Anionic polymerization kinetics

Kinetics. Details of the kinetics of polymerization of THF have been reviewed (6,148). There are five main conclusions. (/) Macroions are the principal propagating species in all systems. (2) With stable complex anions, such as PF , SbF , and AsF , the polymerization is living under normal polymerization conditions. When initia tion is fast, kinetics of polymerizations in bulk can be closely approximated by equation 2, where/ is the specific rate constant of propagation /is time [I q is the initiator concentration at t = 0 and [M q, [M and [M are the monomer concentrations at t = 0, at equiHbrium, and at time /, respectively. 

The mechanism of anionic polymerization of cyclosiloxanes has been the subject of several studies (96,97). The first kinetic analysis in this area was carried out in the early 1950s (98). In the general scheme of this process, the propagation/depropagation step involves the nucleophilic attack of the silanolate anion on the sUicon, which results in the cleavage of the siloxane bond and formation of the new silanolate active center (eq. 17). 

The block copolymer produced by Bamford s metal carbonyl/halide-terminated polymers photoinitiating systems are, therefore, more versatile than those based on anionic polymerization, since a wide range of monomers may be incorporated into the block. Although the mean block length is controllable through the parameters that normally determine the mean kinetic chain length in a free radical polymerization, the molecular weight distributions are, of course, much broader than with ionic polymerization and the polymers are, therefore, less well defined,... 

The kinetics of polymerization and conductometric studies of barium polystyrene with one active end-group per chain were reported by De Groof et al. 79,80). Formation of an unconventional anionic species, Ba2 +, (CH(Ph)CH2—)j, had to be postulated to account for the results. The existence of such a species is supported by the recent study of the kinetics of polymerization of lithium polystyrene performed in the presence of barium polystyrene endowed with two active endgroups 78). The polymerization of the lithium salt is retarded by the presence of the barium salt, and the retardation is accounted for by the formation of the inert aggregated anions,... 

Slomkowski, S., and Penczek, S., Influence of dibenzo-18-crown-6 ether on the kinetics of anionic polymerization of p-propiolactone, Macromolecules, 9, 367-369, 1976. 

Milkovich, R. In Anionic Polymerization Kinetics. Mechanisms and Synthesis ACS Symposium Series No. 166 American Chemical Society Washington, DC, 1981. 

Quite often in the ring-opening polymerization, the polymer is only the kinetic product and later is transformed to thermodynamically stable cycles. The cationic polymerization of ethylene oxide leads to a mixture of poly(ethylene oxide) and 1,4-dioxane. In the presence of a cationic initiator poly(ethylene oxide) can be almost quantitatively transformed to this cyclic dimer. On the other hand, anionic polymerization is not accompanied by cyclization due to the lower affinity of the alkoxide anion towards linear ethers only strained (and more electrophilic) monomers can react with the anion. 

McGrath, J. E.,editor, "Anionic Polymerization Kinetics, Mechanisms and Synthesis," ACS Symposium Series, No. 166, 1981. 

The opposite effects of the initiator on the structure of 1,4-DVB polymers in a range of low (1-16 mol %) and a high (17-200 mol %) concentration of 1,4-DVB were explained by a kinetic model of anionic polymerization of 1,4-DVB [239]. Calculations indicated that, at low concentrations of the initiator, the poly-... 

McGrath JE (ed) (1981) Anionic polymerization kinetics, mechanism and synthesis, vol 166. American Chemical Society, Washington... 

The chemical and kinetic relationships for the anionic polymerization of acrylonitrile follow the same three major steps found for cationic polymerizations (1) initiation, (2) propagation, and (3) termination ... 

Once the robotic system and procedure passed the optimization and reproducibility tests for a certain type of reaction, the researcher has the chance to move on to the most delightful part of a high-throughput experimentation workflow that is to follow the reaction kinetics of the reaction by withdrawing several samples under comparable conditions. The characterization of these samples allows the determination of the apparent rate constants and activation energies in a very reproducible way. As an example, the anionic polymerization of St in cyclohexane initiated by i-BuLi under different reaction conditions was investigated. Several samples were withdrawn during the reaction into small vials which were prefllled with 25 pL of... 

Fig. 8 Semilogarithmic kinetic plot of the anionic polymerization of St (left) and molar mass and PDI values vs conversion plot (right). (Reprinted with permission from [55]. Copyright (2005) John WUey Sons, Inc.)...

Ionic polymerizations, especially cationic polymerizations, are not as well understood as radical polymerizations because of experimental difficulties involved in their study. The nature of the reaction media in ionic polymerizations is often not clear since heterogeneous inorganic initiators are often involved. Further, it is extremely difficult in most instances to obtain reproducible kinetic data because ionic polymerizations proceed at very rapid rates and are extremely sensitive to the presence of small concentrations of impurities and other adventitious materials. The rates of ionic polymerizations are usually greater than those of radical polymerizations. These comments generally apply more to cationic than anionic polymerizations. Anionic systems are more reproducible because the reaction components are better defined and more easily purified. 

It is useful to understand the reasons for the faster reaction rates encountered in many anionic polymerizations compared to their radical counterparts. This can be done by comparing the kinetic parameters in appropriate rate equations Eq. 3-22 for radical polymerization and Eq. 5-84 for anionic polymerization. The kp values in radical polymerization are similar to the fc pp values in anionic polymerization. Anionic fc pp values may be 10-100-fold lower than in radical polymerization for polymerization in hydrocarbon solvents, while they may be... 

Bywater, S., Anionic Polymerization of Olefins, Chap. 1 in Comprehensive Chemical Kinetics, Vol. 15, Non-Radical Polymerization, C. H. Bamford and C. F. H. Tipper, eds., Elsevier, New York, 1976. 

Szwarc, M. and J. Smid, The Kinetics of Propagation of Anionic Polymerization and Copolymerization, Chap. 5 in Progress in Reaction Kinetics, Vol. 2, G. Porter, ed., Pergamon Press, Oxford, 1964. 

Bywater, S., Anionic Polymerization of Olefins, Chap. 1 in Comprehensive Chemical Kinetics, Vol. 15,... 

Table 1.2 Kinetic Constants for Anionic Polymerization of Caprolactam with Different Catalyst and Initiator Systems...

For sodium/hexamethylene-l,6-bis-carbamidocaprolactam system, Sibal et al. [64] found the value of the constant k in Equation 1.4 to be 17.5. Note that the values of the constant k in Equation 1.4 that defines the relative complex viscosity rise during anionic ring opening polymerization of caprolactam are comparable for both caprolactam-magnesium-bromide/isophthaloyl-bis-caprolactam and sodium/hexamethylene-l,6-bis-carbamidocapro-lactam as the catalyst/initiator systems even though the kinetic constants for anionic polymerization for these systems are extremely different (see Table 1.2). 

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