Stereoregularity measurement

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). 

Analytical and test methods for the characterization of polyethylene and PP are also used for PB, PMP, and polymers of other higher a-olefins. The C-nmr method as well as k and Raman spectroscopic methods are all used to study the chemical stmcture and stereoregularity of polyolefin resins. In industry, polyolefin stereoregularity is usually estimated by the solvent—extraction method similar to that used for isotactic PP. Intrinsic viscosity measurements of dilute solutions in decahn and tetraHn at elevated temperatures can provide the basis for the molecular weight estimation of PB and PMP with the Mark-Houwiok equation, [rj] = KM. The constants K and d for several polyolefins are given in Table 8. 

As a measure of the stereoregularity, an index EQ-H% was defined as the precent of the equatorial acetal protons to the total acetal protons. Figure 2 illustrates the temperature dependence of EQ-H% s of the polymer obtained in toluene (A), methylene chloride (B), and 1-nitropropane (C). No significant difference is observed at... 

P-plastomers are semicrystalline, elastomeric copolymers composed predominantly of propylene with limited amounts of ethylene [21]. The concentration of ethylene is typicaUy less than 20 wt%. The placement of the propylene residues is predominantly in a stereoregular isotactic manner. This leads to the crystallinity (which is critical) in the copolymer. The extent of the crystallinity is attenuated by errors in the placement of the propylene and by the incorporation of ethylene. These two strucmral feamres contribute to lower the crystallinity, as measured by the heat of fusion, to less than 40 J g . Copolymers of propylene and ethylene, which have higher levels of crystallinity are... 

The polybutadienes prepared with these barium t-butoxide-hydroxide/BuLi catalysts are sufficiently stereoregular to undergo crystallization, as measured by DTA ( 8). Since these polymers have a low vinyl content (7%), they also have a low gl ass transition temperature. At a trans-1,4 content of 79%, the Tg is -91°C and multiple endothermic transitions occur at 4°, 20°, and 35°C. However, in copolymers of butadiene (equivalent trans content) and styrene (9 wt.7. styrene), the endothermic transitions are decreased to -4° and 25°C. Relative to the polybutadiene, the glass transition temperature for the copolymer is increased to -82°C. The strain induced crystallization behavior for a SBR of similar structure will be discussed after the introduction of the following new and advanced synthetic rubber. 

Rheo-optical Studies. One of the most useful techniques for studying the crystallization behavior of stereoregular polymers is rheo-optical measurements (11). 

A few reviews have dealt with the identification of synthetic polymers by Py-GC/MS [76]. In addition to compositional studies, applications of pyrolysis to synthetic polymers include sequence length characterization in copolymers [77] and tacticity measurements in stereoregular homopolymers [78]. 

The interaction may not be quite as strong as in the case of 2,1 insertion discussed above, but there will always be a tendency of the growing chain to arrive at an isotactic stereochemistry when 1,2 insertion occurs. One example of chain-end control leading to isotactic polymer was reported by Ewen [13] using Cp2TiPh2/alumoxane as the catalyst. The stereoregularity increased with lower temperatures at -45 °C the isotactic index as measured on pentads amounted to 52 %. The polymer contains stereoblocks of isotactic polymer. At 25 °C the polymerisation gives almost random 1,2 insertion and an atactic polymer is formed. 

The study of the stereoregularity of the polymers prepared, provides also Information about the stereoregulating mechanism. The probability of formation of the different types of sequences, was determined on the basis of the resonance of the quaternary carbon of pVP (12). The NMR spectrum performed at 15 MHz allows one to determine the concentration of triads. The values summarized In Table 4 do not agree with those expected for bernoullllan statistics. Hence, more than the last unit of the living chain Is Involved In the process. In order to obtain more precise Information about the process, It is necessary to measure the probability of formation of pentads. Such measurements are possible with spectra performed at 63 MHz (Figure 18). In spite... 

The results of measurements of unperturbed dimensions for a series of stereoregular PMMA are reported. Comparison of the results obtained with those predicted by previous RIS models show good agreement. 

Stereoregularity. The addition of monomer fixes the tacticity of the previous monomer unit. Syndiotactic structure has the adjacent chlorine atoms oriented to opposite sides of the carbon—carbon—carbon plane, whereas isotactic structure has the adjacent chlorine atoms oriented to same side of the carbon—carbon—carbon plane. The potential energy for syndiotactic conformation is 4.2—8.4 kj/mol (1—2 kcal/mol) lower than for isotactic conformation (74,75). Thus the ratio of propagation rates for syndiotactic to isotactic, kjk, must increase with decreasing temperature. Consequently, with decreasing polymerization temperature, the degree of syndiotacticity in PVC should increase. Measured amounts of syndiotacticity are illustrated in Figure 8... 

Currently this technology is of minor commercial significance, but stereoregular forms of numerous polyacrylates have been prepared and characterized These include poly(/-butyl acrylate) (138—141), poly(isopropyl acrylate) (142), and poly(isobutyl acrylate) (143,144). Carefully controlled reaction conditions are usually required to obtain polymers with some measurable degree of crystallinity. In nonpolar solvents the anionic polymerization of acrylates generally yields isotactic polymer, whereas in polar solvents syndiotactie polymerization is favored. The physical and chemical properties of the various forms are often quite different. A general review covers these and other aspects of the anionic polymerization of acrylates (145). 

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