Cis-tacticity

The combination of cis-trans isomerism with iso-syndio and erythro-threo dispositions gives complex stractures as exemplified by the 1,4 polymers of 1-or 4-monosubstituted butadienes, such as 1,3-pentadiene (72, 73), and 2,4-pentadienoic acid (74, 75) and of 1,4-disubstituted butadienes, for example, sorbic acid (76). This last example is described in 32-35 (Scheme 6, rotated Fischer projection). Due to the presence of three elements of stereoisomerism for each monomer unit (two tertiary carbons and the double bond) these polymers have been classed as tritactic. Ignoring optical antipodes, eight stereoregular 1,4 structures are possible, four cis-tactic and four trans-tactic. In each series (cis, trans) we have two diisotactic and two disyndiotactic polymers characterized by the terms erythro and threo in accordance with the preceding explanation. It should be noted that here the erythro-threo relationship refers to adjacent substituents that belong to two successive monomer units. 

Applied difunctional organocyclotetrasiloxanes, both pure cis- and tram-isomers and their mixtures with isomers in various ratios, were of interest as model compounds for synthesizing stereo-regular poly(organocyclotetrasiloxanes) (POCS-4) trans-tactic, cis-tactic and with strict regular alternation of cis- and /ram-scqucnccs. 

In the first two cases, fc-sequences are present in the NMR spectrum due to partial inversion of ini-tial dichloro-derivative. All polymers with different tacticity types displayed high yields, except for the polymer of cis-tactic structure, the yield of which is much lower due to intramolecular cyclizati-on reaction, which induces formation of tricyclosiloxane (in the case of methyl groups at silicon atoms). 

Polymers with double bonds in the chains can occur in cis-tactic (ct) or trans-tactic (tt) configurations according to how the parts of the chain are arranged about the double bond. An example of this is l,4-poly(buta-diene), with the configurations... 

In the free radical polymerization of 1,3-dienes (Figure 20-8, Table 20-12) the difference in activation enthalpies for 1,4-trans and 1,4-cis tactic links is approximately of equal magnitude for butadiene, chloroprene, and... 

SCHEME. 7. CIS-ANTI-CIS-TACTIC AND CIS-ISOTACTIC CONEIGURATION OF PPSSO. 

The influence of cis-tacticity on isotropic crystallization is shown in Figure 14, where the effects of cooling and heating cycles on the storage modulus are illustrated for three polybutadiene raw rubbers. ... 

As previously mentioned, a significant additional improvement in fatigue resistance can be achieved by increasing the polymer molecular weight. Figure 19 shows that a 44% increase of MW improves fatigue life by about 112%. A cis-tactic polymer of this MW can be processed by conventional equipment without difficulty. 

If all the units along the chain are trans or if all the units along the chain are ois the polymer is called tactic in the first case trans-tactic, in the second case cis-tactic. It may be noted that, in the case of 1,4 polyisoprene, these two possibilities correspond to guttapercha and natural rubber respectively tactic polymers of isoprene are naturally occurring polymers. 

Isoprene can be polymerized using free radical initiators, but a random polymer is obtained. As with butadiene, polymerization of isoprene can produce a mixture of isomers. However, because the isoprene molecule is asymmetrical, the addition can occur in 1,2-, 1,4- and 3,4- positions. Six tactic forms are possible from both 1,2- and 3,4- addition and two geometrical isomers from 1,4- addition (cis and trans) ... 

The mechanism of B polymerization is summarized in Scheme 4,9. 1,2-, and cis- and trews-1,4-butadiene units may be discriminated by IR, Raman, or H or nC MMR speclroseopy.1 92 94 PB comprises predominantly 1,4-rra//.v-units. A typical composition formed by radical polymerization is 57.3 23.7 19.0 for trans-1,4- c7a -1,4- 1,2-. While the ratio of 1,2- to 1,4-units shows only a small temperature dependence, the effect on the cis-trans ratio appears substantial. Sato et al9J have determined dyad sequences by solution, 3C NMR and found that the distribution of isomeric structures and tacticity is adequately described by Bernoullian statistics. Kawahara et al.94 determined the microslructure (ratio // measurements directly on PB latexes and obtained similar data to that obtained by solution I3C NMR. They94 also characterized crosslinked PB. 

Diene-based polymers such as polybutadiene have other structural distinctions. The linear versions of these polymers have one residual double bond for each mer. When the double bonds are in the polymer chain, the cis and trans stereoisomers are possible. The double bonds can appear as pendant vinyl groups, which can then exhibit tacticity. 

A common feature of catalysts based on 4 and 5f block elements is that of being able to polymerize both butadiene and isoprene to highly cistactic polymers, independently of the ligands involved. Butadiene, in particular, can reach a cistacticity as high as 99% with uranium based catalysts (3) and cistacticity of > 98% with neodymium based catalysts (4). This high tacticity does not change with the ligand nature (Fig. 1) in contrast to conventional catalysts based on 3-d block elements. A second feature of f-block catalysts is that the cis content of polymer is scarcely... 

