Propagation polymerisation

In this type of polymerisation propagation is not by radical but by either a carbonium ion (in cationic polymerisations) or by a carbanion (in anionic polymerisations). In anionic polymerisation, termination does not take place unless we add a transfer agent. 

The rate of a polymerisation propagated by free ions and paired ions is... 

The central question at issue is still whether pseudocationic polymerisation, propagated by a large concentration of activated ester, is a real phenomenon sui generis or whether all these reactions can be explained in terms of very fast propagation by a very small concentration of conventional carbonium ions. 

In order to deduce the dependence of the polymerisation (propagation) rate and/or molar mass (M ) upon the initiator and/or monomer concentrations, it is necessary to invoke a hypothesis called the Steady State . Employing the Steady State allows one to set the rate of production of the radicals (i. e. Iq [I]) equal to the rate of loss of radicals (i. e. lq[R ] ), which enables one to write Eq. 5.4. 

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... 

Initiation mechanisms are everywhere elucidated by their integration into the complex overall rate behaviour of cationic polymerisation. Propagation rates determine yields and convenfently multiply the measurable effects of initiation. Termination steps contribute their effects to the jigsaw of cationic polymerisation, and even various kinds of cationic grafting reactions are covered. There can be few reviews punctuated by so many specific proposals for timely research, not only by academics, but also sometimes for possible industrial development. 

Cha.in-Tra.nsferAgents. The most commonly employed chain-transfer agents ia emulsion polymerisation are mercaptans, disulfides, carbon tetrabromide, and carbon tetrachloride. They are added to control the molecular weight of a polymer, by transferring a propagating radical to the chain transfer agent AX (63) ... 

Chain-growth polymerisations ate characterised by chains that propagate by adding one monomer molecule at a time, ie, a -mer + monomer — (a + l)-mer. There ate, however, several mechanisms by which this occurs. 

Free-Radical Addition. In free-radical addition polymerisation, the propagating species is a free radical. The free radicals, R-, ate most commonly generated by the thermal decomposition of a peroxide or aso initiator, 1 (see Initiators, free-RADICAl) ... 

Emulsion Polymerization. Emulsion SBR was commercialised and produced in quantity while the theory of the mechanism was being debated. Harkins was among the earliest researchers to describe the mechanism (16) others were Mark (17) and Elory (18). The theory of emulsion polymerisation kinetics by Smith and Ewart is still vaUd, for the most part, within the framework of monomers of limited solubiUty (19). There is general agreement in the modem theory of emulsion polymerisation that the process proceeds in three distinct phases, as elucidated by Harkins (20) nucleation (initiation), growth (propagation), and completion (termination). 

The requirements for a polymerisation to be truly living are that the propagating chain ends must not terminate during polymerisation. If the initiation, propagation, and termination steps are sequential, ie, all of the chains are initiated and then propagate at the same time without any termination, then monodisperse (ie, = 1.0) polymer is produced. In general, anionic polymerisation is the only mechanism that yields truly living styrene... 

Addition polymerisation is effected by the activation of the double bond of a vinyl monomer, thus enabling it to link up to other molecules. It has been shown that this reaction occurs in the form of a chain addition process with initiation, propagation and termination steps. 

The propagation rate is governed by the concentrations of growing chains [M—] and of monomers [M]. Since this is in effect the rate of monomer consumption it also becomes the overall rate of polymerisation... 

In many technical polymerisations transfer reactions to modifier, solvent, monomer and even initiator may occur. In these cases whereas the overall propagation rate is unaffected the additional ways of terminating a growing chain will cause a reduction in the degree of polymerisation. 

Plesch, P. H. The Propagation Rate-Constants in Cationic Polymerisations. Vol. 8, pp. 137-154. Porod, G. Anwendung und Ergebnisse der Rontgenkleinwinkelstreuung in festen Hochpolymeren. Vol. 2, pp. 363-400. 

Plesch, P. H. The Propagation Rate-Constants in Cationic Polymerisations. Vol. 8, pp. 137-154. 

Chain polymerisation typically consists of these three phases, namely initiation, propagation, and termination. Because the free-radical route to chain polymerisation is the most important, both in terms of versatility and in terms of tonnage of commercial polymer produced annually, this is the mechanism that will be considered first and in the most detail. 

Chain polymerisation necessarily involves the three steps of initiation, propagation, and termination, but the reactivity of the free radicals is such that other processes can also occur during polymerisation. The major one is known as chain transfer and occurs when the reactivity of the free radical is transferred to another species which in principle is capable of continuing the chain reaction. This chain transfer reaction thus stops the polymer molecule from growing further without at the same time quenching the radical centre. 

The explanation for autoacceleration is as follows. As polymerisation proceeds there is an increase in the viscosity of the reaction mixture which reduces the mobility of the reacting species. Growing polymer molecules are more affected by this than either the molecules of monomer or the fragments arising from decomposition of the initiator. Hence termination reactions slow down and eventually stop, while initiation and propagation reactions still continue. Such a decrease in the rate of the termination steps thus leads to the observed increase in the overall rate of polymerisation. 

Third generation initiators are based on the NHC system of second generation initiators, but do not contain any phosphine ligand. Instead, one or two pyridine ligands are weakly bound to the ruthenium centre (c/. Fig. 3.28, complexes 73 and 74c). Pyridine dissociates very easily and hardly competes with the olefin for the coordination site. As a result, complete initiation and fast propagation are enabled, therefore living polymerisation is rendered possible. 

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