Living/controlled systems

A similar in situ approach to alkoxide formation employs the readily accessible tris(amide)-Y(NTMS2)3, (293), and PrOI 1.881 In the absence of alcohol the polymerization of CL is fast but not controlled (Mw/Mn > 3). However, upon addition of alcohol, a controlled living system with polydispersities 1.1-1.2 results. At least 50 equivalents of PrOH may be added (the excess effects rapid chain transfer) with molecular weights in good agreement with theoretical values. Similar results have been reported using Nd(NTMS2)3, (294), and PrOH.882 The reactions of PrOH with (292) and (293) have both been studied by NMR, and in both cases Y5(/r5-0)(0 Pr)13 is not... 

The controversies, attention, and active discussions of the mechanisms of new controlled/living systems in the 1990s probably originate in an early... 

Controlled/living systems can be usually obtained when the polymerization is sufficiently slow and when either nucleophilic anions or additives are present (Sections IV and V). This means that the proportion of carbenium ions should be low and conversion to dormant species, fast. Nevertheless, under such conditions cationic species can be detected by dynamic NMR, by ligand exchange, salt, and solvent effects, and by other methods discussed in Chapters 2, 3, and in this section. Under typical controlled/living conditions, dormant species such as onium ions and covalent esters predominate. It is possible that the active species are strongly solvated by monomer and by some additives. These interactions may lead to a stabilization of the carbocations. However, in the most general case, this stabilization has a dynamic sense and can be described by the reversible exchange between carbocations and dormant species. 

In controlled/living systems reactions B and C can be avoided or converted into reversible ones, if ligands such as fluorides are not used, if the concentration of moisture is very low in comparison with initiator and if weakly basic/nucleophilic components (additives, counteranions) are used. Contribution of reaction A is reduced at low temperatures but can not be eliminated completely. 

These equilibria also strongly affect copolymerization. Monomer reactivity ratios in controlled/living systems should be identical to those in conventional cationic copolymerizations, if the comonomers react exclusively with carbocationic species. The equilibrium between active and... 

E. Role of Major Components of Controlled/Living Systems... 

A similar analysis may be applied to this system, but now assuming that the active chains are in equilibrium with a reservoir of dormant chains with Kgq = 10 . Eqn (2.20) shows that now only 1% of the original chains have decomposed in 28 h. However, 95% of the monomer is consumed by these reversibly-deactivated chains in about 100 min. The reversible-deactivation equilibrium has thus transformed a poorly controlled, apparently non-living system into a controlled/living system with practical synthetic possibilities. This analysis also assumes that the ratio ktr,sp/kp was unaffected by whatever system change (solvent, counterion, temperature, etc.) that was carried out to create the large reservoir of dormant chain ends. Very likely, the decrease in ionicity e.g., elimination of free ions in the system) also caused this number to become smaller such that the intrinsic livingness of the system was also improved. 

Most chemically reacting systems tliat we encounter are not tliennodynamically controlled since reactions are often carried out under non-equilibrium conditions where flows of matter or energy prevent tire system from relaxing to equilibrium. Almost all biochemical reactions in living systems are of tliis type as are industrial processes carried out in open chemical reactors. In addition, tire transient dynamics of closed systems may occur on long time scales and resemble tire sustained behaviour of systems in non-equilibrium conditions. A reacting system may behave in unusual ways tliere may be more tlian one stable steady state, tire system may oscillate, sometimes witli a complicated pattern of oscillations, or even show chaotic variations of chemical concentrations. 

Oxygen concentrations can be high because the rates of many of the reactions of oxygen at ambient temperatures are very slow. The reaction of oxygen and fixed carbon by living systems involves control of the rate by a complex... 

Clearly the improved understanding of colloidal behaviour within living systems that we are developing offers the eventual prospect of our being able to manipulate such systems. The control of microarchitecture in both living and synthetic systems has many potential applications. The most important aspect is the ability to define the particular conditions under which a certain pattern or structure will be formed such that the products will be uniform. This clearly happens in Nature, but natural systems have been subject to trial and error for considerably longer than any experiment involving synthetic systems. 

Transdermal controlled-release systems can be used to deliver drugs with short biological half-lives and can maintain plasma levels of very potent drugs within a narrow therapeutic range for prolonged periods. Should problems occur with the system or a change in the status of the patient require modification of therapy, the system is readily accessible and easily removed. 

Biochemical reactions must cater for living systems and as a result are carried out in an aqueous medium within a narrow range of conditions. Each species of microorganism grows best under certain conditions. Temperature, pH, oxygen levels, concentrations of reactants and products and possibly nutrient levels must be carefully controlled for optimum operation. 

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