Slow reinitiation

With most systems polymerization rates have been found to be first order in both transition metal compound and monomer. The few instances reported where polymerizations are second order in monomer cannot be regarded as satisfactorily established as involving two monomer molecules in the propagation reaction. Apparent second order kinetics can, for example, arise as a result of chain transfer with slow reinitiation or from termination reactions. With one system [63] the rate has been found to be independent of monomer concentration. 

Polymerization in solution follows conventional kinetics except for certain solvent-specific side reactions. At monomer concentrations above 2-2.5 M, the reaction order with respect to monomer and initiator has been found to be 1.0 and 0.5, respectively [35]. In DMF, however, a monomer reaction order greater than expected was explained by chain transfer to solvent followed by slow reinitiating by the DMF radical [43]. At higher monomer concentrations, however, the monomer has the effect of adding a nonsolvent to the reaction mixture. Under these conditions, the reaction orders with respect to initiator and monomer can deviate from the expected values. Vidotto et al. [35] found that the reaction became heterogeneous at... 

RAFT agents have been compared for their ability to control MMA polymerization. Tertiary cyanoalkyl trithiocarbonates provide very good control over molecular weight and distribution and there is little retardation. The secondary trithiocarbonate RAFT agent with R=-CHPh(CN) also provides good control but a prolonged inhibition period attributed to slow reinitiation is manifest. The trithiocarbonate... 

The processes with slow reinitiation are called inhibition. In this case, the kinetics and molar masses are described by equations quite different from those for chain transfer to solvent [20]. Nevertheless, our calculations demonstrated that in nonterminating polymerization, in contrast to free radical polymerization, Eqs. (3.7) and (3.8) derived originally for fast initiation remain also valid for slow reinitiation (kri/ kts 1). In fact, the main condition for the validity of Eqs. (3.7) and (3.8) is not the high rate of reinitiation compared to transfer but the quasisteady-state approximation with regard to S, which is valid when (Skn/kp > 1. On the other hand, it was shown in Ref [11] that slow reinitiation leads to the second-order kinetics with respect to monomer. This effect was not, however, observed in anionic polymerization of nonpolar monomers. 

Slow reinitiation with reference to propagation following chain transfer. 

Processes which lead to retardation in conventional radical polymerization also affect RAFT polymerization. Thus, slow reinitiation (k,jt.conventional radical polymerization with irreversible chain transfer. The influence of slow reinitiation can be aggravated by the... 

Good control over the polymerization of LAMs requires use of a less active RAFT agent such as a dithiocarbamate (Z = NR 2) or a xanthates (Z=OR ) with R = alkyl or aryl. The more active RAFT agents Z = R (dithioesters) or SR (trithiocarbonates) strongly retard or inhibit polymerization of LAMs. The choice of the R group is also critical. Inhibition periods due to slow reinitiation are expected for RAFT agents such as 37 (R = 2-cyano-2-propyl) and 38 (R = benzyl). 

Globally, the overall kinetic scheme of B L polymerization involves propagation accompanied by transfer/deprotonation, followed by slow reinitiation reactions. 

Chain transfer, the reaction of a propagating radical with a non-radical substrate to produce a dead polymer chain and a new radical capable of initiating a new polymer chain, is dealt with in Chapter 6. There are also situations intermediate between chain transfer and inhibition where the radical produced is less reactive than the propagating radical but still capable of reinitiating polymerization. In this case, polymerization is slowed and the process is termed retardation or degradative chain transfer. The process is mentioned in Section 5.3 and, when relevant, in Chapter 6. 

The hydroxide ion is usually not sufficiently nucleophilic to reinitiate polymerization and the kinetic chain is broken. Water has an especially negative effect on polymerization, since it is an active chain-transfer agent. For example, C s is approximately 10 in the polymerization of styrene at 25°C with sodium naphthalene [Szwarc, 1960], and the presence of even small concentrations of water can greatly limit the polymer molecular weight and polymerization rate. The adventitious presence of other proton donors may not be as much of a problem. Ethanol has a transfer constant of about 10-3. Its presence in small amounts would not prevent the formation of high polymer because transfer would be slow, although the polymer would not be living. 

The use of protonic compounds such as HC1 or RCOOH in place of ROH or H20 yields a different result in most systems. When such substances take part in the exchange reaction, the result is not exchange as described above but inhibition or retardation since an anion, such as Cl or RCOO, possesses little or no nucleophilicity. Reinitiation does not occur or is very slow. The polymeric alcohols are no longer dormant but are dead. Both the polymerization rate and polymer molecular weight decrease along with a broadening of the polymer... 

Sometimes experimenters are tempted to determine the number of chains formed during polyerization and assume each site makes one chain, but a site terminates and reinitiates chains continuously, making this approach invalid except at very short reaction times. Quick-kill experiments in this laboratory (69) tend to confirm Hogan s number (77), but to actually see the first chain growing with time, the polymerization must be artificially slowed by using noncommercial conditions, and the results are not very reproducible. 

Allylic transfer is also variously named degradative chain transfer, autoinhibition, or allylic termination. The stable radical derived from the monomer by reactions like (6-90) are slow to reinitiate and prone to terminate. Low-molecular-weight products are therefore formed at slow rates and small concentrations of allyl monomers can inhibit or retard the polymerization of more reactive monomers. 

If the new radicals T- or MpQ- do not react readily with more monomer, there will be a decrease in the concentration of reactive radicals and a concurrent reduction in the rate of polymerization. When the rate of reaction (6-91 b) or (6-9 Ic) is very much greater than that of reaction (6-9la) and the new radicals T- or M Q- do not add monomer then high-molecular-weight polymer will not be formed and will beefl ectively zero. This is a case of inhibition. In retardation the polymerization is slowed but not entirely suppressed. This occurs if (I) the rate of either alternative process is close to that of the monomer addition reaction (6-91 a) and the new radicals from steps (6-9lb) or (6-9Ic) do not reinitiate, or (2) if the alternative processes are fast compared to ordinary monomer addition but the new radicals formed reinitiate slowly. 

These ions were considered as non-reactive, i.e. it was assumed that any reinitiation reaction is slow as compared with chain propagation and tmnination reactions. 

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