Isotactic forms

The stereoregularity of the backbone of the polymer chain also influences the as illustrated ia Table 2. The syndiotactic and isotactic forms are of scientific iaterest but have found few commercial uses. 

The isotactic form of propylene has better physical and mechanical properties than the three tactic form mixture (obtained from free radical polymerization). Isotactic polypropylene, in which all of the stereo cen-... 

PIB exists either as a low mw (about 12,000) viscous or atactic liq or as a cryst matl of about 1.00,000 mw. In this latter form the iso tactic (chain) type configuration predominates over the atactic and amorph forms (Ref 8). Since the isotactic form is the form designated by mil spec (Ref 2), its parameters are presented below ... 

Syndiotactic polypropylene became commercially available about ten years ago with the advent of single-site catalysts. Unlike its atactic and isotactic counterparts, its manufacture presented serious challenges to polymer scientists and engineers. Even under the best conditions, its syndiotacticity rarely exceeds 75%, based on pentad sequences. It typically has both a lower melting point (approximately 138 °C relative to approximately 155 to 160 °C) and density (0.89 g/cm3 relative to 0.93 g/cm3) than isotactic polypropylene. Syndiotactic polypropylene crystallites have a much more complex structure than the isotactic form, which impedes its crystallization. Therefore, in general, the syndiotactic form of polypropylene crystallizes very slowly. 

Lactide (LA), the cyclic diester of lactic acid, has two stereogenic centers and hence exists as three stereoisomers L-lactide (S,S), D-lactide (R,R), and meso-lactide (R,S). In addition, rac-lactide, a commercially available racemic mixture of the (R,R) and (S,S) forms, is also frequently studied. PLA may exhibit several stereoregular architectures (in addition to the non-stereoregular atactic form), namely isotactic, syndiotactic, and heterotactic (Scheme 15). The purely isotactic form may be readily prepared from the ROP of L-LA (or D-LA), assuming that epimerization does not occur during ring opening. The physical properties, and hence medical uses, of the different stereoisomers of PLA and their copolymers vary widely and the reader is directed to several recent reviews for more information.736 740-743... 

This production of stereoregular structures has been known for sometime and is especially strong for vinyl ethers. Several general observations have been noted. First, the amount of stereoregularity is dependent on the nature of the initiator. Second, stereoregularity increases with a decrease in temperature. Third, the amount and type (isotactic or syndiotactic) are dependent on the polarity of the solvent. For instance, the isotactic form is preferred in nonpolar solvents, but the syndiotactic form is preferred in polar solvents. 

There are striking differences in physical properties between the atactic and isotactic forms. The atactic material is soft, elastic, somewhat sticky, and rather soluble in solvents such as 1,1,2,2-tetrachloroethane. Isotactic polypropene is a hard, clear, strong crystalline polymer that melts at 175°. It is practically insoluble in all organic solvents at room temperature, but will dissolve to the extent of a few percent in hot 1,1,2,2-tetrachloroethane. That the difference between the atactic and isotactic polymers arises from differences in the configurations of the methyl groups on the chains is shown in a... 

The polymerization is highly stereoselective. Either the isotactic form or the syndiotactic form may be made, by selecting the proper Ziegler-Natta catalyst. 

If the different tactic configurations of a single polymer, for example, poly(methyl methacrylate), are considered the lowest value of Tg corresponds to the isotactic polymer. At T < Tg the specific volume of the isotactic polymer is lower than that of the atactic one, and the free volume fraction is the same for both polymers therefore the volume occupied will be less in the isotactic polymer. Nevertheless, at T > Tg, both tactic configurations have similar specific volume consequently the temperature at which the free volume is equal to 0.025 of the total volume is lower in the isotactic form than in the atactic one. 

The combination, VCl4/AlEt2Cl(Al/V = 2—20), at low temperatures (—78°C) gives the syndiotactic form of polypropene [130] the rate is proportional to vanadium concentration. Addition of AlEt3 to the catalyst changes the structure of the polymer-to the isotactic form. 

Passing now to a brief examination of some properties of PMA beyond the conformational transition and going back to the interesting point of how stereoregularity influences the equilibrium physicochemical properties of PMA in solution, some recent results deserve comment. Differences in the electrophoretic mobility of isotactic and syndiotactic PMA have been reported (77) indicating that the isotactic form is the slower moving one. 

C for the isotactic form. For this reason, among other analyses performed on polymers, the determination of steric structure is sometimes required. 

