Monomer activity measurement conditions

This enantioselective mechanism is also in accordance with the elegant analysis and optical activity measurements by Pino et al.44,45 on the saturated propene oligomers obtained under suitable conditions with this kind of catalysts, proving that the re insertion of the monomer is favored in case of (R, R) chirality of coordination of the C2H4(1-Ind)2 ligand. 

Important information on active centres can be obtained from measurements of polymerization and copolymerization rates of related monomers under various conditions. When the kinetic properties of a centre can be changed by a substituent, its properties can also be changed by a molecule of a non-polymerizing substance (or molecules of non-polymerizing substances)... 

Devolatilization performance is usually measured against the equilibrium amount of volatile in the final polymer. The equilibrium level for the devolatilization conditions used can be calculated using a simplified Flory-Huggins equation for monomer activity in the polymer melt [6]. By equating the partial pressure of the monomer solution to the flash tank partial pressure, the following results ... 

The precise experimental conditions for the measurements of chain lifetimes of polyethylene with the TiCl4/Al(i-Bu2 )H catalyst are not explicitly stated, but there is clear evidence for a steady increase in lifetime with polymerization time. For an average lifetime of 4 min after 40 min polymerization time, the instantaneous values were 4 min after 18 min polymerization and 10 min after 40 min polymerization. As the concentration of active centres remains almost steady after a sharp initial fall, the increase cannot be accounted for wholly by changes in the monomer/active sites ratio. The explanation may lie in a reduced rate of chain transfer with increase in conversion, as has been found for propene with a-TiClj/AlEt2 Cl [121]. In accord with this view average chain lifetimes of polypropene have been calculated to increase with conversion [123]. 

Oxidation of Mixtures of Monomers. The method most likely to yield random copolymers of DMP and DPP is the simultaneous oxidation of a mixture of the two phenols, although this procedure may present problems because of the great difference in reactivity of the two phenols. The production of high molecular weight homopolymer from DPP is reported to require both a very active catalyst, such as tetramethylbutane-diamine-cuprous bromide, and high temperature, conditions which favor carbon-carbon coupling and diphenoquinone formation (Reaction 2) from DMP (II). With the less active pyridine-cuprous chloride catalyst at 25 °C the rate of reaction of DMP, as measured by the rate of oxygen... 

Cationic polymerization of 2-methylpropene at temperatures about 170 K may be almost flash-like the transformation of tetrahydrofuran to an equilibrium polymer-monomer mixture may last tens to hundreds of hours at 260 K. Evidently the overall polymerization rate is a function of many factors which may be interconnected or may act separately. The aim of kinetic measurements is to describe the polymerization, and to find conditions under which it would proceed in the desired manner. This is usually only possible after the various factors and their consequences have been isolated and investigated. The rate of monomer consumption during polymerization mostly depends on the generation rate of active centres, and on their concentration and reactivity. 

The yield of the catalyst, 0, was measured at various ethylene concentrations (see Fig. 10). According to the results, initiation is rapid and the catalytic system maintains full capacity for a long time, for at least 1 h. In this interval, the polymeric particles increase their size 5-10 fold. Thus the monomer supply into the pores of the particles by diffusion cannot be hindered. In the subsequent phase, activity already decreases. Either the conditions for monomer transport to the centres by diffusion are deteriorating, and/or the centres are slowly decaying. The polymerization rate, i>pol, can be determined from the slopes of the curves in Fig. 10. The determined values of the initial rates are directly proportional to monomer concentration (except for the lowest values of [M]), as shown in Fig. 11. 

At the same time, the equilibrium constants have been determined as equal to K = ka/k,i = 3103 mol L [76], From these sets of data it follows that at the conditions of measurements ([M] = 1 mol-L-1, [M ] = 10-3 mol-L-1), carbenium active species would constitute 0.03% of all active species, but would account for —3% of propagation (in terms of monomer consumed). 

Tognacci et al. [ 183] discussed various methods for measuring the monomer concentration in the polymer particles. The method proposed by the authors is a direct estimation of the solvent activity by the GC (gas chromatography) measurement of its partial pressure in the gas phase at equilibrium with the polymer particle, monomer droplet (if any) and aqueous phase in the latex. They proposed an original measuring technique and carried out measurements for different monomers (St, MMA, and VAc) and polymeric matrices (PSt and MMA-VAc copolymer), both above and below saturation conditions (corresponding to Intervals II and III). They compared the experimental data with that predicted by the monomer partitioning relationships derived by Maxwell et al. [166,170] and Noel et al. [172]. 

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