Poly isotactic glass transition

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. 

Commercial poly(vinyl ethers) range from viscous oils to soft tacky resins and elastomeric solids (specific viscosity r)sp = 0.1-6). Due to the polymerization conditions (initiator, low-polarity medium), they are amorphous and atactic with a majority of isotactic triads [96,97], Glass-transition temperature is -34° C for PVM, -42° C for PVE, and - 19° C for... 

Isotactic poly(methylmethacrylate) is characterized by its low glass transition temperature (Table 1) [15,16], and susceptibility to induced crystallization in the presence of organic vapors—methanol in particular. 

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 12.6 Plot of the glass transition temperature as a function of log r, where x is the number of chain atoms or bonds in the backbone. Data for (-0-) poly(a-methyl styrene), (-A-) poly(methyl methacrylate) (- -) polystyrene, (- -) poly(vinylchloride), (-A-) isotactic poly-propylene, (- -) atactic polypropylene, and (- -) poly(dimethylsiloxane). (From Cowie, J.M.G., Eur. Polym. J., 11, 297, 1975. With permission of Pergamon Press.)... 

Very little research has been done on the relations between glass transition temperatures and tacticity. Atactic and isotactic poly(styrenes) almost always have the same glass transition temperatures, and this is also the case for at- and it-poly(methacrylate). The glass transition temperature of it-poly(methyl methacrylate) (42° C), on the other hand, is distinctly lower than that of the atactic product (103°C). 

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. 

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