Syndiotactic form

Poly(acrylic acid) and Poly(methacrylic acid). Poly(acryHc acid) (8) (PAA) may be prepared by polymerization of the monomer with conventional free-radical initiators using the monomer either undiluted (36) (with cross-linker for superadsorber appHcations) or in aqueous solution. Photochemical polymerization (sensitized by benzoin) of methyl acrylate in ethanol solution at —78° C provides a syndiotactic form (37) that can be hydrolyzed to syndiotactic PAA. From academic studies, alkaline hydrolysis of the methyl ester requires a lower time than acid hydrolysis of the polymeric ester, and can lead to oxidative degradation of the polymer (38). Po1y(meth acrylic acid) (PMAA) (9) is prepared only by the direct polymerization of the acid monomer it is not readily obtained by the hydrolysis of methyl methacrylate. 

Pure amorphous polymers, being homogeneous materials, are transparent. Atactic polystyrene is a good example. The crystalline syndiotactic form is not transparent. Alack of transparency does not necessarily indicate crystallinity, however. It can also be caused by inorganic fillers, pigments, gas bubbles (as in a foam), a second polymer phase, etc. 

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. 

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. 

Tacticity of the PVC varies according to the particular reaction conditions but generally manufactures favor a syndiotactic form with many PVC materials being about 50%i sPVC. The reported amount of crystallinity is in the range of 5%i-10%i. This allows for a material with some strength, but one with sufficient amorphous regions to retain good flexibility. 

Poly(acrylic acid) and Poly(rnethacrylic acid). Poly(acrylic acid) (PAA) may be prepared by polymerization of the monomer with conventional free-radical initiators using the monomer either undiluted (widi cross-linker for superadsorber applications) or in aqueous solution. Photochemical polymerization (sensitized by benzoin) of methyl acrylate in ethanol solution at —78°C provides a syndiotactic form that can be hydrolyzed to syndiotactic PAA. 

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

Polystyrene (PS) in its atactic and syndiotactic forms is a brittle thermoplastic, even in an orientated state [4]. To improve the toughness of aPS, impact modification has been practised for a long time, either by polymerizing the styrene in the presence of a polybutadiene rubber leading to high-impact polystyrene, commonly called HIPS, or by blending the polystyrene with multi-block copolymers, mainly of the styrene-butadiene-styrene (S-B-S) type. 

PS, in either its atactic or syndiotactic form, is a polymer which shows no segmental mobility of chain segments below its glass transition temperature. Secondary relaxation processes which can be attributed to mobility in the main chain are missing. Therefore, these materials do not exhibit long-range energy... 

It has been established from the polymerization of l-dj-deutero-propene that the double bond opens in the cis direction [71], in conformity with the proposed mechanism. A repetition of this process with the monomer molecules approaching the active centre in the same direction would give the isotactic polymer while alternate approach to mirror im e positions would give the syndiotactic form. 

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. 

At least two features of these potentiometric plots (7) deserve comment. The isotactic polyacid is seen to behave as a weaker acid over the whole range of degrees of neutralization, a, as compared with the syndiotactic form. Similar results have also been obtained by Nagasawa et al. (2), and an indication of this fact was earlier given by Loebl and O Neill (3). It is interesting to recall that no difference has been observed in the potentiometric behavior of stereoisomeric samples of poly-(acrylic acid) (4). 

Polymer molecules may be linear or branched, and separate linear or branched chains may be joined by crosslinks. Extensive crosslinking leads to a three-dimensional and often insoluble polymer network. Polymers in which all the monomeric units are identical are referred to as homopolymers those formed from more than one monomer type are called copolymers. Various arrangements of the monomers A and B in the copolymer molecules (Fig. 8.1) can be produced with consequent effects on the physical properties of the resulting polymer. Synthetic polymers may have their main chains substituted in different ways, depending on the conditions of the reaction, such that atactic (random), isotactic or syndiotactic forms are produced, as diagrammatically represented in Fig. 8.1. 

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). 

The cationic polymerization of several para-substi-tuted a-methylstyrenes initiated by various Friedel-Crafts catalyst-cocatalyst combinations has been studied for the effects of catalyst type, monomer substituent and reaction solvent polarity on polymer structure and properties. By using solvent mixtures, the tacticity of the resulting polymers could be varied over a wide range, the syndiotactic form being favored in the more polar mixtures. 

Both the isotactic and the syndiotactic forms of polypropylene are known as stereoregular polymers, becanse each is characterized by a precise stereochemistry at the carbon atom that bears the methyl group. There is a third possibility, shown in Figure 7.17c, which is described as atactic. Atactic polypropylene has a random orientation of its methyl groups it is not a stereoregular polymer. 

The 1,2-products can exist in the stereoregular isotactic or syndiotactic forms and the irregular atactic forms. The stereoregular forms are rigid, crystalline materials, and the atactic forms are soft elastomers. 

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