Propagation and Transfer

According to the above discussed initiation mechanism, the alkyl halide determines the initiating entity of the kinetic chain whereas the alkylaluminum compound determines the counter-ion  

Cationic polymerizations conducted in solvents of medium or low polarity most likely proceed by encumbered cations which are part of carbenium ion/counter-ion pairs. Cons juently the main molecular weight determining event, chain transfer to monomer, will also be proceeding via ion pairs and therefore will be affected by the nature of the counter-ion. On the other hand, the yield rif polymer will depend particularly on the number of active centers and on the rate of propagation relative to the rate of termination. The latter too is affected by the coimter-ion. 

The fact that the lines for each of the two diffoient ts of initiator systems are parallel, but not superimposable, is indication for furtter, more subtle and yet systematic differences, which must be attributed to the relationship of the different counter-ions to the propa ting cation. 

In another publication (31) it was concluded that the reason for such a vertical displacement of two parallel lines of the Arrhenius type is due to differences in the entropy of the jH opagating carbenium-ion/counter-ion pair and that, more specifically, the C Al(C2H5)Qf ion pair is probably loc r than the — C AlCl pair. 

In this context, it might be mentioned that the slope of the logM versus 1/T line for the BF3/isobutylene system is essoitiaUy parallel with the AICI3 and Al(C2H5)Cl2 lines shown in Fig. 4 BF3 is another initiator, the coinitiator requirement of which is extranely low (52). 

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Scheme 7 Proposed chain propagation and transfer process in bis(imino)pyridine iron and cobalt catalysts...

We propose to explain the mechanism of the propagation and transfer reactions in terms of even-membered cyclic transition states such as structure (I) for the propagation step (ii), structure (II) for reaction (iii), and structure (III) for reaction (iv). 

Chain-transfer reactions would be expected to increase in rate with increasing pressure since transfer is a bimolecular reaction with a negative volume of activation. The variation of chain-transfer constants with pressure, however, differ depending on the relative effects of pressure on the propagation and transfer rate constants. For the case where only transfer to chain-transfer agent S is important, Cs varies with pressure according to... 

The major approach to extending the lifetime of propagating species involves reversible conversion of the active centers to dormant species such as covalent esters or halides by using initiation systems with Lewis acids that supply an appropriate nucleophilic counterion. The equilibrium betweem dormant covalent species and active ion pairs and free ions is driven further toward the dormant species by the common ion effect—by adding a salt that supplies the same counterion as supplied by the Lewis acid. Free ions are absent in most systems most of the species present are dormant covalent species with much smaller amounts of active ion pairs. Further, the components of the reaction system are chosen so that there is a dynamic fast equilibrium between active and dormant species, as the rates of deactivation and activation are faster than the propagation and transfer rates. The overall result is a slower but more controlled reaction with the important features of living polymerization (Sec. 3-15). 

Secondly, to explain the different mechanisms of propagation and transfer and the evolution of the microstructures. The experimental conditions are summed up in Table 2. 

For N-vinylcarbazole in methylene chloride solutions cycloheptatrienyl ion has been shown to be a very efficient initiator, reacting by a rapid and direct addition to the olefin (82). A mechanistic scheme involving virtually instantaneous and quantitative initiation, rapid propagation (and transfer) and no true termination appears to operate, enabling rate constants for propagation kp, to be determined very simply from initial slopes of conversion/time curves. Under the experimental conditions used the initiators were almost totally dissociated and there seems every reason to suppose that the propagating cations are similarly dissociated (Section II.C.2). The derived rate constants therefore refer to the reactivity of free poly-(N-vinylcarbazole) cation, kp, and relevant data are summarised in Table 7. 

A hypothesis which may explain the experimental observations can be developed as follows Transfer has been assumed to occur by proton transfer to monomer. Previous studies (18,19) indicate that propagation and transfer have similar transition states in cationic polymerizations. For this reason it is possible that these two processes may both occur within the ion-counterion-monomer complex. Termination has been assumed to occur by ion-counterion collapse (20), for example, for EtAlCl2 ... 

These reactions have transition states different from those of propagation and transfer and can occur in the absence of monomer. Termination via ion collapse may be hindered in the ion-counterion-monomer complex by the presence of monomer, that is, the ion-counterion pair is in fact separated by complexed monomer. Reactions (17>—(19) may occur only in the uncomplexed ion-counterion pair. In that case ... 

Furthermore, since most molecular chains must be terminated by regenerative chain transfer, and hence the molecular weight is governed by the competition between propagation and transfer, by the assumed mechanism, no dependence of DP on dose rate would be expected. Metz et al. (25) have reported preliminary data which would question this. 

Experimental methods for determining the reactivity of a radical active centre are based on kinetic studies reactivity can be estimated from rate constant values, e.g. in propagation and transfer (see Chap. 8, Sect. 1.2). 

