Vinyl polymers Polystyrene, syndiotactic

Draw a short segment of each polymer (a) isotactic poly(vinyl chloride) (b) syndiotactic polyacrylonitrile (c) atactic polystyrene. 

Heterogeneous Emulsion dispersion Suspension Precipitation Solid catalyzed Vinyl polymers (styrene, MMA, PVC) Vinyl polymers (styrene, MMA, PVC) Vinyl polymers (PVC, PAN, PVDC) Polyacetals, vinyls Polyolefins, syndiotactic polystyrene Polyamides, solid state polymerization... 

Syndiotactic polystyrene, s-PS, is a highly stereoregular, semicrystalline vinyl polymer that normally melts at 270 °C [45,46], S-PS shows a large number of crystalline polymorphs [47-50] obtained by melt crystallization and solvent-exposure techniques. However, s-PS assumes only two distinct crystalline conformations [all trans, planar zigzag (... tttttttt...) and 2 -helical (... ttggttgg. ..) t= trans and g = gauche], which are characterized by fiber repeats of... 

Until 2003, Chen s [28], Qu s [29-31], and Hu s [32] groups independently reported nanocomposites with polymeric matrices for the first time the. In Hsueh and Chen s work, exfoUated polyimide/LDH was prepared by in situ polymerization of a mixture of aminobenzoate-modified Mg-Al LDH and polyamic acid (polyimide precursor) in N,N-dimethylactamide [28]. In other work, Chen and Qu successfully synthesized exfoliated polyethylene-g-maleic anhydride (PE-g-MA)/LDH nanocomposites by refluxing in a nonpolar xylene solution of PE-g-MA [29,30]. Then, Li et al. prepared polyfmethyl methacrylate) (PMMA)/MgAl LDH by exfoliation/adsorption with acetone as cosolvent [32]. Since then, polymer/LDH nanocomposites have attracted extensive interest. The wide variety of polymers used for nanocomposite preparation include polyethylene (PE) [29, 30, 33 9], polystyrene (PS) [48, 50-58], poly(propylene carbonate) [59], poly(3-hydroxybutyrate) [60-62], poly(vinyl chloride) [63], syndiotactic polystyrene [64], polyurethane [65], poly[(3-hydroxybutyrate)-co-(3-hydroxyvalerate)] [66], polypropylene (PP) [48, 67-70], nylon 6 [9,71,72], ethylene vinyl acetate copolymer (EVA) [73-77], poly(L-lactide) [78], poly(ethylene terephthalate) [79, 80], poly(caprolactone) [81], poly(p-dioxanone) [82], poly(vinyl alcohol) [83], PMMA [32,47, 48, 57, 84-93], poly(2-hydroxyethyl methacrylate) [94], poly(styrene-co-methyl methacrylate) [95], polyimide [28], and epoxy [96-98]. These nanocomposites often exhibit enhanced mechanical, thermal, optical, and electrical properties and flame retardancy. Among them, the thermal properties and flame retardancy are the most interesting and will be discussed in the following sections. 

The most obvious effect of stereoregularity in polymers concerns their crystallizability and hence the potential existence of a melting transition with a concomitant heat of fusion. It is generally accepted that with one or two possible exceptions (eg. polyvinyl alcohol), substantial stereoregularity is required to permit crystallization in vinyl polymers and, indeed, the appearance of a melting transition is sometimes adopted as a crude assay of synthetic procedures. It should be noted, of course, that the absence of a fusion transition cannot in itself be taken as evidence for atacticity, since thermal and solvent exposure history strongly influence the extent of crystallization in isotactic and syndiotactic systems. For example, isotactic polystyrene (T = 240°C) can be bulk annealed to yield material with a degree %f crystallinity (x. ) between 0.4 and 0.5, and... 

Certain regularities have been observed in the T s of substituted vinyl polymers of the type 4CH2CXY-M9). Thus, if X=H (i.e. monosubstituted vinyl polymers) there is no effect of tacticity on the T s of the respective polymers. Examples are polystyrene (Y -(. ), polypropylene (-CH-), alkyl acrylates (-C00R) and in the longer branched o-olefTns (-(CH2) CH-). In contrast, in unsymmetrically disubstituted vinyl polymers there is a large effect of tacticity on T. The best documented examples are in the methacrylate polymers (X = -CH., Y = -COOR) and in the a-methyl styrenes (X - -CH, Y = -()>)." In the latter polymers, the T of the syndiotactic is invariably substantially above that of the isotactic isomer. This effect must ultimately be related to the conformational properties of the macromolecu-lar chains and as a first approximation may be regarded as an intra-molecular effect. 

There are other important commercial thermoplastics beyond polyolefins. There are the various vinyl polymers. Both atactic polystyrene and syndiotactic polystyrene have a Tg of 100 C. Syndiotactic polystyrene has a crystalline melting point of 270 °C. Poly(vinyl chloride) has both atactic (-85%) and syndiotactic (-15%) sections of chains depending upon polymerization conditions. Its Tg is 65 C and is higher than 200 °C. In addition to vinyl polymers, there is poly(methyl methacrylate), which is atactic and has Tg about 110 °C. 

The differences in reactivities in poly(vinyl alcohol)s between isotactic meso) and syndiotactic (trans acetals [26-28] is another example. In extending this to model compounds, reactions of stereo isomers of pentane-2,4-diol and heptane-1,4,6-triol with formaldehyde take place much faster for the meso than for the dl-diol portions [26-28]. Even more important are the steric effects imposed by restricted rotations. For instance, quatemizations of chloromethylated polyether sulfmies exhibit decreasing rates at high degrees of substitution. This can be attributed to restricted rotations of the polymeric chains, because this phenomenon is not observed with more flexible chloromethylated polystyrene under identical conditions [23, 24]. 

It is well-known that many polymers, synthetic and natural, form physical, thermoreversible aggregates in dilute solutions, whereas in moderately concentrated solutions gels can be formed. Examples are poly(vinyl chloride), polyacrylonitrile, poly(vinyl alcohol), atactic polystyrene, mixtures of syndiotactic and isotactic poly(methyl methacrylates), liquid crystalline polymers, gelatin, agarose, carrageenans etc. 

S. Cimmino, E. D. Pace, E. Martuscelli, C. Silvestre, D. M. Rice, and F. E. Karasz, Miscibility of syndiotactic polystyrene/poly(vinyl methyl ether) blends. Polymer, Vol. 34, 214-217, 1993. 

Table 17.4 shows the well-known fiber polymers, and a few which seem unsuitable as fiber material, namely polystyrene and poly(vinyl chloride). The latter finds limited fiber application, but a special syndiotactic grade with some crystallinity is then used. Polypropylene has a fairly low melting point but is nevertheless a large fiber product, because the material is cheap and versatile. Polyethylene is even lower melting and is used only as a superstrong fiber at ambient temperature. 

Polystyrene is imusual among commodity polymers in that we can prepare it in a variety of forms by a diversity of polymerization methods in several types of reaction vessel. Polystyrene may be atactic, isotactic, or syndiotactic. Polymerization methods include free radical, cationic, anionic, and coordination catalysis. Manufacturing processes include bulk, solution, suspension, and emulsion polymerization. We manufacture random copolymers by copolymerizing styrene directly vith comonomers containing vinyl groups. In addition, we can polymerize styrene in the presence of polymer chains containing unsaturation in order to create block copolymers. Crosslinked varieties of polystyrene can be produced by copolymerizing styrene vith difunctional monomers, such as divinyl benzene. 

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