Anionic polymerisation kinetics

R. MUkovich, in J. E. McGrath, A., Anionic Polymerisation Kinetics, Mechanisms, and Synthesis, ACS Symposium Series, No. 166, American Chemical Society, Washington, D.C., 1981, p. 41. 

The described experimental rig for the anionic polymerisation of dienes has been shown to behave as an ideal CSTR. The mathematical model developed allows the prediction of the MWD at future points in the reactor history, once suitable kinetic parameters have been estimated. 

It is possible to illustrate the kinetics of anionic polymerisation by the polymerisation of styrene with potassium amide in liquid ammonia. The first step involves the bread-down of initiators into ions. 

With regard to ion-pairs, there is much more information both from small molecule kinetics and from the anionic polymerisations, but it must be remembered that the solvents usable for the latter cover only a very small range of polarities, and an effective transfer of skills between the two domains remains to be achieved. 

Anionic polymerisation of hydrocarbon monomers is initiated by lithium butyl to produce a living polymer the association number of which in solution is required to elucidate the kinetics. When the living polymer (for example polystyryl lithium) is terminated, the polystyrene can be isolated and a solution then made to determine its molecular weight, M. If the living polymer is associated in solution, the ratio of its... 

Thus, in the case of PO anionic polymerisation with complexed potassium cations, the kinetic equation for propagation reaction (4.10) becomes ... 

No.23, 8th Nov. 1993, p.6237-42 DETERMINATION OF SOLUTION POLYMERISATION KINETICS BY NEAR-INFRARED SPECTROSCOPY. I. LIVING ANIONIC POLYMERISATION PROCESSES Long T E Liu H Y Schell B A Teegarden D M ... 

Details are given of the preparation of SAN latexes by micro-emulsion polymerisation using sodium dodecyl sulphate as an anionic surfactant. Kinetics of copolymerisation was studied at different temperatures using different concentrations of potassium persulphate and hydrogen peroxide/ascorbic acid. The latexes were characterised for particle size and number of particles by dynamic light scattering and TEM. Products were examined using NMR and thermal analysis. 20 refs. INDIA... 

The redox potential and the reactivity of this oxidation state depend strongly upon the anion (Table 11). Strong complexes are formed with SO ". Even in perchloric acid, hydrolysis and polymerisation greatly complicate kinetics. The co-ordination number of Ce(lV) in solution is not established . 

The initiation of the cationic polymerisation of alkenes is examined in detail by means of simple thermodynamic concepts. From a consideration of the kinetic requirements it is shown that the ideal initiator will yield a stable, singly charged anion and a cation with a high reactivity towards the monomer by simple, well defined reactions. It must also be adequately soluble in the solvent of choice and for the experimental method to be used. The calculations are applied to carbocation salts as initiators and a method of predicting their relative solubilities is described. From established and predicted data for a variety of carbocation salts the position of their ion molecule equilibria and their reactivity towards alkenes are examined by means of Born-Haber cycles. This treatment established the relative stabilities of a number of anions and the reason for dityl, but not trityl salts initiating the polymerisation of isobutene. 

Polyatomic ions (as opposed to neutral molecules) may also be unstable with respect to decomposition, polymerisation or disproportionation. However, ions cannot be scrutinised in isolation. In a crystalline solid, there are always counter-ions of opposite charge to be considered, and in solution an ion is surrounded by solvent molecules. The intimacy of the chemical environment of any ion must influence its viability. For example, redox reactions involving electron transfer between cation and anion, or between ion and solvent, may find easy kinetic pathways. We look here at some examples of unstable oxoanions. 

If, as in the preceding scheme, the hydrogen dichloride anion is the relevant species, the equilibrium concentration of (paired) carbenium ions formed will be governed by the same equation, and the rate of polymerisation will also follow the same kinetic laws, assuming that protonation is rate determining. 

Rooney has recently revived work on this monomer in an investigation of its polymerisation by trityl hexafluoroantimonate - He used a spectroscopic stop-flow apparatus to follow initiation and an adiabatic calorimeter to measure rates of polymerisation. Propagation was shown to compete effectively with initiation to the point that some initiator was often present at the end of the polymerisations. These observations cast some doubts on the assumption made in the paper by the Liverpool school discussed above. A kinetic analysis of the initiation reaction showed it to be bimolecular, with a rate constant of about 130 sec at 20°C. The determination of the propagation rate constant was less strai tforward despite the fact that further monomer-addition experiments seemed to rule out any appreciable termination. The kp values fluctuated considerably as the initial catalyst concentration was varied, a fact which induced Rooney to propose that the empirical constant was a composite function of kp and kp. Experiments with a common-anion salt supported this proposal and their kinetic treatment led to the individual values of kp = 6 x 10 sec and kp = 5 x 10 sec. It is difficult to assess the reliability of these values in view of the following statement by the author the reaction at a 5 x 10 M concentration of initiator, thought to proceed exclusively through paired ions. .. . This statement is certainly incorrect as far as the initiator is concerned for which the proportion of ion pairs for a concentration 5 x 10 M at 20°C is only about 20% in methylene chloride However, the experiments... 

---