Syndiotactic polymers, glass transition temperatures

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. 

Polymerization. Poly (methyl methacrylate) was obtained commercially. The polymers of other methacrylates and their copolymers were prepared in toluene with 2,2 -azobisisobutyronitrile (AIBN) at 60 °C. All the polymers prepared free radically were syndiotactic or atactic. Isotactic poly(a,a-dimethylbenzyl methacrylate) was obtained using C6H5MgBr as the initiator in toluene at 0°C. Poly(methacrylic acid) was prepared in water using potassium persulfate at as the initiator 60 °C. The molecular weights, glass transition temperatures and tacticities of the polymethacrylates are summarized in Table I. 

Simultaneously, thermomechanical investigations were performed [28], obtained under continuous impact of compressing stress (100 g/cm3) on the sample in accordance with the technique [62], Glass transition temperatures, shown in the Tables, are determined from the primary deviation of thermomechanical curve run in the area of positive deformations. Depending on the spatial structure of the cycle, synthesized atactic and syndiotactic polymers are characterized by almost equal glass transition temperatures, Tg. 

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... 

Metallocenes and methylalumoxanes can further be used to synthesize isotactic polypropylene [70, 71], syndiotactic polypropylene [38], other propylene polymers or oligomers [72], ethylene/cycloolefin copolymers [10-13], syndiotactic polystyrene [14, 61], and ethylene/styrene copolymers [64]. Cycloolefin copolymers are amorphous, with high glass transition temperatures [10-13]. The syndiotactic polystyrenes are semicrystalline polymers with a glass transition temperature around 100 °C and a melting point of 270 °C [14]. 

Table 6.6. Glass transition temperatures of syndiotactic, isotactic and atactic polymers, in degrees Kelvin. Many of the values listed for the syndiotactic and isotactic polymers are extrapolations from measurements on series of polymers with differing tacticities. Some of the listed Tg values are averages of two or more published values. Some of the Tg values listed for atactic polymers differ from those listed in Table 6.2 because they are from different sources.

The control of PMMA stereochemistry is important because the glass transition temperature of PMMA strongly depends on the microstmcture [121], The measured for 99% mm PMMA is reported to be 50 °C, and that for PMMA with 96-98% r diads is 135 °C. To obtain PMMA with higher upper use temperature, polymers with the highest syndiotactic microstructure are desired hence the interest in developing anionic systems for MMA at higher temperatures [118, 119, 190]. 

Examples of these three types of structural arrangements are known in general, stereoregular polymers are synthesized by the use of coordination catalysts, whereas atactic polymers are formed by uncoordinated catalysts such as free radicals or free ions. Stereoregular polymers are often partially crystalline, and usually, even the isotactic and syndiotactic isomers have different properties. For example, isotactic poly(methyl methacrylate) (PMMA) has a glass-transition temperature of 35 °C, while that of the syndiotactic polymer is 105 °C. 

The mutual repulsion between substituents may cause some displacement. As a result, the plane of symmetry is bent in the form of a helix. This occurs also in biopolymers (double-helix of deoxyribonucleic acid (DNA)). Different stereoisomers have different mechanical and thermal properties. For example, atactic polystyrene is an amorphous polymer whereas syndiotactic polystyrene is a crystalline substance. The chemical design of macromolecules determines their properties as extent of crystallization, melting point, softening (glass transition temperature), and chain flexibility which in turn strongly influence mechanical properties of the materials. 

Catalysts that form ethylene-propylene copolymers can be used to produce a material known as ethylene-propylene rubber (EPR). If an isospecific catalyst for the formation of polypropylene incorporates a small amount of ethylene, a crystalline copolymer is formed that has a lower melting point than isotactic PP. If an aspecific catalyst is used, or if more ethylene is incorporated into the polymer, an amorphous ethylene-propylene rubber (EPR) is formed. EPR generally has a lower glass transition temperature than atactic PP, and is a useful material for low-temperature applications. Catalysts that form isotactic or syndiotactic polypropylene can also generate polymers possessing defined stereochemistry within the propylene units in an EP copolymer. 

Table 3. Glass-Transition Temperatures of Syndiotactic, Isotactic, and Atactic Polymers ...

Table 2. Glass-Transition Temperatures (°C) of Atactic, Syndiotactic, and Isotactic Methacrylic Ester Polymers ...

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