The Effect of Tacticity

III Molecular Engineering of Side Chain Liquid Crystalline Polymers 

Discussion of the effect of the polymer backbone in Sec. 3.4 of this chapter already provided examples of highly isotactic po-ly(f )-endo,exo-5,6-di [n-[4 -(4 -methox-yphenyl)phenoxy]alkyl]carbonyl bicy-clo[2.2.1]hept-2-ene s [190] and syndiotactic poly n-[4 -(4 -methoxyphenyl)phen-oxyjalkoxy methacrylate s [42, 44] which crystallize and form more ordered meso-phases than those of the corresponding atactic polymers (Fig. 15). Although more flexible backbones are more able to achieve the conformation necessary to order, the side chains are evidently already attached to the polymer backbone of these tactic polymers with the proper configuration to order, which obviates the need to distort their conformation for such purposes. 

As discussed in Sec. 2.1 of this chapter, isotactic poly n- [4 -(4 -methoxyphenyl)-phenoxy] alkyl methacrylate s have also been prepared, although the isotactic polymerizations are much less controlled than the syndiotactic polymerizations [42, 44]. 

The thermotropic behavior of both the isotactic and syndiotactic poly n-[4 -(4 -methoxyphenyl)phenoxy]alkyl methacrylate s is summarized in Table 11. All of the tactic polymers crystallize. With the exception of poly 2-[4 -(4 -methoxyphe-nyl)phenoxy]ethyl methacrylate] (n=2), the melting temperature of the isotactic polymers is almost independent of the spacer length. In contrast, the syndiotactic polymers melt with a large odd-even alternation. However, only the syndiotactic polymers with at least four carbons in the spacer exhibit an enantiotropic smectic mesophase, which occurs over only a very narrow temperature range. The greater order of the isotactic polymers is evidently due to the greater segmental mobility of isotactic versus syndiotactic polymethacrylate backbones [246, 247]. 

Surprisingly, the mesogenic groups of syndiotactic [(rr) = 0.70-0.78] poly 6-[4 -(4 -methoxyphenoxycarbonyl)phenoxy]-hexyl methacrylate s are not in the proper 

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Millan (98) studied the effect of tacticity on the ionic dehydrochlorination and chlorination of PVC. For the dehydrochlorination reaction, both the reaction rate and the polyence sequence distribution depend markedly on the syndiotactic content. Chlorination appeared to be easier through heterotactic parts than through syndiotactic sequences as shown by C-NMR. 

Millan and coworkers (99-101) also studied the effect of tacticity on the nucleophilic substitution reactions of PVC. Sodium thiophenate and phenol were used for these reactions. The central chlorine in isotactic triads and, to a lesser extent, in heterotactic triads was found to be most reactive. It was concluded that initiation of degradation may occur by normal structures, and polyene build-up may be favored by syndiotic sequence. This... 

The term tactidty refers to the configuration of polymer chains when their constituent monomer residues contain a steric center. Figure 1.8 illustrates the three principal classes of tacticity as exemplified by polypropylene. In isotactic polypropylene, the methyl groups are all positioned on the same side of the chain, as shown in Fig. 1.8 a). In syndiotactic polypropylene, the methyl groups alternate from one side to the other, as shown in Fig. 1.8 b). Random placement of the methyl groups results in atactic polypropylene, which is shown in Fig. 1.8 c). We can readily observe the effects of tacticity on the properties of polypropylene isotactic polypropylene is hard and stiff at room temperature, syndiotactic polypropylene is soft and flexible, and atactic polypropylene is soft and rubbery. 

The effect of tacticity of the polymeric backbone on properties of POCT-4 has been searched for [26], Figure 4 shows curves for PMCS-4 with different backbone structure, obtained by the diffe-rential scanning calorimetry (DSC) method (curves 1 - 4). For atactic polymer the only transition, corresponded to Tg, is observed on curve 1, above which, in accordance with the data of X-ray dif-fraction analysis, the polymer is amorphous (two amorphous haloes at 20 = 8-11° and 20-35°). As the polymeric backbone is enriched with... 

Results and Discussion. As a first step in the analysis of the absorption behaviour of SAN copolymers, the effects of tacticity and molecular weight on the extinction coefficients of well-characterized polystyrene samples were investigated. 

