 

Synthetic polymers in general can be classified (1) by thermal behavior, i.e., thermoplastic and thermosetting (2) by chemical nature, i.e., amino, alkyd, acrylic, vinyl, phenolic, cellulosic, epoxy, urethane, siloxane, etc. and (3) by molecular structure, i.e., atactic, stereospecific, linear, cross-linked, block, graft, ladder, etc. Copolymers are products made by combining two or more polymers in one reaction (styrene-butadiene). See cross-linking block polymer epitaxy homopolymer plastics. 

Significant developments in synthetic rubber began at this time. Outstanding were the introduction of polychloroprene (neoprene) by Carothers, and of the oil-resistant polysulfide rubber Thiokol by Patrick. These were soon followed by styrene-butadiene copolymers, nitrile rubber, butyl rubber, and various other types, some of which were rushed into production for the war effort in the early 1940s. The stereospecific catalysts researched by Ziegler and Natta aided this development, including synthesis of true rubber hydrocarbon (polyisoprene). Since 1935 synthetic rubbers have been referred to as elastomers. 

C are only slightly crystalline and it may be assumed that only syndlotactic sequences crystallize. Polymers where only sequences with a stereospecific configuration can crystallize may. in the context of this paper, be considered as copolymers. 

Stereospecific polymerization of 1,3-dienes (10-18) (to butadiene) and isoprene homo- and copolymers), dimerization of propene (19) and recently stereospecific polymerization of acetylene (20) to high cis-content polyacetylene have all been reported using lanthanide catalysts. Sen (21) has reported the preparation of cationic europium systems (which perhaps function as cationic initiators) for polymerization of norbornadiene and 1,3-cyclohexa-diene. 

We have been concerned with the precision and accuracy of NMR data of polymers since we first started NMR studies on polymers.1-4 Using continuous-wave (CW) spectrometers, the effects of measurement conditions including temperature, sample concentration and radiofrequency (rf) field strength, were examined using several polymer and copolymer samples. Since our research group have been deeply involved in stereospecific polymerization of methacrylates, one of the main concerns about NMR measurement was the precision of tacticity determination by NMR. The errors in determining the tacticity of poly(methyl methacrylate) (PMMA) and those in the results of polymerization of methyl methacrylate (MMA) by radical and anionic initiators were examined and found to be satisfactorily small.4 Although there... 

The on-line GPC/NMR technique is also useful for studying the molecular weight dependence of polymer properties such as tacticity and copolymer composition. This type of information is very important for an understanding of the mechanism of polymerization. The polymerization of MMA by anionic initiators often involves multiple active species with different reactivities and stereospecificities. For example, Grignard reagent exists in the Schlenk equilibrium ... 

Vizen and Kissin (94) suggested that the distribution of active centres of a heterogeneous catalyst determines the distribution of compositions in the copolymer they derived formulas for calculating both composition and weight of copolymer fractions in relation to the stereospecificity of the active centres. The theoretical relationships were compared with experimental data, obtained in part by IR measurements, on composition distribution in a copolymer prepared with the catalytic system VC13—A1(C2H5)3. 

Finally, IR spectroscopy contributed much to the finding of Zam-belli et al. (96) that C2-C3 copolymers tend to have an alternating sequence distribution if prepared by syndiospecific catalysts (i.e. catalysts that polymerize propylene to syndiotactic polymer) and a random distribution when prepared by isospecific or non-stereospecific catalysts. 

Synthetic polyisoprene, prepared by free-radical polymerization of isoprene monomer, is a copolymer of six structurally distinct kinds of isoprene chain units. Unlike natural rubber, which is a regularly repeating Class I structure (cis-1,4), such synthetic polyisoprene does not crystallize. On the other hand, by the use of the appropriate stereospecific catalyst, isoprene monomer can be converted to a regular Class I polymer with the same structure as natural rubber (. ... 

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