Termination during living radical polymerization

It remains a common misconception that radical-radical termination is suppressed in processes such as NMP or ATRP. Another issue, in many people s minds, is whether processes that involve an in-eversible termination step, even as a minor side reaction, should be called living. Living radical polymerization appears to be an oxymoron and the heading to this section a contradiction in terms (Section 

In any processes that involve propagating radicals, there will be a finite rate of termination commensurate with the concentration of propagating radicals and the reaction conditions. The processes that fall under the heading of living or controlled radical polymerization (e.g. NMP, ATRP, RAFT) provide no exceptions. 

In conventional radical polymerization, the chain length distribution of propagating species is broad and new short chains are formed continually by initiation. As has been stated above, the population balance means that, termination, most frequently, involves the reaction of a shorter, more mobile, chain with a longer, less mobile, chain. In living radical polymerizations, the chain lengths of most propagating species are similar i.e. i j) and increase with conversion. Ideally, in ATRP and NMP no new chains are fonned. In practice, 

It can also be noted that reversible chain transfer, in RAFT and similar polymerizations, and reversible activation-deactivation, in NMP and ATRP, provide other mechanisms for reaction diffusion. 

Knowledge is also important in designing polymer syntheses. For 

---

Termination in heterogeneous polymerization is discussed in Section 5.2.1,5 and the more controversial subject of termination during living radical polymerization is described in Section 5.2.1.6. Termination in copolymerization is addressed in Section 7.3. 

While in most of the reports on SIP free radical polymerization is utihzed, the restricted synthetic possibihties and lack of control of the polymerization in terms of the achievable variation of the polymer brush architecture limited its use. The alternatives for the preparation of weU-defined brush systems were hving ionic polymerizations. Recently, controlled radical polymerization techniques has been developed and almost immediately apphed in SIP to prepare stracturally weU-de-fined brush systems. This includes living radical polymerization using nitroxide species such as 2,2,6,6-tetramethyl-4-piperidin-l-oxyl (TEMPO) [285], reversible addition fragmentation chain transfer (RAFT) polymerization mainly utilizing dithio-carbamates as iniferters (iniferter describes a molecule that functions as an initiator, chain transfer agent and terminator during polymerization) [286], as well as atom transfer radical polymerization (ATRP) were the free radical is formed by a reversible reduction-oxidation process of added metal complexes [287]. All techniques rely on the principle to drastically reduce the number of free radicals by the formation of a dormant species in equilibrium to an active free radical. By this the characteristic side reactions of free radicals are effectively suppressed. 

With the recent development of living radical polymerization, the problem of gel formation during radical polymerization possibly can be controlled. This is because termination by radical chain coupling is virtually eliminated. Thus Hawker reported the preparation of soluble hyperbranched polystyrene using alkoxyamine IV as a living radical polymerization initiator [12]. 

The reactions associated with RAFT equilibria are in addition to those that occur normally during conventional radical polymerization (i.e. initiation, propagation and termination). The RAFT agent is a transfer agent. Since radicals are neither formed nor destroyed as a consequence of the RAFT process, termination is not directly suppressed. Retention of the macromonomer end group in the polymeric product is responsible for the living character of RAFT polymerization and renders the process potentially suitable for synthesizing block copolymers and end functional polymers. 

Bengough and Norrish observed this behaviour during vinyl chloride polymerization. They explained it by transfer to polymer chains on which immobile, long-lived and propagating radicals are formed. These centres decay by transfer to monomer or by termination with untrapped radicals from the liquid phase [47], According to these two authors, the acceleration is proportional to the surface area of the solid particles. A similar acceleration of polymerization was observed by Bamford et al. [18] with acrylonitrile... 

Radical polymerizations are almost always considered as kinetically stationary. However, the stationarity conditions are not always fulfilled. Living polymerizations with rapid initiation are stationary, but the character of the medium should not significantly change during polymerization in order to prevent shifts in the equilibria between ion pairs and free ions. All other polymerizations are non-stationary even, to some extent, living polymerizations with slow initiation. It is usually very difficult to define initiation and termination rates so as to permit exact kinetic analysis. When the concentration of active centres cannot be directly determined, indirect methods must be applied, and sometimes even just a trial search for best agreement with experiment. 

The absence of termination during a living polymerization leads to a very narrow molecular-weight distribution with polydispersities as low as 1.06. By comparison, polydispersities above 2 and as high as 20 are typical in free radical polymerization. 

---