A new correlation will be developed in Section 6.B, to enable the prediction of reasonable values of Tg without requiring and thus being limited by the availability of group contributions. This correlation will account for the effect of the composition and the structure of a polymer, as reflected in the chain stiffness and the cohesive forces between different chains, on Tg. A new correlation will be presented for the effects of the average molecular weight on Tg in Section 6.C. The effects of plasticization on Tg will be discussed in Section 6.D. A new correlation will be presented for the effects of crosslinking on Tg in Section 6.E. The effects of tacticity on Tg will be discussed in Section 6.F. Secondary relaxations will be discussed in Section 6.G. The crystalline melting temperature will be discussed in Section 6.H. Finally, the ratio Tm/Tg... 

One example of the effects of tacticity on polymer properties is that stereoregular polymers can have high crystallinity, while atactic polymers are amorphous. This tendency is the result of the greater ease of packing stereoregular chains into a periodic crystalline lattice. 

Tacticity can also have a large effect on Tg, and hence on all properties which depend on Tg. Literature data [100,144-152] for the effects of tacticity on Tg are summarized in Table 6.6. 

The effects of tacticity on Tg have been studied most extensively for acrylic polymers, whose structural variants make up 15 of the 20 polymers listed in Table 6.6. 

The Tg s of styrenic polymers do not vary much with tacticity. The effects of tacticity on the properties of these polymers are mainly due to its effects on the crystallinity. For example,... 

The effects of tacticity on Tg are associated intimately with its effects on chain stiffness, which was shown earlier to be a key factor in determining Tg. These effects are difficult to quantify entirely in terms of empirical quantitative structure-property relationships, and require instead examining chain stiffness from a computationally more sophisticated perspective. 

The use of more sophisticated calculations based on rotational isomeric state theory [9,10] allows calculation of CM(T), but this capability still does not enable us to predict Tg accurately since CJTg) cannot be calculated without having some idea of what Tg should be before the calculation. Despite this limitation, Equation 6.18 and the general theoretical framework which led to its development hold out the hope that further extensions of this approach or similar methods may be able to allow accurate predictions of the effects of tacticity on Tg in the future. 

Finally, it is worth noting that Suzuki et al [47] have provided a theoretical analysis of the effects of tacticity variations on Tg, treating homopolymers containing triads of different tacticities as steric copolymers . 

Recently, high resolution solution and solid state 29Si NMR studies have been conducted on the polysilanes (RR Si)w124 127. It is obvious that this technique will be a powerful method for determining the microstructure of these polymers. Figure 12 shows a comparison between the 29SiNMR spectrum of poly(methyl-n-hexylsilane) and poly(di-n-hexylsilane)124. In the former case the effects of tacticity can be seen in the fine structure of the spectrum whereas in the latter case for which this is impossible only a single peak is observed. 

There are some important structural aspects of the polymer which are necessary to take into account in the analysis of the polymer behavior in mixture solvents, such as its polarity, chemical structure, microtacticity, molecular weight. The analysis of these properties shows that they are determinant factors in preferential adsorption phenomena involved. It has been pointed out that the effect of tacticity, and particularly the molecular weight, is a complex problem. In the case of poly(2-vinylpyridine), when the polar solvent is preferentially adsorbed, preferential solvation is independent of molecular weight but when the non-polar solvent is adsorbed, there is a dependence on the molecular weight. ... 

As yet, no comparison has been made of the effect of tacticity on molecular dimensions, and the validity of using the a-PPs as molecular mass standards for i-PP remains to be proved. The tacticity of PP is readily determined by H- and C-NMR spectroscopy (see Figure 4.6). 

Liquori et al. [23] first discovered that isotactic and syndiotactic PMMA chains form a crystalline stereocomplex. A number of authors have since studied this phenomenon [24]. Buter et al. [25,26] reported the formation of an in situ complex during stereospecific replica polymerization of methyl methacrylate in the presence of preformed isotactic or syndiotactic PMMA. Hatada et al. [24] reported a detailed study of the complex formation, using highly stereoregular PMMA polymers with narrow molecular weight distribution. The effect of tacticity on the characteristics of Langmuir-Blodgett films of PMMA and the stereocomplex between isotactic and syndiotactic PMMA in such monolayers at the air-water interface have been reported in a series of papers by Brinkhuis and Schouten [27,27a]. Similar to this system, Hatada et al. [28] reported stereocomplex formation in solution and in the bulk between isotactic polymers of / -(+)- and S-(—)-a-methylbenzyl methacrylates. 

