Activation temperature polymerization activity

Fig. 5. Activity and isotacticity vs polymerization temperature. Polymerization occurs in hexane at 0.7 MPa (7 bat) for 4 h with a superactive...

The mechanism of ion polymerization in formaldehyde crystals proposed by Basilevskii et al. [1982] rests on Semenov s [1960] assumption that solid-phase chain reactions are possible when the arrangement of the reactants in the crystal prepares the configuration of the future chain. The monomer crystals capable of low-temperature polymerization fulfill this condition. In the initial equilibrium state the monomer molecules are located in the lattice sites and the creation of a chemical bond requires surmounting a high barrier. However, upon creation of the primary dimer cation, the active center shifts to the intersite, and the barrier for the addition of the next link... 

Attempts to synthesize transition metal alkyl compounds have been continuous since 1952 when Herman and Nelson (1) reported the preparation of the compound C H6>Ti(OPri)3 in which the phenyl group was sigma bonded to the metal. This led to the synthesis by Piper and Wilkinson (2) of (jr-Cpd)2 Ti (CH3)2 in 1956 and a large number of compounds of titanium with a wide variety of ligands such as ir-Cpd, CO, pyridine, halogen, etc., all of which were inactive for polymerization. An important development was the synthesis of methyl titanium halides by Beerman and Bestian (3) and Ti(CH3)4 by Berthold and Groh (4). These compounds show weak activity for ethylene polymerization but are unstable at temperatures above — 70°C. At these temperatures polymerizations are difficult and irreproduceable and consequently the polymerization behavior of these compounds has been studied very little. In 1963 Wilke (5) described a new class of transition metal alkyl compounds—x-allyl complexes,... 

The catalyst activity is so high that uranium concentration lower than 0.1 millimoles per liter allows a complete conversion of butadiene to be obtained in a few hours, at 20°C, The transfer reaction of uranium based catalyst is similar to that of conventional 3d-block elements (titanium, cobalt, nickel) so that the molecular weight of the polymer is affected by polymerization temperature, polymerization time and monomer concentration in the customary way. This is in contrast, as we shall see later on, to some catalysts based on 4 f-block elements. Uranium based catalysts are able to polymerize isoprene and other dienes to high cis polymers the cis content of polyisoprene is 94%, somewhat inferior to titanium based catalysts. In contrast, with 3d-block elements an "all cis", random butadiene-isoprene... 

Cationic diorganoaluminum species with non-coordinating anions are activators in the high-temperature polymerization of olefins, while mixtures of AlEt3/B(C6F5)3 also show moderate activity for the polymerization of ethylene. Neutral magnesium, calcium, and zinc compounds, in turn, have been used as catalysts for the polymerization of... 

Evidence in support of a carbonium ion type of mechanism for low temperature polymerization was also obtained in an investigation of the kinetics of the homogeneous liquid phase polymerization of propene in the presence of aluminum bromide and hydrogen bromide at about —78° (Fontana and Kidder, 89). The rate of reaction is approximately proportional to the concentration of the promoter, no polymerization occurring in its absence. During the main portion of the reaction, the rate is independent of the monomer concentration toward the end, it decreases, due apparently to the low-concentration of the monomer, addition of more olefin resulting in an increase in the rate. It was concluded that the reaction involves an active complex, which may be regarded as a carbonium ion coupled with an anion ... 

Considerable efforts have centered on carrying out the synthesis of polybenzimidazoles at more moderate temperatures. Polymerization of the isophthalic acid or its diphenyl ester have been successfully carried out in polyphosphoric acid or methanesulfonic acid-phosphorous pentoxide at 140-180°C, but the reaction is limited by the very low solubilities (<5%) of the reactants in that solvent. The lower reaction temperature is a consequence of activation of the carboxyl reactant via phosphorylation. Lower reaction temperatures are also achieved in hot molten nonsolvents such as sulfolane and diphenyl sulfone, but the need to remove such solvents by a filtration or solvent extraction is a disadvantage. 

The reduced transition metal compound so prepared, for instance TiCh, was diluted and activated by adding an alkylaluminum compound without a preceding washing or filtration treatment. After 5 minutes of aging at reaction temperature, polymerization was started by introducing gaseous propylene into the diluted catalyst slurry. In all polymerizations, concentration of the transition metal compound was 5 mmole/liter. 

Figure 22 shows that the rate of reaction is not directly proportional to ethylene pressure. When ethylene was adsorbed only on the active sites but its concentration in the gas phase was negligible, no polymerization was observed even after 68 hours at room temperature. Polymerization therefore does not occur by migration and interaction of the ethylene molecules adsorbed on active sites. It is possible that the physically adsorbed ethylene participates in the reaction. However, no simple rate expression could be found to fit the observed dependence on ethylene pressure. 

In 1964 a short investigation was made of EPR signals of platinum on alumina by F. Nozaki, D. Stamires and Turkevich.(70) The relation of catalytic activity of transition metal oxides to their EPR properties was studied. Thus in 1967 Kazanski investigated the chromium oxide on silica and its ability to carry out low temperature polymerization of ethylene. 

Monomers containing active hydrogen (e g., adds and hydroxy eompoimds) are not suitable for GTP, but aeiylates, acrylonitrile, and Af,Af-dimethyl aerylamide can be polarized quite readily. The reaction is very susceptible to impurities, and all reagents and solvents must be scrupulously dried. Although both low- and high-temperature polymerization reactions are possible, a range of 270 to 320 K is preferred. 

In special case when constants of chain propagation and active sites deactivation rates do not depend on temperature, polymerization kinetic scheme corresponds to fast initiation and to the first orders of chain propagation reaction on monomer and active sites deactivation on their concentration the expression for polymer yield was received [1,62] ... 

Phosphate esters based on alkanols, polyethoxy-lated alkanols and alkylphenols are favourably applied in emulsion polymerization processes. Especially in low-temperature polymerization, high requirements are demanded of emulsifiers for solubility and surface activity at ca. 5°C. 

Figure 3.11 Polymerization kinetics of Cr-based catalysts supported on silica-titania and activated with one thermal activation step at the indicated temperatures. Polymerization rate from 1 to 10 Kg/g catalyst/hr vs polymerization time from 0-150 minutes. Reprinted from [12] with permission from Elsevier Publishing.

The same catalyst was also very active for polycondensation of benzyl chloride. Thus, when 0.1 g of the catalyst was added to 5 cm of benzyl chloride at room temperature, polymerization occurred immediately with violent evolution of hydrogen chloride and completed in less than 10 s. The yield of methanol - insoluble polymer was about 70%. Elemental and NMR analyses indicated that the product is predominantly linear para-substituted polybenzyl. The molecular weight as determined by vapor pressure osmometry was 8175, the degree of polymerization being ca, 90. 

In this section we discuss the initiation step of free-radical polymerization. This discussion is centered around initiators and their decomposition behavior. The first requirement for an initiator is that it be a source of free radicals. In addition, the radicals must be produced at an acceptable rate at convenient temperatures have the required solubility behavior transfer their activity to... 

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