A number of halogenated polystyrenes are used in practice, either for specific applications as thermoplastics or in copolymers. Among the halogenated polystyrenes, the most common are the poly(chlorostyrenes). Poly(4-chlorostyrene) can be obtained in isotactic form (CAS 24991-47-7) or in syndiotactic form (CAS 62319-29-3) and is represented by the formula [-CH2CH(p-C6H4CI)-]n. Other poly(chlorostyrenes) include poly(2-chlorostyrene) with CAS 26125-41-7, and poly(3-chlorostyrene) with CAS 26100-04-9, CAS 116002-24-5 (isotactic), and CAS 107830-48-8 (syndiotactic). 

When the propylene (monomer) polymerises, the pendent CH can lie in either direction.The randomly disposed CHj group (atactic form) does not form good fibre. In the isotactic form the methyl groups are on the same side of the polymer backbone, but in syndiotactic arrangement the methyl groups lay attemately on either side of the polymer chain (Fig. 1-30). To produce this form of polypropylene, special catalysts are used. 

It was not until the technique of stereospecific polymerization of propylene was developed that it was possible to spin useful fibres from polypropylene. The isotactic form gives fibres with a melting point of 170°C and a tenacity of 7/8 g per denier, which compares with steel wire of equivalent cross-section. 

The FT-IR-ATR absorption spectrum of the control polypropylene (untreated) is shown in Figure 1. It is a typical polypropylene spectrum with absorption bands due to asymmetric and symmetric stretching of CH3 and CH2 groups around 2900 cm-1. The absorption bands at 1460 nd at 1380 cm-1 represent the asymmetric and symmetric bending of CH3 respectively. Absorption bands at 2878 cm-- - (CH3 stretching) and at 841 cm-- (Methylene rocking modes) suggest that the polypropylene membrane is of the isotactic form. 

Vinyl polymers of the type CH2CHX and vinylidene polymers of the type CH2CXY are capable of existing in different stereoregular forms. These are generally referred to as isotactic, syndiotactic, and atactic forms it is here assumed that the reader is familiar with this nomenclature and the specific structures associated with it. It has been shown that steric isomerism has little effect on the Tg of vinyl polymers but profoundly affects the Tg of vinylidene polymers.12 In the latter case, the isotactic form invariably is associated with the lowest Tg values. This result has also been rationalized on the basis of the Gibbs-DiMarzio theory, but again the details are beyond the scope of this book. 

Certain soluble catalysts polymerize propylene to highly syndiotactic polymers that are free of the isotactic form. Natta, Pasquon, and Zambelli (7 8) showed that VCl (or vanadium triacetylacetonate) in combination with AlRnX-type metal alkyls and anisole, polymerize propylene to highly syndiotactic polypropylene. These apparently homogeneous catalysts were used at low temperatures (-40 to -78 °C). Stereochemistry was initially explained by a mechanism involving repulsion between the methyl groups on the last added and new monomer unit to achieve orientation (83. 84). 

But, 1,2- or vinyl BR can be polymerised in the atactic, the syndiotactic or the isotactic form. Hence, five different configurations can be obtained by polymerisation reactions with butadiene (CH2=CH-CH=CH2) as monomer. The product obtained depends on the catalyst system used but is usually a mixture of 1,4 cis-, 1,4 trans- and atactic 1,2-BR. The commercial processes using Co-, Ni- or Ti-based catalyst systems, for instance, produce BR with a 1,4 cis-BR content higher than 90 %wt. But butyllithium initiated homopolymerisation of butadiene results in a product with 1,4 trans-BR contents up to 60 %wt. 

The 1,2- and 3,4-IR can in theory be polymerised in the atactic, the syndiotactic and in the isotactic form. Hence eight different IR modifications might be possible. 

These homopolymers have structure units as shown in Fig. 2.1, where the asterisk indicates asymmetric carbon atoms. Thus, polypropylene (PP), poly (butene-1) (PB1), and poly(4-methylpentene-l) (P4MP1) have different tactic forms. The most important commercial polyolefins are polyethylene, polyisobutene, and the isotactic forms, that is, iPP, iPBl, and iP4MPl. Polyisobutene was first polymerized by the IG Farbenindustries (BASF) in the late 1920s. Polyethylene was first polymerized by ICI in the late 1930s in a branched form (1). Linear polyethylene... 

The very large dipole moment of polymers results in strong intermolecular forces in solution. Atactic and isotatic polymers have different dipole moments. The dipole moment of the atactic poly(vinyl isobutyl ether) is 10% lower than that of the isotactic form, showing that the isotactic polymer adopts a more ordered structure with group dipoles tending to align parallel to each other. 

Stereochemistry is also important in determining properties of polyblands. For example, syndiotactic poly(methyl methacrylate) is miscible with poly(vinyl chloride) at certain concentrations, whereas the isotactic form is immiscible over the entire composition range [66]. 

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