Stereospecific polymerizations dependence

Stereospecific polymerization has particular significance for the preparation of stereoregular polymeric dienes. In the radical polymerization of butadiene or isoprene the molecular chains always consist of varying proportions of adjacent cis- and trans-1,4-units as well as 1,2- and 3,4- linked units, depending on the polymerization conditions but it is now possible, using particular ionic initiation systems to make a synthetic natural rubber that contains more than 90% cfs-l,4-isoprene repeating units (see Example 3-21). 

The propylene oxide complex not only dissociated into its components but also transformed to either an oligomer or a polymer of propylene oxide when it was allowed to stand in solution. This transformation could be followed by H-NMR techniques with the use of a-deuterated propylene oxide instead of the non-deuterated one. Its rate depended on the nature of solvent and on the temperature. This experimental result implies that the monomer liberated by dessociation of the complex is polymerized by the catalyst, that only a minute fraction of the organozinc component of the complex actually acts as a catalyst for polymerization, and that the rate of propagation is far faster than that of initiation. These implications together with the evidence that coordination of the monomer to the catalyst is a prerequisite for the stereospecific polymerization led us to the detailed studies of the bulk polymerization, that is, the polymerization of propylene oxide in propylene oxide solution. 

Fig. 15. Temperature dependence of polymer composition in the stereospecific polymerization of propylene oxide by EtZnNBu ZnEt [Oguni el. al. (90)]...

Boor (65) showed that the specificity and activity of a propylene catalyst depended on both catalyst components. The addition of tri-alkylamines to titanium trichloride catalysts made with diethylzinc, ethylzinc chloride, or diethylcadmium, increased the specificity of the catalyst. There was also a simultaneous decrease in activity. Boor ascribed this phenomena to the amine deactivating catalysts which cause the non-stereospecific polymerization. The amine either destroyed the... 

It is in the stereospecific polymerization of propylene that metallocene complexes display their astonishing versatility. Commercial Ziegler-Natta catalysts for isotactic polypropylene - based on combinations of TiCU, MgCl2, Lewis bases and aluminum alkyls - depend on a metal-centered chirality which exists at specific edge and defect sites on the crystal lattice to direct the incoming monomer in a particular orientation. These catalysts produce small amounts of undesirable atactic material due to the presence of achiral active sites. 

For stereospecific polymerization of a-olefms such as propene, a chiral active center is needed, giving rise to diastereotopic transition states when combined with the prochiral monomer and thereby different activation energies for the insertion (see Figure 2). Stereospecificity may arise form the chiral /0-carbon atom at the terminal monomer unit of the growing chain - chain end control - or from a chiral catalyst site - enantiomorphic site control . The microstructure of the polymer produced depends on the mechanism of stereocontrol as well as on the metallocene used [42-44]. 

Precatalyst 4(Sm) was utilized as a standard system [60]. The mechanism follows a coordination anionic polymerization via an eight-membered transition state (Scheme 3, see p. 985). Formation of a metal enolate turned out to be essential for the initiation of the MMA polymerization and was confirmed by the initiation activity of the enolate complex [(C5H4SiMe3)2Y(OCH=CH2)]2- The rate of polymerization is directed by steric factors depending on the metal (Sm > Y > Yb > Lu) and the auxiliary ligand (Cp > Cp ). Ethyl, isopropyl and f-butyl methacrylates are also stereospecifically polymerized, but the rate of poly-... 

In this section we present the elements of chirality relevant to the stereospecific polymerization of propene with group 4 metallocenes. First of all, coordination of a prochiral olefin, such as propene, gives rise to nonsuperimposable coordinations. To distinguish between the two propene coordinations, we prefer the nomenclature re, si—defined for specifying heterotopic half-spaces— instead of the nomenclature R,. defined for double or triple bonds jr-bonded to a metal atom—in order to avoid confusion with the symbols R and S used for other chiralities at the same catalytic site, or the nomenclature Re, Si-defined for reflection—variant units—and used by Pino and co-workers in refs 83—86. The use of the si, re nomenclature can be confusing when different monomers are considered, because the name of a fixed enantioface of an 1-olefin depends on the bulkiness of the substituent in position 1. However, since propene is the only monomer considered in this review, this problem does not exist here. We only remark that the re and si coordinations sketched in Scheme 3 correspond to the R and S coordinations, respectively. 

The most important mechanism of stereospecific polymerization is isospecific enantiomorphic site control, which allows today the production of more than 25 million tons per year of isotactic polypropene and its copolymers, in a wide range of molecular weights and crystallinities. As already mentioned in section II, the molecular architecture of polypropenes obtained from ansa-zirconocenes is strongly dependent on the biscyclopentadienyl ligand structure. 

The nature of the above eqiulibria and the predominant species among three dimeric forms A, B and C will vary with the group R, solvent, temperature and ctmcen-tration. It was suggested from the NMR spectra of the catalysts that the stereospecificity of polymerization depends not only on the structure of the R group but also on the form of the dimeric species. 

Butadiene can also be stereospecifically polymerized to give either of two isomeric polymers. The stereochemistry depends on the conditions of the reaction and the Ziegler-Natta catalyst used. 

Depending upon the nature of R, they exhibit a wide variety of properties. They are obtained either by radical or cationic polymerization of the corresponding monomer. Under certain conditions, the process can be a controlled and/or stereospecific polymerization (see Sections 8.4 and 8.7.5). 

As a result of polymerization of 1,3-butadiene and its derivatives one obtains synthetic rubbers (elastomers) whose properties depend on the structure of the formed products. The stereospecific Ziegler-Natta catalysts offer new opportunities for the synthesis of rubbers with a defined structure. It turned out that by choosing an appropriate catalyst one can be obtain the following polymer structures cis-1.4, trans-1.4, isotactic 1.2, and syndiotactic 1.2, all in a relatively pure form. The influence of the catalyst structure on the stereospecific polymerization of butadiene is shown in Table 8.11. 

It is thought that polymerization proceeds in the same way as proposed for lanthanide catalysts, and the catalyst systems using non-bridged bis(cy-clopentadienyl)zirconium also afford syndiotactic poly(MMA). Some examples of stereospecific polymerization of MMA are siunmarized in Table 2 and Fig. 16. Complexes with non-bridged indenyl or fluorenyl Hgands, 42 and 43, were ejq)lored,but syndiotacticity did not depend on the hgands [134]. 

The course of stereospecific olefin polymerization was studied by using the molecular mechanics programs, MM-2 and Biograph, based on the optimized geometries of the ethylene complex and the transition state [13,203]. Interestingly, the steric interaction at the transition state mainly controls the stereochemistry in polymerization, which proceeds specifically isotactic or syndiotactic depending on the kind of catalyst. 

As was found for the polymerization of styrene, CpTiCT/M AO and similar half-sandwich titanocenes are active catalysts for the polymerization of conjugated 1,3 dienes (Table XX) (275). Butadiene, 1,3-pentadiene, 2-methyl-l,3-pentadiene, and 2,3-dimethylbutadiene yield polymers with different cis-1,4, trans-1,4, and 1,2 structures, depending on the polymerization temperature. A change in the stereospecificity as a function of polymerization temperature was observed by Ricci et al. (276). At 20°C, polypen-tadiene with mainly ds-1,4 structures was obtained, whereas at -20°C a crystalline, 1,2- syndiotactic polymer was produced. This temperature effect is attributed to a change in the mode of coordination of the monomer to the metallocene, which is mainly cis-rf at 20°C and trans-rj2 at -20°C. 

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