Poly syndiotactic glass transition

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. 

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. 

Figure 10-19. Dependence of the melting temperature Tm and glass transition temperature Tg of a 90% syndiotactic l,2-poly(butadiene) on the X-ray crystallinity, olx.

The atactic form of poly(methyl methacrylate) is amorphous and exhibits only one transition temperature (Tg). The stereoregular isotactic and syndiotactic forms are partially crystalline and undergo both a glass transition and melting. 

Special comphcations arise when the stereospecific homopolymers show different glass transitions for isotactic (1), syndiotactic (S), and heterotactic (H) chains. An example is the poly(methyl methacrylate). The iso- and syndiotactic stereoisomers of PMMA have glass transition temperatures of 315 and 400 K, respectively. Treating the stereoisomers as copolymers with a modified Barton equation of Fig. 7.70 ... 

Thus, it was not until 1990 that the group of Kaminsky and Arndt-Rosenau took a more detailed look at the homopolymerization of norbornene and the structures of the resulting polymers. Driven by the growing interest in copolymers with high norbornene contents and high glass transition (Tg) temperatures, as well as the unusual properties of PNBs, Arndt-Rosenau et al. used the hydrooligomerization technique to produce saturated model norbornene dimers and trimers with metallocene catalysts known to produce atactic, isotactic, and syndiotactic poly(a-olefins) (1-3, Table 16.1). 

Ute, K. Miyatake, N. Hatada, K. Glass transition temperature and melting temperature of uniform isotactic and syndiotactic poly(methyl methacrylate)s from 13 mer to 50 mer. Polymer 1995, 36, 1415-1419. 

The presence of a third transition between the Tg and Tm in the case of the isotactic poly(methyl chloroacrylate) has been commented on previously and evidence has been presented for the assignment of this phenomenon to the glass transition of a stereocomplex of isotactic and syndiotactic polymer molecules (3). It is not possible to confirm or reject this hypothesis on the basis of available evidence. 

The relaxation labelled a for the isotactic poly(methyl o chloroacrylate) in Table II is thought to correlate with the transition observed by DSC at 150° and is tentatively assigned to motions accompanying the glass transition of the "stereocomplex" postulated to occur in these polymers, composed of isotactic and syndiotactic chains. 

Tsige and Taylor s work on syndiotactic poly(methyl methacrylate) (68) indicated that atomistic molecular dynamic simulations can provide useful information about the nature of the glass transition in polymers via computational runs of modest size, especially if an uncomplicated force field is used in order to help reach the difiiisive regime with modest computational resources. It was also found that the subtraction of the oscillatory contribution to the motion is a useful tool for revealing the time variation of diffusive displacements. 

Figure 8. Dependence of the glass ( , O) and nematic-isotropic ( , ) pha.se transition temperatures of syndiotactic [(/r)=0.70-0.77] poly 6-[4 -(4"-me-Ihoxyphenoxycartxmyhphenoxy ]hexyl methacrylate) as a function of the number average degree of polymerization ( , ) and the inverse number average degree of polymerization (O, ) [45]. Infinite molecular weight transitions G 44 N 105 I.

The G-SmA-N-I transition temperatures of syndiotactic poly(6-[4 -(4"- -bu-toxyphenoxycarbonyl)phenoxyl)phenoxy]-hexyl methacrylate prepared by aluminum porphyrin initiated polymerizations also level off at approximately 25 repeat units [91]. Similarly, the glass and nematic-isotropic transition temperatures of poly[6-(4 -methoxy-4"- Z-methylstilbeneoxy)hexyl methacrylate] prepared by group transfer polymerization become independent of molecular weight at approximately 20 repeat units [48]. Both polymethacrylates reach the same transition temperatures as the corresponding polymers prepared by radical polymerizations, which have nearly identical tacticities. 

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