Isotactic polymers, glass transition

The value of the glass-transition temperature, T, is dependent on the stereoregularity of the polymer, its molecular weight, and the measurement techniques used. Transition temperatures from —13 to 0°C ate reported for isotactic polypropylene, and —18 to 5°C for atactic (39,40). 

In the crystalline region isotactic polystyrene molecules take a helical form with three monomer residues per turn and an identity period of 6.65 A. One hundred percent crystalline polymer has a density of 1.12 compared with 1.05 for amorphous polymer and is also translucent. The melting point of the polymer is as high as 230°C. Below the glass transition temperature of 97°C the polymer is rather brittle. 

In contrast, the curve E2 (isotactic polypropylene) is characteristic for partially crystalline polymers. The modulus is three decades higher than in an elastomer. At the glass transition temperature [T (2) 0 °C] the decay of the E modulus is small it does not drop to the lower level of the molten state before the melting point. 

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. 

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]. 

Polymerization to Isotactic Polymers high melting point COC-Copolymerization of Ethene and Cyclic Olefins to transparent polymers high glass transition point... 

Thin films of isotactic or elastomeric [51] polypropylene can be prepared from pellets by melt pressing. For this purpose several iPP or ePP pellets are placed between two sheets of thick aluminum foil and are subjected to pressure in in a hot press (T - 175°C pressure ca. 1-10 tons). Careful Wear appropriate safety shields and gloves After cooling the sample down to ambient temperature, the foil is unwrapped. If the polymer sticks to the foil, the film can be removed after cooling it down below the glass transition temperature e.g. in liquid nitrogen. Careful For... 

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.

Recent experiments on isotactic polypropene irradiation in the presence of methyl mercaptan were performed by Geymer [318-320]. Polypropene samples of initial Mw = 440,000 were irradiated to a total dose of 40 Mrad at 77°K, maintained at various temperatures and then heated with methyl mercaptan. The results given in Table 20 show that at —196°, only main-chain scission occurred as the temperature and time of ageing increased, Mw increased and gel was formed. The glass-transition temperature of polypropene is 0°C. Strong inhibition of gel formation was observed when the polymer was irradiated at room temperature in the presence of methyl mercaptan. Since p0/q0 was not determined in the... 

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