Living/equilibrium polymer

Cationic polymerization of THF fulfills all the requirements of living polymerization. With several initiators, the initiation is relatively fast and quantitative, and propagation proceeds without transfer or termination. The dependence DP = ([M]0 - [M)e)/[I)0 holds up to high polymerization degrees. The only limitation is that, due to the reversibility of propagation, the molecular weight distribution is broadened and reaches the value of MjMn = 2 in equilibrium. Polymers with narrower MWD were obtained by terminating the polymerization at lower conversion [56]. 

In conclusion, a conversion of a cyclic monomer into a living linear polymer is thermodynamically allowed, provided that the reaction reduces the free energy of the system. However, the ultimate state of equilibrium corresponds to a mixture of cyclic oligomers and living linear polymers in appropriate proportion. 

Living (or equilibrium) polymers are formed by primary units (unimers) connected with reversible bonds. In the simplest case the living polymerization process can be thought of as a one-after-one attachment of unimers at the end of a growing chain (Figure 11(a)). 

Chemically activated equilibrium polymers (CAEPs). If the monomers are inactive unless activated through chemical reaction with initiator molecules, it obeys a chemically activated equilibrium polymerization. The amount of initiator added determines the number of activated monomers that in turn fixes the number of EPs in the system. The living polymerization of poly(a-methylstyrene) is an example of a CAEP, provided the polymerization reaction is not (chemically) terminated [18]. 

The above MC moves allow the formation of micellar structures from stable monomers. In the case of simpler reversible aggregating systems like linear living or equilibrium polymers one may integrate scission-fusion moves directly into the algorithm. We cannot discuss the different algorithms at this point and refer the reader to the literature (e.g.. Ref. 21 is a good starting point). 

If monomers can be added at the ends of polymeric units only one refers to the polymers as living polymers if polymeric chains can undergo scission and fusion anywhere along their contour they are called equilibrium polymers. 

Sedlak s results taken together unambiguously indicate that the polyelectrolyte slow mode in the systems he studied arises from long-lived equilibrium structures. The structures are much larger than a single polymer chain. The polymer concentration within a domain is larger than the concentration in the surrounding medium. 

Generally, the models used for simulation of living polymers can be divided roughly into two classes, focused on static or dynamic properties of the LP or GM. The static models are mainly designed to study equilibrium conformational properties of the polymer chains, critical behavior at the polymerization transition, and molecular weight distribution... 

In contrast to statics, the relaxational kinetics of living polymers and of giant wormlike micelles is unique (and different in both cases). It is entirely determined by the processes of scission/recombination and results in a nonlinear approach to equilibrium. A comparison of simulational results and laboratory observations in this respect is still missing and would be highly desirable. 

The equilibrium between monomer and living polymer is dynamic and therefore the molecular weight distribution of the polymer will change with time until the equilibrium distribution is reached. This is a peculiar process in which the amount of polymer present in the system, as well as its number average molecular weight is constant. This means also that, the number of polymeric... 

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. 

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