Slow Reinitiation by

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... 

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. 

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... 

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

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. 

Rapid heating of the fracture to 120"C, under conditions of constant effective stress, sharply reduces the fracture aperture to 4.4 pm within 60 hours followed by a slow decrease of a few tenths of a micron spread over the following 100 hours (at constant temperature). The slow monotonic reduction in the fracture aperture is attributed to the redistribution of quartz within the fracture. This could result from the combined effects of dissolution of contacting fracture asperities or propping grains, and precipitation in flow-throats. Two sharp decreases in fracture aperture are apparent at 103 h and 115 h and are correlated with shutdown of the water injection pump for 2 min and 1.5 hours, respectively. A third resulted from pump failure. In all instances, changes are sufficiently rapid (visible instantly after the flow reinitiated) that asperity breakage under elevated effective stress is the likely cause. These stress-related reductions in aperture are irreversible. The... 

Some chain transfer agents yield radicals with low activity and if the reinitiation reaction is slow, the polymerization rate decreases because there is a buildup of radicals leading to increased termination by couphng. When this happens the substance responsible is said to be a retarder, e.g., nitrobenzene acts in this way with styrene. 

In the case of difunctional polymers, captodative olefins rapidly scavenge propagating radicals to give new radicals which are unable, or slow, to reinitiate polymerization. These radicals have no p-hydrogens and therefore are unable to decay by disproportionation. Thus they react exclusively by combination. The functional groups are again provided by the initiator. 

Another form of retardation occurs when a radical species formed from transfer (e.g., S in Scheme 3.2) reinitiates at a slow rate. The termination of S with other radicals in the system needs to be considered, as well as the slower reaction rate of S with monomer to form a polymer radical, as shown by the network of reactions. 

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