The scheme of propagation and transfer is strikingly similar to the scheme of copolymerization with a non-polymerizing comonomer [compare eqn. (1) with Chap. 5, eqn. (62)]. Our experience with copolymerization can therefore be applied to transfer. An undoubtedly favourable circumstance is the availability of the corresponding values of the apparent transfer constant Ctr. Using... 

The elementary reactions of carbocationic polymerizations can be separated into three types. Deactivation of carbenium ions with anions and transfer to counteranion are ion-ion reactions, propagation and transfer to monomer are ion-dipole reactions, and ionization is a dipole-dipole reaction [274]. Ion-ion and dipole-dipole reactions with polar transition states experience the strongest solvent effects. Carbocationic propagation is an ion-dipole reaction in which a growing carbenium ion adds electro-philically to an alkene it should be weakly accelerated in less polar solvents because the charge is more dispersed in the transition state than in the ground state [276]. However, a model addition reaction of bis(p-methoxyphenyl)carbenium ions to 2-methyl- 1-pentene is two times faster in nitroethane (e = 28) than in methylene chloride (e = 9) at - 30° C [193]. However, this is a minor effect which corresponds to only ddG = 2 kJ morit may also be influenced by specific solvation, polarizability, etc. [276,277]. 

Using the expression DP = p/(fc,r[M] + kt) and molecular weight-temperature data, the authors derive activation energies (A.E.) for propagation and transfer ... 

Table 7, Number of polymerization centers and values of propagation and transfer rate constants for ethylene polymerization... 

Each of these centers, upon which polymerization would take place according to the Burfield model, is probably characterized by different constants regarding the elementary propagation and transfer process, by different adsorption constants for the species present in the reaction phase, and by different intrinsic stability. Besides by these parameters, the kinetics is regulated by the equilibria between organo-aluminum and donor and their reaction products which determine the effective concentration of the components and, therefore, their effect on the active centers. 

In this section a brief review of quantum-chemical studies on the electron structure of the active center and the nature of the elementary steps of the chain propagation and transfer reactions for olefin polymerization is given. 

In the patent literature there are numerous indications to obtain polymers with broad MWD using catalysts containing different active centres. These catalysts are prepared by combining compounds of two or more transition metals (Ti, V, Zr, Cr, etc.). This supports the hypothesis that such centres give rise to independent polymerization processes characterized by different rate constants of the elementary stages of the propagation and transfer reactions. 

Doi et al. studied the effect of different aluminium alkyls on the polydispersity of syndiotactic polypropylene obtained with the V(acac)3-alkyl aluminium halide soluble catalytic system, at temperatures below —65 °C. These authors found that, by varying the type of aluminum alkyl not only the propagation and transfer rates are changed, but also the polymer polydispersity index decreases in the following order ... 

The comparable values of the polydispersity index for both isotactic and atactic polymers led Keii to conclude that the same active centre distribution exists according to the ratio between propagation and transfer rate constants. Furthermore, the low values of the polydispersity index would suggest good active centre homogeneity. 

More recently it has been shown that in the polymerization with TT-crotylnickel iodide the order in monomer falls from a value close to unity at [M] below 0.5 mole 1" to below 0.5 at [M] > 4 mole 1 . These observations have been interpreted in terms of scheme (c) on p. 162, namely coordination of two monomer molecules with the catalyst and with most of the catalyst existing in the complex (inactive) state. The molecular weights of the polymers are double those calculated from the kinetic scheme put forward [61] and this is attributed to coupling of live polymer chains on termination [251]. Molecular weight distributions are binodal consistent with slow propagation and transfer. 

It is also to be expected that pressure will affect the rate of chain transfer reactions to monomer, polymer, and solvent. In the polymerization of allyl acetate, where degradative chain transfer to monomer occm s, the rates of the propagation and transfer reactions increase by about the same amoimt for a given increase in pressure (17). The transfer reaction becomes less degradative—i.e., the allyl acetate radicals become more reactive—as pressure is increased. 

It is seen from Fig. 9.10 that chemisorbed ethylene disappears in initiation, propagation, and transfer with monomer. Therefore,... 

Kinetic analysis has shown that the experimental data can only be described if kp = f (q), kt = f (q), kx = const, and km = const. Although it is difficult to explain the finding of asymbathic kp and km variations within the framework of the physical interactions between chain propagation and transfer to the monomer, the researchers obtained a satisfactory agreement between the calculated and experimentally measured MWD at Pw/Pn varying from 1.92 to 6.0. 

Figure 1. The average degree of polymerization of the product, n, is a function of the relative rates of ethylene consumption and transfer. By invoking the steady-state assumption, it can be shown that the average degree of polymerization is governed by the competitive rates of propagation and transfer.

Deng. Margl, and Ziegler find transfer to monomer to be quite competitive with insertion for the recently reported iron ethylene polymerization catalyst. 5. The highest points on the propagation and transfer to monomer pathways differed by only 0.8 kcal/mol. not a good way to make polymers. 

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