Polymerization constants, various monomers

Table 18-3. Equilibrium Constants for the Dissociation of Ion Pairs into Free Ions in the Anionic Polymerization of Various Monomers with a Series of Gegenions (According to a Collection by S. Bywater)...

In general, carbocationic homopolymerization is characterized by low activation enthalpy, low concentration of active species (10 -10 mol/dm ), high propagation rate constants (10 -10 dm /mol/sec), and short lifetime of the car-benium ion (94). Table 4 lists propagation rate constants determined in bulk carbocationic polymerization of various monomers initiated by y-radiation, electron pulse, or field ionization techniques. 

Table 1. Polymerization and Chain-Transfer Constants for Various Monomers ...

Although the basic mechanisms are generally agreed on, the difficult part of the model development is to provide the model with the rate constants, physical properties and other model parameters needed for computation. For copolymerizations, there is only meager data available, particularly for cross-termination rate constants and Trommsdorff effects. In the development of our computer model, the considerable data available on relative homopolymerization rates of various monomers, relative propagation rates in copolymerization, and decomposition rates of many initiators were used. They were combined with various assumptions regarding Trommsdorff effects, cross termination constants and initiator efficiencies, to come up with a computer model flexible enough to treat quantitatively the polymerization processes of interest to us. 

AH values for various monomers. The AS values fall in a narrower range of values. The methods of evaluating AH and AS have been reviewed [Dainton and Ivin, 1950, 1958], These include direct calorimetric measurements of AH for the polymerization, determination by the difference between the heats of combustion of monomer and polymer, and measurements of the equilibrium constant for the polymerization. The overall thermodynamics of the polymerization of alkenes is quite favorable. The value of AG given by... 

These results show that the big differences in the polymerization rate in the various solvents are caused mainly by the position of the equilibria, and only to a small extent by the direct interaction of the solvent. Figure 11 shows the equilibria as functions of temperature, calculated with the parameters of Table II and the corresponding rate constants for monomer addition. 

The frequency of the transfer reactions (4 a) and (4 b) depends on the competition between substrate (polymer) and monomer for the radicals present in the solution, i.e. on the competition between growth and transfer. This competition is usually characterized by a transfer constant which is equal to the ratio of the transfer rate constant to the propagation rate constant, and which can be determined by measurements of the degree of polymerization at various concentration ratios of transfer agent... 

The counter radical method has been studied with various monomers more or less successfully. However, the synthesis of only few block or grafted copolymers is effectively described. This is a strong indication that a true control of the polymerization is still not achieved with all monomers although progress is constant. Nevertheless, it is clear that the possibility of reversibly controlling the termination step offers a tool of choice for the synthesis of well-defined and pure block copolymers and many studies are still necessary to understand properly the precise mechanism of macroradical end capping in order to control the reversible character and possible secondary reactions. 

Another way of determining whether the observed plateau does really correspond to [M]e is to perform polymerization at various ratios of [M to [I]0. The properly determined value of [M]e should be independent of this ratio, provided that [M]o/[I]o is not too small. Thus, for non-living systems it is necessary to carry out polymerizations with increasing initial initiator concentration until a constant ultimate monomer conversion is reached. This method of approaching the equilibrium concentration gave reliable thermodynamic parameters for the cationic polymerization of cyclic esters of phosphoric acid, in spite of termination observed in these systems 11 ... 

The majority of commercial methacrylic ester polymers are produced by free-radical initiators. Peroxides and azo compounds ftinction as t5ipical initiators for this type of polymerization. Other possible routes for producing methacrylic polymers with radicals include photoinitiation and radiation-induced polymerization. Both Y ray and electron-beam radiation have been employed in the production of methacrylic ester polymers (36-38). At constant temperature, there is a first-order dependence of the polymerization rate on monomer concentration and a one-half-order dependence on initiator concentration. Rate data for the polymerization of various methacrylic monomers using the azo compoimd 2,2 -azobisisobut5ironitrile [78-67-1] (AIBN) are shown in Table 8. 

With agitation at 500 rpm by an impeller with a rectangular blade, the polymerization is carried out at 60°C. Samples are taken at various degrees of conversion. At about 70% conversion, the pressure in the reactor begins to drop from its constant value. The limit of conversion by this procedure is about 85%. Beyond this point, polymerization involves residual monomer that can diffuse only with great difficulty in the nonswollen, dense polymer. Porous resins are evidently formed by this procedure at conversions up to approximately 70%,... 

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