Polymerization Performance

In the following sections, the performance of the imine complexes is discussed. It must be noted [Ila] that comparing catalyst activity data from different studies is only approximate due to their dependence on experimental conditions, that is reactor geometry, stirring procedure, polymerization time, pressure and the like. Therefore it is advantageous to include known complexes for a direct comparison ( benchmarking ) under identical experimental polymerization conditions. It follows that the catalytic activity and productivity obtained for these known catalysts may differ from the ones reported in the literature they are characteristic of the equipment and screening procedure used. 

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Table 3.13 Radical Polymerizations Performed with Initiators Labeled with...

To find more information about the influence of steric effects on the polymerization performance, catalyst 2 (5,6 triptycene indenyl hafnocene dichloride) was developed. 

The exchange reaction places an upper limit on the polymer molecular weight that is possible for polymerizations performed in the presence of alcohols or other protonic substances. There are very few reports of polymers with molecular weights above 10,000 for ethylene... 

Suspension polymerization may be the most important particle-forming polymerization from an industrial viewpoint. The system is very simple, composed of monomer, initiator, stabilizer, and medium (water in most cases). The monomer droplets with dissolving initiator are dispersed in water and the stabilizer exists at the interface. But suspension polymerization is regarded as a kind of homogeneous polymerization because the polymerization occurs only in monomer droplets and water does not affect the polymerization. Water contributes only to dividing the polymerization locus into small droplets and absorbing the heat evolved by polymerization. On the contrary, in emulsion polymerization, which is another type of polymerization performed in water and as practically important as suspension polymerization, water affects the polymerization significantly. In this section, emulsion polymerization is first discussed, and then some modified emulsion polymerizations such as soap-free emulsion polymerization and micro and mini emulsion polymerizations are described. 

Polymerizations performed at a constant heating rate are typical of characterization techniques such as DSC. In these cases, a small mass of material is employed so that the temperature remains uniform in the whole specimen but varies linearly with time ... 

The ceiling temperatures listed in Table 3 refer to bulk conditions, whereas the equilibrium monomer concentrations refer to polymerizations performed at 25° C. As discussed in Section F, the equilibrium monomer concentration varies with the polymerization temperature, just as the ceiling temperature depends on the monomer concentration. Table 4 demonstrates that the equilibrium monomer concentration of a-methylstyrene... 

Lewis acids based on titanium tend to aggregate and form dimers which are usually more reactive than their monomeric precursors (cf., Chapter 2). The degree of aggregation depends on the solvent, temperature, and the ligands attached to titanium no dimerization was detected by cryoscopy at -95° C in CH2CI2 [174], However, kinetic measurements of isobutene and styrene polymerizations indicate that polymerization is second order in titanium chloride [175,176], perhaps due to formation of a low concentration of the more reactive dimer or more stable Ti2Cl9-anions. However, polymerizations performed at lower [TiCl4] were reported to be first order in titanium chloride [105]. 

The large concentrations of initiator may lead to the precipitation of a portion of the macroions this was indeed observed when polymerization performed in CCI4 was rechecked The precipitation of the low molecular macroions has also recently been reported for dicationic species The precipitated macroions become undetectable by NMR but can still be reactive in propa tion. Obviously, this will result in the wrong ccmclusion that all of the observed growth is due to macroesters and will give a value for k larger than the real oi e. 

Emulsion polymerization is an important commercial process because, in contrast to the same free-radical polymerization performed in the bulk, molecular weight and reaction rate can be increased simultaneously (1-3). Furthermore, the lower viscosity of an emulsion system compared with that of the corresponding bulk process provides better control over heat transfer. Commercial emulsion processes usually use a surfactant/water/monomer system... 

Figure 65 Ethylene polymerization performances of Hf- and Zr-based pyrrolide-imine-based catalysts.

Of course, if the only polymerizations performable were those of styrene, complex bases would be of little interest in the anionic polymerization field. 

We succeeded in polymerizing ethylene in PHTP in a steel tube at 20 C and 50 atm and propylene at 20 C and 10 atm. As far as we know these are the first examples of inclusion polymerization performed under high pressure. 

Almost aU of the pyrrole-, indole-, and carbazole-containing systems afforded stable Fe(ll) and Co(II) complexes. The only exceptions are hgands 44 and 47 which contain only aryl substituents. This indicates that two pairs of o-phenyl groups on the N-pyrrolyl substituents impose too much of a steric hindrance and/ or are too electron-withdrawing for the corresponding [N,N,N] metal complex to form. The two Fe(lII) precatalysts 54c, 55c were synthesized to evaluate the dependence of the polymerization performance on the oxidation state of the metal center. In total, twenty-one different N-azolyl complexes were prepared aiming at a thorough structure-activity evaluation (vide infra). 

All of this data suggest strongly that (// -toluene)Ni(C6F5)2 should be thought of as an initiator of norbornene polymerization rather than a catalyst. A full paper describing the chemistry and polymerization performance of this initiator system will be pubhshed shortly [62]. 

Batch polymerization Polymerizations performed in a closed reactor, with all the ingredients introduced at the beginning of a single step. This method, apart from certain exceptions, is of little interest, as a large part of the functional monomer is consumed, providing substantial quantities of water-soluble polymers that disturb nucleation and the hnal stabilization of the particles. 

H NMR studies have shown that metal-catalyzed cross-coupling polymerizations of 3-alkylthiophenes such as reactions 6.4, generally lead to regiorandom polythiophenes. A similar situation holds for polymerizations performed by the oxidation... 

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