Chain transfer solvent system

An interesting situation is obtained when the catalyst-solvent system is such that the initiator is essentially 100% dissociated before monomer is added and no termination or transfer reactions occur. In this case all chain initiation occurs rapidly when monomer is added, since no time-dependent initiator breakdown is required. If the initial concentration of catalyst is [AB]o,then chain growth starts simultaneously at [B"]q centers per unit volume. The rate of polymerization is given by the analog of Eq. (6.24) ... 

Suspension polymerization of VDE in water are batch processes in autoclaves designed to limit scale formation (91). Most systems operate from 30 to 100°C and are initiated with monomer-soluble organic free-radical initiators such as diisopropyl peroxydicarbonate (92—96), tert-huty peroxypivalate (97), or / fZ-amyl peroxypivalate (98). Usually water-soluble polymers, eg, cellulose derivatives or poly(vinyl alcohol), are used as suspending agents to reduce coalescence of polymer particles. Organic solvents that may act as a reaction accelerator or chain-transfer agent are often employed. The reactor product is a slurry of suspended polymer particles, usually spheres of 30—100 pm in diameter they are separated from the water phase thoroughly washed and dried. Size and internal stmcture of beads, ie, porosity, and dispersant residues affect how the resin performs in appHcations. 

Metal chelates afford a better initiating system as compared to other redox systems since the reactions can be carried out at low temperatures, thus avoiding wastage reactions due to chain transfer. Homopolymer formation is also minimum in these systems. It was observed by Misra et al. [66,67] that the maximum percentage of grafting occurs at a temperature much below the decomposition temperature of the various metal chelates indicating that the chelate instead of undergoing spontaneous decomposition receives some assistance either from the solvent or monomer or from both for the facile decomposition at lower temperature. The solvent or monomer assisted decomposition can be described as ... 

On the basis of these studies we decided to carry out a series of AMI and IMA experiments (2) with the TMPCl/EtAlCl2/DtBP combination. Figures 1 and 2 show the results. The M versus Wp (g of poly(P-PIN) formed) plots and the N (number of moles of poly(P-PIN) formed) versus Wp plots (insets) indicate increasing deviation from the theoretical values (calculated for Ieff = 100%). According to these results chain transfer proceeds in these polymerizations, i.e., the systems are nonliving. Further experimentation would be necessary to develop satisfactory living conditions, in particular to investigate the effect of solvent polarity, temperature and electron donors on the mechanism. 

Free radical copolymerizations of the alkyl methacrylates were carried out in toluene at 60°C with 0.1 weight percent (based on monomer) AIBN initiator, while the styrenic systems were polymerized in cyclohexane. The solvent choices were primarily based on systems which would be homogeneous but also show low chain transfer constants. Methacrylate polymerizations were carried out at 20 weight percent solids... 

Chain transfer reactions with solvent or some other component of the reaction system also occur (Eq. 33). 

The adventitious chain-breaking reactions are those which involve the adventitious components of the polymerization system in other words, the impurities. The inherent chain-breaking reactions are those which are characteristic of the system, such as reactions between cation and anion, monomer transfer, solvent transfer. Each system has its own inherent chain-breaking reactions and for any one monomer the relative importance of these can be changed by changing the solvent, catalyst or co-catalyst [27b, 101]. 

Chain transfer can also occur with initiator, impurity, solvent, or other additive present in the polymerization system. While the average chain length is equal to Rp divided by the sum of all termination reactions, it is customary to control all termination steps except the one that is being studied. Chain transfer to all other molecules, except solvent or some special additive, is typically negligible. 

Monomer and initiator must be soluble in the liquid and the solvent must have the desired chain-transfer characteristics, boiling point (above the temperature necessary to carry out the polymerization and low enough to allow for ready removal if the polymer is recovered by solvent evaporation). The presence of the solvent assists in heat removal and control (as it also does for suspension and emulsion polymerization systems). Polymer yield per reaction volume is lower than for bulk reactions. Also, solvent recovery and removal (from the polymer) is necessary. Many free radical and ionic polymerizations are carried out utilizing solution polymerization including water-soluble polymers prepared in aqueous solution (namely poly(acrylic acid), polyacrylamide, and poly(A-vinylpyrrolidinone). Polystyrene, poly(methyl methacrylate), poly(vinyl chloride), and polybutadiene are prepared from organic solution polymerizations. 

Monomers may also be polymerized in solution using good or poor solvents for homogeneous and heterogeneous systems, respectively. In solution polymerizations, solvents with low chain transfer constants are used to minimize reduction in chain length. 

For any specific type of initiation (i.e., radical, cationic, or anionic) the monomer reactivity ratios and therefore the copolymer composition equation are independent of many reaction parameters. Since termination and initiation rate constants are not involved, the copolymer composition is independent of differences in the rates of initiation and termination or of the absence or presence of inhibitors or chain-transfer agents. Under a wide range of conditions the copolymer composition is independent of the degree of polymerization. The only limitation on this generalization is that the copolymer be a high polymer. Further, the particular initiation system used in a radical copolymerization has no effect on copolymer composition. The same copolymer composition is obtained irrespective of whether initiation occurs by the thermal homolysis of initiators such as AIBN or peroxides, redox, photolysis, or radiolysis. Solvent effects on copolymer composition are found in some radical copolymerizations (Sec. 6-3a). Ionic copolymerizations usually show significant effects of solvent as well as counterion on copolymer composition (Sec. 6-4). 

The autoxidation mechanism by which 9,10-dihydroanthra-cene is converted to anthraquinone and anthracene in a basic medium was studied. Pyridine was the solvent, and benzyl-trimethylammonium hydroxide was the catalyst. The effects of temperature, base concentration, solvent system, and oxygen concentration were determined. A carbanion-initi-ated free-radical chain mechanism that involves a singleelectron transfer from the carbanion to oxygen is outlined. An intramolecular hydrogen abstraction step is proposed that appears to be more consistent with experimental observations than previously reported mechanisms that had postulated anthrone as an intermediate in the oxidation. Oxidations of several other compounds that are structurally related to 9,10-dihydroanthracene are also reported. 

Secondary reactions usually proceed in addition to template polymerization of the system template-monomer-solvent. They influence both kinetics of the reaction and the structure of the reaction products. Depending on the basic mechanism of reaction, typical groups of secondary reactions can take place. For instance, in polycondensation, there are such well known reactions as cyclization, decarboxylation, dehydratation, oxidation, hydrolysis, etc. In radical polymerization, usually, in addition to the main elementary processes (initiation, propagation and termination), we have the usual chain transfer to the monomer or to the solvent which change the molecular weight of the product obtained. Also, chain transfer to the polymer leads to the branched polymer. 

Solvent polarity and temperature also influence ihe results. The dielectric constant and polarizability, however, are of little predictive value for the selection of solvents relative to polymerization rates and behavior. Evidently evety system has to he examined independently. In cationic polymerization of vinyl monomers, chain transfer is the most significant chain-breaking process. The activation energy of chain transfer is higher than that of propagation consequently, the molecular weight of the polymer increases with decreasing temperature. 

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