Polystyrene stereochemistry

The stereoregularity of polystyrenes prepared by anionic polymerization is predominantly syndiotactic (racemic diad fraction P = 0.53-0.74) and the stereoregularity is surprisingly independent of the nature of the cation, the solvent, and the temperature, in contrast to the sensitivity of diene stereochemistry to these variables [3, 156]. The homogeneous alkyllithium-initiated polymerization of styrene in hydrocarbon media produces polystyrene with an almost random (i.e., atactic) microstructure for example, was 0.53 for the butyllithium/toluene system [3, 191, 192]. A report on the effect of added alkali metal alkoxides showed that polystyrene stereochemistry can be varied from 64% syndiotactic triads with lithium f-butoxide to 58% isotactic triads with potassium f-butoxide [193]. 

Neither PVC nor polystyrene is very crystalline and polystyrene often has poor mechanical strength. Both of these maybe results of the stereorandom nature of the polymerization process. The substituents (Cl or Ph) are randomly to one side or other of the polymer chain and so the polymer is a mixture of many diastereoisomers as well as having a range of chain lengths. Such polymers are called atactic. In some polymerizations, it is possible to control stereochemistry, giving (instead of atactic polymers) isotactic (where all substituents are on the same side of the zig-zag chain) or syn-diotactic (where they alternate) polymers. 

An atactic polymer is a regular polymer with macromolecules composed of a certain number of statistically distributed configurational units. The constitutional unit is a type of atom or group of atoms composing the macromolecule (e. g. —[CH2—CHPhJ- or —[CHPh— in polystyrene). The configurational unit is a constitutional unit with one or several stereoisomer-ic centres. These definitions would require a more detailed explanation. In this volume they will only rarely be used, the stereochemistry of jjolymers is a special branch of macromolecular chemistry. More information can be found in the original literature [2]. 

Table V compiles the ZIE product isomer ratios of reactions from Table I for which such information has been reported. With 0.5-2% cross-linked polystyrene supptxis, the isomer ratios are similar to those obtained from Wittig reactions in solution. The exceptions appear to be the reactions on more highly cross-linked supports shown in Table IV. There is a clear trend toward greater E selectivity as the degree of cross-linking of the polymer increases. This probably should be explained as an environmental effect, but comparison with solvent effects on stereochemistry of Wittig reactions in the literature reveals no tendency for aromatic solvents, structurally similar to polystyrene, to increase E selectivity.

HIPS) is produced commercially by the emulsion polymerization of styrene monomer containing dispersed particles of polybutadiene or styrene-butadiene (SBR) latex. The resulting product consists of a glassy polystyrene matrix in which small domains of polybutadiene are dispersed. The impact strength of HIPS depends on the size, concentration, and distribution of the polybutadiene particles. It is influenced by the stereochemistry of polybutadiene, with low vinyl contents and 36% d5-l,4-polybutadiene providing optimal properties. Copolymers of styrene and maleic anhydride exhibit improved heat distortion temperature, while its copolymer with acrylonitrile, SAN — typically 76% styrene, 24% acrylonitrile — shows enhanced strength and chemical resistance. The improvement in the properties of polystyrene in the form of acrylonitrile-butadiene-styrene terpolymer (ABS) is discussed in Section VILA. 

Impact polystyrene is produced commercially by dispersing small particles of butadiene rubber in styrene monomer. This is followed by mass prepolymerization of styrene and completion of the polymerization either in mass or in aqueous suspension. During prepolymerization, styrene starts to polymerize by itself, forming droplets of polystyrene with phase separation. When nearly equal phase volumes are obtained, phase inversion occurs, and the droplets of polystyrene become the continuous phase in which the rubber particles are dispersed. The impact strength increases with rubber particle size and concentration, while gloss and rigidity are decreasing. The stereochemistry of the polybutadiene has a... 

Styrsn St6r60Ch6miStry. The effect of counterion, solvent, and temperature on the stereochemistry of anionic polymerization of polystyrene is shown in Table 13. The principal conclusion is that the stereoregularity of polystyrenes... 

Table 13. Stereochemistry of Polystyrenes Prepared with Anionic Initiators ...

The physical and mechanical properties of a polymeric material critically depend on many factors, one of which is stereochemistry. Polymers that have chiral centers in the repeated unit can exhibit two structures of maximum order, isotactic and syndiotactic [27]. Sequential stereocenters of isotactic polymers are of same relative stereochemistry whereas those of syndiotactic polymers are of opposite relative configuration. Due to their stereoregularity, isotactic and syndiotactic polymers are typically crystalline, which is an important feature for many applications. Isotactic polymers are used in a wide range of applications. Typical examples include isotactic polyolefins and almost all natural polymers. In contrast, syndiotactic polymers have limited applications mainly due to their hard productivity and inherently alternating stereochemistry. The properties of syndiotactic polymers are usually similar to or in some cases better than isotactic counterparts according to the studies on syndiotactic polystyrene and other syndiotactic polyolefins [28]. Syndiotactic PLA is expected to be a versatile polymer with controllable stereochemistry. 

The production of durable functional products without using petroleum-based raw materials is a focus of much academic research today but it is also prioritized by many industries. Many questions still remain concerning the use, production and properties of bio-based and/or degradable polymers and whether or not they are more environmentally friendly than oil-based products. Polylactide is a bio-based compostable thermoplastic that is considered as one of the most promising materials for replacement of traditional volume plastics. The properties of polylactide can be tuned to resemble polystyrene, polyfethylene terephthalate) or polyolefins by controlling the stereochemistry by copolymerization or blending. This chapter reviews the life-cycle of polylactide based materials as well as the properties and applications. The recent trends in the area are also discussed. 

As an extension of this work, the same authors have used polystyrene-supported proline as a recyclable catalyst in the Morita-Baylis-Hillman reaction of a range of aryl aldehydes with methyl or ethyl vinyl ketone. These reactions were performed in the presence of imidazole and provided a series of Morita-Baylis-Hillman adducts in moderate to high yields (17 88%) combined with high enantioselectivities of up to 95% ee (Scheme 2.55). This study represented the first example of supported proline as heterogeneous catalyst for the Morita-Baylis-Hillman reaction. In addition, Zhou et al. reported that these reactions could be eatalysed by combinations of L-proline with chiral tertiary amines derived from various readily available chiral sources, such as L-proline or (5)-a-phenylethylamine. In these conditions, the Morita-Baylis-Hillman adducts were obtained in reasonable chemical yields (34-97%) and low to good enantioselectivities (12 83% ee). In this study, it was demonstrated that the proline stereochemistry was the sole factor to determine the eonfiguration of the newly formed chiral centre. 

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