Termination, ionic radical

Ionic polymerizations are characterized by a wide variety of modes of initiation and termination. Unlike radical polymerization, termination in ionic polymerization never involves the bimolecular reaction between two propagating polymer chains of like charge. Termination of a propagating chain occurs by its reaction with the counterion, solvent, or other species present in the reaction system. 

Rate constants for ionic propagation reactions vary but some are higher than those in free-radical systems. This is particularly true in media where the ionic active center is free of its counterion. The high reactivity of ionic active centers, which yields fast propagation rates, also results in a greater propensity toward side reactions and interference from trace impurities. Low temperatures favor propagation over competing reactions which cause chain termination. Ionic polymerizations are thus performed often at much colder temperatures than those used in free-radical processes, which would be impossibly slow under the same conditions. 

The nature of the radical within the latex particle determines its fate ie, its propensity to desorp, propagate, chain transfer, or terminate. It seems reasonable that an ionic radical will not penetrate deeply into a latex particle but rather anchor its ionic head on the surface or palisade of the latex particle, much the way a surfactant molecule does. Once anchored, the nonpolar tail containing the radical will penetrate into the particle, and reactively diffuse throughout the poljmier and monomer solution imtil either the ionic radical desorbs back into the aqueous phase, the ionic radical terminates with another radical within the particle, or the ionic radical undergoes a chain transfer event with either the monomer, polymer, or a chain-transfer agent within the latex particle. Once a chain transfer event occurs, the new radical becomes nonionic and has a markedly different solublity in the particle and aqueous phases. As the nonionic radical grows in chain length within the particle, it becomes even less soluble in the aqueous phase and becomes less likely to desorb. Such a qualitative description of radical fate was quantified... 

In order to reduce the contribution of termination processes (Scheme 2.19), controlled/living radical methods establish a fast equilibrium between dormant and active chain ends. Essentially, this equilibrium decreases the relative proportion of propagating radicals and allows them to reversibly deactivate rather than permanently terminate. The radical chain end concentration for CRPs (10 -10" M) is also much less than for ionic polymerizations (10 -10 M). 

Both modes of ionic polymerization are described by the same vocabulary as the corresponding steps in the free-radical mechanism for chain-growth polymerization. However, initiation, propagation, transfer, and termination are quite different than in the free-radical case and, in fact, different in many ways between anionic and cationic mechanisms. Our comments on the ionic mechanisms will touch many of the same points as the free-radical discussion, although in a far more abbreviated form. 

The molecular weight distribution for a polymer like that described above is remarkably narrow compared to free-radical polymerization or even to ionic polymerization in which transfer or termination occurs. The sharpness arises from the nearly simultaneous initiation of all chains and the fact that all active centers grow as long as monomer is present. The following steps outline a quantitative treatment of this effect ... 

Even though the catalyst may be only partially converted to H B", the concentration of these ions may be on the order of 10 times greater than the concentration of free radicals in the corresponding stationary state of the radical mechanism. Likewise, kp for ionic polymerization is on the order of 100 times larger than the sum of the constants for all termination and transfer steps. By contrast, kp/kj which is pertinent for the radical mechanism, is typically on the order of 10. These comparisons illustrate that ionic polymerizations occur very fast even at low temperatures. 

Copolymer composition can be predicted for copolymerizations with two or more components, such as those employing acrylonitrile plus a neutral monomer and an ionic dye receptor. These equations are derived by assuming that the component reactions involve only the terminal monomer unit of the chain radical. The theory of multicomponent polymerization kinetics has been treated (35,36). 

A factor in addition to the RTD and temperature distribution that affects the molecular weight distribution (MWD) is the nature of the chemical reaciion. If the period during which the molecule is growing is short compared with the residence time in the reactor, the MWD in a batch reactor is broader than in a CSTR. This situation holds for many free radical and ionic polymerization processes where the reaction intermediates are very short hved. In cases where the growth period is the same as the residence time in the reactor, the MWD is narrower in batch than in CSTR. Polymerizations that have no termination step—for instance, polycondensations—are of this type. This topic is treated by Denbigh (J. Applied Chem., 1, 227 [1951]). 

A number of important addition polymers are produced by ionic mechanisms. Although the process involves initiation, propagation and termination stages the growing unit is an ion rather them a radical. 

Even within the small numbers of studies conducted to date, we are already seeing potentially dramatic effects. Free radical polymerization proceeds at a much faster rate and there is already evidence that both the rate of propagation and the rate of termination are effected. Whole polymerization types - such as ring-opening polymerization to esters and amides, and condensation polymerization of any type (polyamides, polyesters, for example) - have yet to be attempted in ionic liquids. This field is in its infancy and we look forward to the coming years with great anticipation. 

The block copolymer produced by Bamford s metal carbonyl/halide-terminated polymers photoinitiating systems are, therefore, more versatile than those based on anionic polymerization, since a wide range of monomers may be incorporated into the block. Although the mean block length is controllable through the parameters that normally determine the mean kinetic chain length in a free radical polymerization, the molecular weight distributions are, of course, much broader than with ionic polymerization and the polymers are, therefore, less well defined,... 

The thermal (or photochemical) decomposition of the azo group gives rise to a radically initiated polymerization. The reactive site F, the transformation site, however, can, depending on its chemical nature, initiate a condensation or addition type reaction. It can also start radical or ionic polymerizations. F may also terminate a polymerization or even enable the azo initiator to act as a monomer in chain polymerizations. 

Low-molecular weight azo compounds have frequently been used in cationic polymerizations producing azo-containing polymers. Thus, the combination of ionically and radically polymerizable monomers into block copolymers has been achieved. Azo compounds were used in all steps of cationic polymerization without any loss of azo function as initiators, as monomers and, finally, as terminating agents. 

The elementary reactions comprising the chain reaction mechanism are generally classified as initiation, propagation, or termination reactions. In the initiation reaction an active center or chain carrier is formed. Often these are atoms or free radicals, but ionic species or other intermediates can also serve as chain carriers. In the propagation steps the chain carriers interact with the reactant molecules to form product molecules and regenerate themselves so that the chain may continue. The termination steps consist of the various methods by which the chain can be broken. 

Chain polymerisation involves three major steps (i.e., initiation, propagation and termination). This process of chain polymerisation can be brought about by a free radical, ionic or coordination mechanism. 

The mutual termination of growing chains which prevails in radical polymerizations must be ruled out for all ionic systems in which the opposite ions form separate kinetic units because of the electrostatic repulsion between like ions. However, in solvents of low DC in which the growing end of the polymer chain consists of an ion pair, a mutual termination by interaction of two such ion pairs is at least conceivable. 

The apparent rate constant for polymerization increased with decreasing monomer concentrations of [16 n = 3 and 6], whereas [16 n = 2] did not show this effect. Thus, the phenomenon must be caused by a diminished termination rate because of an increased electrostatic repulsion of the ionic polymer radicals. Unfortunately, however, the properties of these polymers are similar to those of ordinary polymer micelles. Therefore, immobilization appears to be inefficient for systems in dynamic equilibrium. 

Living free-radical polymerization has recently attracted considerable attention since it enables the preparation of polymers with well-controlled composition and molecular architecture previously the exclusive domain of ionic polymerizations, using very robust conditions akin to those of a simple radical polymerization [77 - 86]. In one of the implementations, the grafting is achieved by employing the terminal nitroxide moieties of a monolith prepared in the presence of a stable free radical such as 2,2,5,5-tetramethyl-l-pyperidinyloxy (TEMPO). In this way, the monolith is prepared first and its dormant free-... 

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. 

---