Conformational factors. The most important conformational factor is the tacticity of vinyl-type polymers. A polymer such as poly(methyl methacrylate) can have quite different values of Tg, depending on whether it is isotactic, syndiotactic, or atactic. See Table 3 for a collection of literature data (1) on the effects of tacticity on Tg. A theoretical analysis of the effects of tacticity variations on Tg has been provided (52). 

We can illustrate the effect of tacticity by considering density. The more crystalline the polymer, the higher its density. As shown in Table 1-10, distinct... 

It had been known that PMMA was compatible with PVC under some conditions, but contrary to earlier reports it has recently been found that a wide range of polyacrylates and polymethacrylates show compatibility with PVC. Such polymers are believed to be compatible due to a specific interaction between the carbonyl group in the ester and the hydrogen or halogen in the PVC. Similarly, it was known that poly(vinylidene fluoride) was compatible with PMMA and poly(ethyl methacrylate), it has now also been shown to be compatible with poly(vlnyl acetate), poly(vinyl methyl ketone), and polyacrylates. This work has also been extended to show the effect of tacticity on the compatibility of poly(ethyl methacrylate) where ail isomers are compatible but the isotactic form phase separates on heating."... 

During the period covered by this article a number of books and review articles have been published. Some of these are fairly general. Others deal with one polymer, for example polyisoprene, or one aspect of the subject such as the effects of tacticity in polymer reactions or pKjlymer modifications with polymerizable monomers. ... 

Chemical modification of poly(vinyl chloride) (PVQ provided an important feature in a recent symposium devoted to this polymer. Contributions included the effects of tacticity on ionic dehydrochlorination and chlorination of PVC, and chemical methods, such as isotopic chlorine exchange, for the determination of labile chlorine. ... 

The effect of tacticity on Tg may be significant, as illustrated in Table 8.12 (140,141). Karasz and MacKnight (141) noted that the effect of tacticity on Tg is expected in view of the Gibbs-DiMarzio theory (Section 8.6.3.1). In disub-stituted vinyl polymers, the energy difference between the two predominant rotational isomers is greater for the syndiotactic configuration than for the iso-... 

The miscibility between several (meth)acrylate polymers and phenoxy (PHE) has been reported.[155,156] PEMA was noted to be partially miscible and PBMA to be phase separated in PHE blends. [155] An investigation of the effect of PMMA s tacticity on the phase behavior of PMMA/PHE blends showed aPMMA, sPMMA and iPMMA to all exhibit miscibility with PHE. [156] The miscibility of PMMA with a vinylidene chloride-acrylonitrile (20 wt% AN) copolymer was shown to be a function of the PMMA tacticity (aPMMA and iPMMA miscible with the copolymer and sPMMA immiscible). [157] Hsu et fl/.[158] reported the effect of tacticity of PMMA on the miscibility with PVAc. This study showed a limited effect of tacticity and solvent choice on the phase behavior of PMMA/PVAc blends and all blends were analyzed as being immiscible based on the observation of two Tgs. Modest areas of miscibility were established for MA, EA, nBA, and 2-ethylhexyl acrylate based acrylate-acrylic acid in the respective copolymers. No miscible combinations were found for MMA-AA or acrylate-MAA copolymers with vinyl acetateethylene (VAE) or PVAc. 

The effects of tacticity can be clearly identified from changes of the acoustic relaxation amplitude with temperature tacticity will lead to... 

Finally, Honeycutt" has applied blend PRISM theory at an atomistic RIS model level to study the effect of tacticity (stereochemical differences) on the phase behavior of a commercially important binary polymer mixture. Tacticity is found to result in significant changes of the computed spinodal boundaries, which serves to again emphasize the importance of monomer structure and local packing on the free energy of mixing. 

The effect of tacticity (i.e., the stereochemical arrangement of the units in the main chain of a polymer) on the properties of polymers and polymer blends has long been recognized with such basic differences as in the Tg, miscibility, crystallization, and blend characterization, including their mesoscale morphologies. In general, isotactic polymers (where all substituents are located on the same side of the polymer backbone) are semicrystalline in nature, whereas atactic polymers (where all substituents are placed randomly along the backbone) are amorphous. 

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