Stereoregular polymer polymers Stereoselective polymerization

Stereoselective polymerizations yielding isotactic and syndiotactic polymers are termed isoselective and syndioselective polymerizations, respectively. The polymer structures are termed stereoregular polymers. The terms isotactic and syndiotactic are placed before the name of a polymer to indicate the respective tactic structures, such as isotactic polypro-pene and syndiotactic polypropene. The absence of these terms denotes the atactic structure polypropene means atactic polypropene. The prefixes it- and st- together with the formula of the polymer, have been suggested for the same purpose it-[CH2CH(CH3)] and st-[CH2 CH(CH3)] [IUPAC, 1966],... 

Mechanisms have been proposed to explain stereoselectivity in 1,3-diene polymerizations, but these processes are less understood than the polymerization of ethylene and 1-alkenes [Peluso et al., 1997]. The ability to obtain cis 1,4-, trans 1,4-, and st 1,2-polymers from 1,3-butadiene, each in very high stereoregularity, hy using different initiators has great practical utility for polymer synthesis even if it is not well understood why a particular initiator gives a particular stereoregular polymer. 

The versatility of polymerization resides not only in the different types of reactants which can be polymerized but also in the variations allowed by copolymerization and stereoselective polymerization. Chain copolymerization is the most important kind of copolymerization and is considered separately in Chap. 6. Other copolymerizations are discussed in the appropriate chapters. Chapter 8 describes the stereochemistry of polymerization with emphasis on the synthesis of polymers with stereoregular structures by the appropriate choice of initiators and polymerization conditions. In the last chapter, there is a discussion of the reactions of polymers that are useful for modifying or synthesizing new polymer structures and the use of polymeric reagents, substrates, and catalysts. The literature has been covered through early 2003. 

By assuming that stereoselectivity is connected to the regular heterogeneous catalyst surface, Natta introduced violet T1CI3 and used preformed heterogeneous catalysts in further studies.234,240 These complexes may act as stereoselective catalysts and polymerize propylene to crystalline stereoregular polymers. 

The free-radical polymerization of chloroprene (2-chloro-l,3-butadiene) is a stereoselective polymerization that has been known for many years. However, this stereochemical fact has been largely ignored or superficially treated in some organic texts. One will note that six stereoregular polymers are possible. A 1,4 addition can give either cis-1,4- or tra%s-l,4-polychloroprene. The 1,2 and 3,4-additions account for the other four possibilities since each addition can be either isotactic or syndiotactic. The macromolecule, which is produced by free-radical synthesis, is practically all trans-l,4-polychloroprene. 

Polymer rac-84b is prepared from the coupling of rac-13 and rac-15. This polymer can be either a stereoregular polymer or a mixture of diastereomeric polymers. In order to find out whether the polymerization of the racemic monomers rac-13 and rac-15 is stereoselective or the polymer rac-84b actually contains randomly distributed R and S binaphthyl units, the Suzuki coupling of two equivalents of rac-15 with one equivalent of (/ )-13 has been studied. After the reaction is completed, an oligomer 85 is obtained that shows = 3600 and = 2300 (PDI = 1.5) as measured by GPC. Its specific optical rotation is [aJu = -97.7 (c = 1.0, THF). About 20% of unreacted rac-15 is recovered that has a specific optical rotation of [q ]d = -0.7. Thus there is no enrichment of either R or S enantiomer at all after the coupling reaction. This result demonstrates that the coupling of rac-... 

The open transition state—-hybridized carbocations— has low stereoselectivity, thus mostly atactic polymers are formed (94). At the same time, reportedly the very first synthetic stereoregular polymer was an isotactic poly(isobutyl vinyl ether) prepared by carbocationic polymerization initiated with BF3/0(C2115)2 at -78°C (95). Other examples of stereoregular crystalline polymers synthesized by homogeneous and heterogeneous carbocationic polymerization are polymers of vinyl ether derivatives, and 2,5-dimethyl styrene or a-methylstyrene (95-100). Stereoregular vinyl ether polymers have also been synthesized using... 

Stereoselective or stereoelective polymerization of racemic monomers. Stereoregular polymers have been obtained by stereoselective polymerization from many cyclic, racemic monomers (100,121,127-140). Some of these monomers are indicated in Table 2. The identification of the stereoselective character of a polymerization process, which leads to isotactic optically inactive polymers, requires a very accurate characterization of the polymer obtained. For the polymers reported in Table 2, elution chromatography on an optically active support, spectroscopic (IR, or NMR) measurements, comparison of the X-ray pattern with that of the corresponding optically active polymer, and enzymic degradation have been used. In many cases the stereoselectivity of the polymerization process has been confirmed by the stereo-electivity observed when using the catalyst in an optically active form. 

Stereoselective or stereoelective polymerization of racemic monomers. The stereoselective polymerization of racemic monomers was first investigated in the case of racemic a-olefins (184). When the asymmetric carbon atom was in the a-position with respect to the double bond, essentially stereoregular polymers were obtained, the single macromolecules having isotactic main chain and carbon atoms with mostly the seune configuration in the lateral chains. The stereoselectivity decreases when the asymmetric carbon atom of the monomer is in the 3-position with... 

Since the bulk properties of PLA are highly dependent on the stereoregularity or the tacticity of the polymer, i the development of catalysts for rac-lactide (or meso-lactide) polymerization has been focused on achieving stereoselectivity. 2 2 " Numerous organocatalysts such as A-heterocyclic carbenes (NHCs) and phosphine-based compounds " have been investigated for the controlled ROP of lactide. 

According to an early hypothesis21, stereoregular isotactic polymerization requires the presence of chirality within the catalyst. Thus, with achiral metallocenes mostly atactic polymers are obtained. Chiral ansa-metallocenes with substituted cyclopentadienyl ligands, or especially with indenyl and tetrahydroindenyl ligands, are effective in stereoselective polymer synthesis. 

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