Isotactic 4 loss

Figure 6 Loss tangent (<5) plotted as a function of complex modulus G for a series of syndiotactic and isotactic polypropylene. Reproduced with permission from Rojo et al. [32]. Copyright Wiley-VCH Verlag GmbH Co. KgaA. iPP, isotactic PP sPP, syndiotactic PP Met-iPP, metallocene-catalyzed PP (see Ref. [32] for full details).

Fig. 2.22. Dependence of the elastic modulus E and the mechanical loss factor 6 on temperature for various polymers. Curves 1 elastomer (statistical copolymer of ethylene and propylene) curves 2 isotactic polypropylene (semicrystalline)...

The increased ionic freedom between the propagating polymer ion and its gegen ion occurs concurrently with increased space separation between the two ion species. The studies of Schuerch and co-workers and of Yoshino and co-workers (98) with deuterated acrylates and by Natta and co-workers (99) with sorbic esters show that this increased separation allows trans addition to mono olefins and 1,4 trans addition to conjugated dienes before complete loss of isotactic steric control at the end of the chain. The increased freedom between the propagating ion and the less closely associated gegen ion appears to result in a distortion of the cyclic transition state which permits backside attack at the beta position of the incoming acrylate monomer and 1,4 attack on the incoming sorbate monomer. 

Fig. 5. Temperature dependence of the shear loss modulus of poly(pivaloyl-2-oxyethyl methacrylate) atactic (—), isotactic (---)...

A final example of a stereoselective heterogeneous catalytic system is the work of Laycock, Collacott, Skelton and Tchir.17 Layered double hydroxide (LDH) synthetic hydrotalcite materials were used to stereospecifically polymerize propylene oxide [PO] to crystalline isotactic and liquid atactic poly(propyleneoxide) [PPO]. These authors suggest that the LDH surface acts as other inorganic or organometallic coordination initiators or catalysts by providing specific surface orientations for propylene oxide monomer. X-ray powder diffraction showed some loss of crystallinity after calcination and X-ray photoelectron spectroscopy showed an enhancement of Mg/Al content due to restructuring of the Mg and A1 surface atoms. The surface was also rich in Cl ... 

FIG. 13.29 Frequency-temperature correlation map for polystyrene, (o, ) mechanical loss peaks ( , ) dielectric loss peaks (A, ) NMR narrowing and T, (i.e. spin-lattice relaxation time) minima for (open symbols) atactic an (filled symbols) isotactic polystyrenes. From Yano and Wada (1971). Courtesy John Wiley Sons, Inc. 

Sterically Restricted Poly (methacrylate ester)s. It was recognized by Okamoto and coworkers150 that the anionic polymerization of tri-phenylmethyl methacrylate (TrMA, 41) (Chart 8) at low temperature in the presence of an optically active initiator results in the formation of an isotactic, optically active polymer. The helical conformation of the backbone in these macromolecules is the result of steric interactions between the bulky trityl groups, as was shown by the loss of optical activity upon their conversion to methyl ester groups. This class of bulky... 

The catalysts which are useful for the preparation of isotactic polymers are heterogeneous, i.e., they are insoluble in the solvent, or diluent, in which they are prepared. Their activity and stereoregulating ability are greatly affected by the components, and method, used for their preparation. The inclusion of electron donors such as Lewis bases (e.g., amines, ethers, ketones, and esters) during preparation of the catalyst also can improve stereospecificity, often but not always with a loss of activity. 

Enhancement of mechanical properties is of interest only if it is not accompanied by a loss of other important properties of the blend. Of particular concern for such polymer blends is stiffness, because most means of increasing impact strength also reduce stiffness (14-19). But this is not the case for the iPS-fc-iPP-iPS-iPP blends studied here as seen in Table II. It is clear that the enhancement in toughness just described is not accompanied by a loss of stiffness, but it is essentially unaffected by the compatibilizer. And the stiffness of iPS-fc-iPP-iPS-iPP is higher than that of iPP and HIPS. The impact-modulus behavior seems to be due to the tough (or rigid) characteristics, morphologies of phases, and semicrystalline isotactic structure of each block in the iPS-b-iPP diblock copolymer. 

Fig. 34. Weight loss curves of isotactic polymethacrylic acid, o, 190°C 200°C x,...

The thermal degradation of irradiated isotactic PMMA has also provided information on the structure of the end groups formed at the site of main-chain scission [408]. The weight loss of non-irradiated PMMA at 250°C has been shown by Grassie and Melville [409] to be mainly due to depolymerization initiated at the carbon—carbon double bonds situated at the chain ends. A small proportion of randomly initiated depolymerization also occurs at this temperature. In agreement with this mechanism, the rate of volatilization has been found to be much higher for atactic than for isotactic PMMA, the latter having no double bonds at the chain ends. If 4.3 scissions per chain are produced by 7-irradiation in the isotactic sample, the rate of monomer evolution is identical to that of the initial unirradiated isotactic sample. This proves that chain ends of the type... 

Polypropylene is a polyolefin found in high concentrations in the plastic waste stream. Of the different types of PP, isotactic polypropylene is the one most widely used on a commercial scale and so is the type predominant in plastic wastes. Compared to PE, the backbone of the PP molecule is characterized by the presence of a side methyl group at every second carbon. This fact implies that half of the carbons in a PP chain are tertiary carbons and so, as a consequence of their higher reactivity, PP is thermally degraded at a faster rate than PE. Thus, as can be seen in Figure 4.7, the PP weight loss in TGA measurements starts at a lower temperature compared to both HDPE and LDPE. 

Common PS is atactic and amorphous. It has good optical clarity, low dielectric loss factor, modulus E = 3.2 GPa, strength o = 45-65 MPa, density p = 1050 kg/m and CUT = 50-70°C. Because of brittleness and low chemical resistance the demand for neat PS has decreased, and except for foaming, PS is rarely used. PS can also be polymerized into crystalline forms isotactic (iPS) or syndiotactic (sPS) with T = 230 or 272°C, respectively. The former was polymerized using Ziegler-Natta catalyst [Ishihara et al., 1986], while the latter using a single-site metallocene titanium-based catalyst [Imabayashi et ah, 1994]. 

Sample (% isotactic dyads) (gmi ) Heat of fusion (J g Degradation time (h) Weight loss (%)... 

The reaction is accompanied by a loss of molecular weight. Nitration of isotactic polystyrene yields a more crystalline product (about 1.6 N02/ring) than the parent compound. Here too, however, a loss in molecular weight accompanies the reaction. Polystyrene can be nitrated under mild conditions using acetyl nitrate. The product contains approximately 0.6 nitro groups per each benzene ring. ... 

The chlorotitanium calix[4]arene 83, 84 complexes were reported to be active using conventional heating and microwave (MW) irradiation for the ROP of LA under solvent-free conditions (Scheme 6.7). Nearly full conversion (95%) was observed after only 80 min under MW-heating versus 180 min for the thermal heating method. Unfortunately, a loss of control was observed with the PLAs molecular weights and PDIs. The lowest activity was observed for the dimeric complex 85, presumably due to the severe steric congestion at both titanium centres but resulting in isotactic enriched PLA up to 72% (Pm). ... 

For further characterization, degradation of the polymer was studied using the Mettler thermogravimetric apparatus. Figure 10 shows the weight loss curves for atactic PPBA when heated at 6°C/min. No appreciable weight loss occurred at temperatures up to 613°K. Isotactic polymers showed similar traces and no appreciable degradation occurred at temperatures less than 573°K. 

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