In particular, the increase of tacticity and the possibility of achieving the synthesis of "all cis" diene copolymers of high molecular weight allows the possibilities of broadening the range of products attainable in conventional, solution polybutadiene plants,... 

The polymerization of MMA has been shown to be subject to enantiomorphic site control when the Ci-symmetric a .va-lanthanocene complexes (196) and (197) are employed as initiators.463 When the (T)-neomenthyl catalyst (196) is used, highly isotactic PMMA is produced (94% mm at — 35 °C), whereas the (-)menthyl derived (197) affords syndiorich PMMA (73% rr at 25 °C). NMR statistical analysis suggests that conjugate addition of monomer competes with enolate isomerization processes, and the relative rate of the two pathways determines the tacticity. 

Stereoselective ROMP has been reported with monomer (213). Initiator (211) affords highly stereoregular polymer with >98% trans C=C bonds in the polymer backbone.534 However, when (210) is used, >98% cis-poly-(213) is obtained.535 A similar situation occurs for the diester monomer (214). Furthermore, a rapidly equilibrating mixture of (210) and (211) can be used to allow intermediate cis/trans contents to be manipulated by the stoichiometry of the initiator mixture. 13C NMR536 and dielectric analyses537 suggested that trans-poly-(213) is highly syndio-tactic (92% r dyad content). The ROMP of other fluorinated olefins has been recently reviewed.538... 

As for the stereochemistry, for the case of complete cyclization, besides the usual tacticity (possibly, isotactic or syndiotactic, referred to relative configurations of equivalent stereogenic carbons of subsequent monomeric units), the cis or trans configuration of the 1,3-cycloalkane rings which are present in the polymer main chain also has to be considered.70,74... 

According to a widely accepted cyclopolymerization mechanism,67,75 there are two distinct stereochemical events for these cyclopolymerizations the already discussed stereoselectivity and stereospecificity of olefin insertion, which determine the tacticity of the polymer, and the stereoselectivity of the cyclization step, which determines the cis or trans configuration of the rings. 

Double bonds present along a polymer chain are stereoisomeric centers, which may have a cis or trans configuration. Polymers of 1,3-dienes with 1,4 additions of the monomeric units contain double bonds along the chains and may contain up to two stereoisomeric tetrahedral centers. Stereoregular polymers can be cis or trans tactic, isotactic or syndiotactic, and diisotactic or disyndio-tactic if two stereoisomeric tetrahedral centers are present. In the latter case erythro and threo structures are defined depending on the relative configurations of two chiral carbon atoms.1... 

The first report of ROMP activity by a well-characterized Mo or W species was polymerization of norbornene initiated by W(CH-t-Bu)(NAr)(0-f-Bu)2 [122]. In the studies that followed, functionality tolerance, the synthesis of block copolymers, and ring-opening of other monomers were explored [30, 123]. Two important issues in ROMP concern the cis or trans nature of the double bond formed in the polymer and the polymer s tacticity. Tacticity is a consequence of the presence of two asymmetric carbons with opposite configuration in each monomer unit. The four ROMP polymers (using polynorbornene as an example) that have a regular structure are shown in Scheme 3. 

The butadiene polymers represent another cornerstone of macromolecular stereochemistry. Butadiene gives rise to four different types of stereoregular polymers two with 1,2 linkage and two with 1,4. The first two, isotactic (62) and syndiotactic (25), conform to the definitions given for vinyl polymers, while the latter have, for eveiy monomer unit, a disubstituted double bond that can exist in the two different, cis and trans, configurations (these terms are defined with reference to the polymer chain). If the monomer units all have the same cis or trans configuration the polymers are called cis- or trans-tactic (30 and 31). The first examples of these stereoisomers were cited in the patent literature as early as 1955-1956 (63). Structural and mechanistic studies in the field have been made by Natta, Porri, Corradini, and associates (65-68). 

The same type of addition—as shown by X-ray analysis—occurs in the cationic polymerization of alkenyl ethers R—CH=CH—OR and of 8-chlorovinyl ethers (395). However, NMR analysis showed the presence of some configurational disorder (396). The stereochemistry of acrylate polymerization, determined by the use of deuterated monomers, was found to be strongly dependent on the reaction environment and, in particular, on the solvation of the growing-chain-catalyst system at both the a and jS carbon atoms (390, 397-399). Non-solvated contact ion pairs such as those existing in the presence of lithium catalysts in toluene at low temperature, are responsible for the formation of threo isotactic sequences from cis monomers and, therefore, involve a trans addition in contrast, solvent separated ion pairs (fluorenyllithium in THF) give rise to a predominantly syndiotactic polymer. Finally, in mixed ether-hydrocarbon solvents where there are probably peripherally solvated ion pairs, a predominantly isotactic polymer with nonconstant stereochemistry in the jS position is obtained. It seems evident fiom this complexity of situations that the micro-tacticity of anionic poly(methyl methacrylate) cannot be interpreted by a simple Bernoulli distribution, as has already been discussed in Sect. III-A. 

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