Rate constant anionic chain polymerization

The determination of the various rate constants (ki, kp, kt, kts, ktr) for cationic chain polymerization is much more difficult than in radical chain polymerization (or in anionic chain polymerization). It is convenient to use Rp data from experiments under steady-state conditions, since the concentration of propagating species is not required. The Rp data from non-steady-state conditions can be used, but only when the concentration of the propagating species is known. For example, the value of kp is obtained directly from Eq. (8.143) from a determination of the polymerization rate when [M J is known. The literature contains too many instances where [M" "] is taken equal to the concentration of the initiator, [IB], in order to determine kp from measured Rp. (For two-component initiator-coinitiator systems, [M" ] is taken to be the initiator concentration [IB] when the coinitiator is in excess or the coinitiator concentration [L] when the initiator is in excess.) Such an assumption holds only if Ri > Rp and the initiator is active, i.e., efficiency is 100%. Using this assumption without experimental verification may thus lead to erroneous results. 

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 kinetics of radiation-induced polymerization of bulk nitroethylene was also studied at 10° C by the use of hydrogen bromide as an anion scavenger (27). The value of Gt (yield of the initiation by 100 eV energy absorbed) was found to be about 3, which was much larger than the value obtained for many radiation-induced cationic polymerizations. The propagation rate constant, kp, was estimated to be 4 x 107 M-1 sec-1. The large kp value was attributed to the concept that the propagating chain ends were free ions in contrast to the existence of counter ions in catalytic polymerization. 

Anionic polymerization differs from radical polymerization in that no chain termination of the propagating polymers with each other occurs ( living polymers ). Furthermore, the rate constant of the propagation kp is not so high that this process is controlled by diffusion. 

Rate Constants for the Transitions between the Different Ionic States of the Active Chain End in the Anionic Polymerization of Styrene. The cases treated in this section demonstrate the high information capacity of MWDs which makes this determination extremely useful for solving kinetic problems. In addition, these examples reveal how it is possible to test experimentally determined MWDs by kinetic measurements. 

As mentioned previously, most continuous anionic polymeri tion studies have been conducted at relatively low temperatures ( < 50 °C). Even then, mixing kinetics have been of considerable concern due to the fast polymerization kinetics. In the recent anionic polymerization studies of Priddy and Pirc [1,73], the polymerization temperature range of 80-140 °C was studied (typical free radical temperature range). At these temperatures, the polymerization kinetics are extremely fast Also, the high polymerization temperature results in significant thermal termination of active polystyryl chains. Kem et aL [74] found that the termination reaction involved liberation of lithium hydride (1) and was first order. They found the apparent rate constant K at 65, 93, and 120°C are 0.15, 0.78, and 1.3 h respectively. 

Additional well-defined side-chain liquid crystalline polymers should be synthesized by controlled polymerizations of mesogen-ic acrylates (anionic or free radical polymerizations), styrenes (anionic, cationic or free radical), vinyl pyridines (anionic), various heterocyclic monomers (anionic, cationic and metalloporphyrin-initiated), cyclobutenes (ROMP), and 7-oxanorbornenes and 7-oxanorbornadienes (ROMP). Ideally, the kinetics of these living polymerizations will be determined by measuring the individual rate constants for termination and... 

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