Poly -a-methylsty rene

The polydispersity of polymers prepared in this way is usually very low for example, a value MJM of 1.05 was found for a sample of poly(a-methylsty-rene). Living polymers can also be used for the preparation of block copolymers after the consumption of the first monomer, a second anionically polymerizable monomer is added which then grows onto both ends of the initially formed block. By termination of the living polymer with electrophilic compounds the polymer chains can be provided with specific end groups for example, living polystyrene reacts with carbon dioxide to give polystyrene with carboxylic end groups. 

For polymers with low ceiling temperature, such as poly (a-methylsty-rene) and poly (methylmethacrylate), unzipping is the predominant degradative process. However, this is unimportant for pyrolysis of polypropylene at low temperature. Unzipping becomes more important above 800°C but still is not the dominant reaction. 

Patterns in TOF-SIMS spectra of polystyrenes. A. Poly(4-methylstyrene), B. Poly(a-methylsty-rene). (Reprinted with permission from Ref. 35, Copyright 1992 American Chemical Society)... 

Sedimentation experiments on semi-dilute solutions are appropriate and many experiments have been performed on neutral polymers like polystyrene and poly(a-methylsty-rene) in good solvents It has been found that the effective exponent Xj increases from 0.59 up to 0.8 as the concentration rises from 0.1 to 10%. Good solvents used in these experiments (benzene, bromobenzene and toluene) are far from athermal conditions (x — 0.45). Two monomers, belonging to a subchain of size and separated by n monomers, experience excluded volume effects when n > n, where iic oc (1 - 2x). As the concentration decreases, the number of monomer per subchain g, increases and excluded volume effects become more and more important. The effective exponent Xs, which is a combination of effective dynamic and static exponents tends monoti-cally to the asymptotic value 0.5 (g > He). Inversely, if the concentration increases, g decreases when g < He, the subchain exhibits purely Gaussian behaviour, and v = 0.5 which leads to oc and Sd °o This cross-over between excluded volume and Gaussian behaviour qualitatively explains the increase of Xj, if p increases. Detaib on the dependence of x, on the concentration can be found in Ref. 110. Whatever the exact value of the exponent, these experiments show that the frictional properties of semi-dilute solutions depend only on the concentration they are independent of the molecular weight of the polymer used 1. 

To avoid such ambiguity, these structures are designated poly(a-methylstyrene) (1), poly(o-methylsty-rene) (2), and poly(p-methylstyrene) (3), respectively. 

It has been shown that in highly crystalline polymers having multiple transitions, such as poly(ethylene terephthalate), the glass transitions may be determined only by gas chromatography [170, 174, 177, 204]. The glass transition was also detected by gas chromatography in the copolymers acrylonitrile-vinyl acetate [201], acrylonitrile-a-methylsty-rene and the terpolymers of these monomers with vinyl acetate [207], polystyrene-butadiene [199], and styrene-tetrahydrofuran block copolymers [208]. 

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