Polystyrene Syndiotactic, tacticity

Remarkable differences in the G values of F12 were also recorded in atactic and syndiotactic polystyrene. The tacticity itself was also affected by the irradiation. Fligh-temperature irradiation with y-rays caused the reduction of isotacticity of isotactic PMMA... 

Although syndiotactic polystyrene (SPS) consists of pure polystyrene, the tacticity has a dramatic effect on chain dynamics and causes crystallization to occur. This will be examined in a separate chapter. 

Polymers that incorporate steric centers into their backbones can display various types of tacticity. The three principal types of tacticity are isotactic, syndiotactic, and atactic, as illustrated in Fig. 1.8 for polypropylene. Other polymers that display tacticity include polystyrene and poly a-olefins,... 

Small amounts of iso tactic polystyrene have been synthesized in the laboratory using noncommercial polymerization techniques. These polymers are capable of partially crystallizing, albeit at a very slow rate. Syndiotactic polystyrene was available commercially for several years, but its continued production proved unprofitable. 

A third factor influencing the value of Tg is backbone symmetry, which affects the shape of the potential wells for bond rotations. This effect is illustrated by the pairs of polymers polypropylene (Tg=10 C) and polyisobutylene (Tg = -70 C), and poly(vinyi chloride) (Tg=87 C) and poly(vinylidene chloride) (Tg =- 19°C). The symmetrical polymers have lower glass transition temperatures than the unsymmetrical polymers despite the extra side group, although polystyrene (100 C) and poly(a-meth-ylstyrene) are illustrative exceptions. However, tacticity plays a very important role (54) in unsymmetrical polymers. Thus syndiotactic and isoitactic poly( methyl methacrylate) have Tg values of 115 and 45 C respectively. 

Although PS is largely commercially produced using free radical polymerization, it can be produced by all four major techniques—anionic, cationic, free radical, and coordination-type systems. All of the tactic forms can be formed employing these systems. The most important of the tactic forms is syndiotactic polystyrene (sPS). Metallocene-produced sPS is a semicrystalline material with a of 270°C. It was initially produced by Dow in 1997 under the trade name Questra. It has good chemical and solvent resistance in contrast to regular PS that has generally poor chemical and solvent resistance because of the presence of voids that are exploited by the solvents and chemicals. 

The tacticity of anionically prepared polystyrenes has been the subject of extensive study by a number of groups of workers, mostly by means of 13C-NMR spectroscopy. From a study of the aromatic Cl resonances, Matsuzaki and coworkers found 234) that there is a tendency towards syndiotacticity when using -butyl-Iithium in toluene as initiator. From the sensitivity of the CMR spectrum to the nature of the solvent employed it was concluded that the polymerization did not conform to Bernoullian statistics. Randall examined the methylene resonances in the CMR spectrum and concluded that butyllithium initiated polystyrene is essentially atactic 235) and that propagation is Bernouillian. Uryu et al.236) examined polystyrene... 

The affect of polymer stereoregularity in the chains on the PAL data has also been studied. Hamielec et al [56] found what appears to be an increased lifetime (hole size) with increased randomness of the chain configuration in a series of polyvinlychloride (PVC) polymers, despite the large degree of scatter in the sample (probably due to the fact that a series of commercially available products were used.). They however found little correlation with tacticity in polypropylene. More recently a PAL study on a series of very well characterized polystyrene and poly(p-methlystyrene) samples of differing tacticity [57] was performed. In addition to finding that the polystyrene samples have smaller free volume holes than the poly(p-methylstyrene) samples, they found that the syndiotactic samples had broader hole distributions than the attactic samples. 

In specifying the tacticity of the polymer, the prefixes it and st are placed before the name or structure to indicate isotactic and syndiotactic structures, respectively. For example, it-polystyrene means that the polystyrene is isotactic. The absence of these terms denotes that the polymer has an atactic structure. 

In polystyrene. Cl is split into three peaks (upfield intensities with ratio of 1 1.3 1.8), reflecting the presence of isotactic, heterotactic and syndiotactic triads in the chain [56]. In polyAOTcp, the aromatic carbon Cl is not affected by tacticity, and is not split. In the copolymer. Cl of styrene units is split into four peaks (1 1.2 3.3 1.7) whereas Cl of AOTcp units is split into two peaks (upfield intensities of 1.5 1). Similarly, the carbonyl carbon is split into two peaks in polyAOTcp (relative upfield ratios 1 1.6), and into three peaks in... 

Diene polymerization may involve either or both of the double bonds. Geometric and structural isomers of butadiene, for example, are indicated by using appropriate prefixes — cis or irons, 1,2 or 1,4 — before poly, as in cw-l,2-poly(l,3-butadiene). Tacticity of the polymer may be indicated by using the prefix i (isotactic), s (syndiotactic), or a (atactic) before poly, such as 5-polystyrene. Copolymers are identified by separating the monomers involved within parentheses by either alt (alternating), b (block), g (graft), or co (random), as in poly(styrene-g-butadiene). 

The highly ordered planar stmctures, as obtained by stereospecific polymerization, were mentioned in Section 2.3. In contrast to the branched chains, we deal here with linear chains where side substitute groups appear regularly on the same side of the plane, or alternatively. These structures are named isotactic or syndiotactic, respectively, and the degree of order is termed "tacticity." In both cases, good conditions for crystallization exist, even when the polymer itself is known as difficult to crystallize (as in the case of polystyrene or polymethylmethacrylate). Any orderless structure is named atactic. 

Tacticity a property that describes the steric order of a polymer chain. Polymers with a unique way of coupling the monomer units are called isotactic. If the coupling of monomer units varies, but in a regular way, the polymers are called syndiotactic. Atactic polymers are polymers with an irregular steric structure. Polystyrene, for example, can adopt all types of tacticities, depending on the process of synthesis. 

Ti(0-n-Bu)4 systems, the tacticity of the obtained polystyrene strongly depends on the support material (i.e., the presence of Cl in the carrier). When MgCh-supported Ti(0-n-Bu)4 is activated with MAO, the soluble part of the catalyst in toluene can afford sPS, whereas the insoluble part of the catalyst gives iPS. This is also true for the Ti(0- -Bu)4/Mg(0H)Cl system when activated with MAO. For Ti(0-n-Bu)4/Mg(OH)2/MAO, which has no Cl in the carrier, the insoluble part gives syndiotactic polystyrene, and the soluble part is almost inactive for styrene polymerization. For the TiCls (AA)/MAO-system (AA = aluminum-activated), iPS (95% mmmm pentad, melting point 221 °C) and sPS are obtained from the insoluble and soluble part of the catalyst respectively. ... 

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