Reversible deactivation polymerization

Chain Breaking Reactions in Reversible-Deactivation Polymerizations... 

Figure 2.5 shows plots of Mw/Mn vs. conversion for polymerization systems possessing a Poisson molecular weight distribution (eqn (2.30)) and three distributions predicted by eqn (2.31) (P = kd/kp[l]f) with kpjk = 5, 2, and 0.5 L moU For the system with slow exchange kpjk = 5 L moU ), the final distribution at p=1.0 is 1.05, significantly broader than Poisson. For A p/ (j = 0.5LmoU the distribution of the reversible-deactivation polymerization is very nearly Poisson at high conversions but deviates from Poisson at low conversions. For p/ d<0-5L mol , eqn (2.31) predicts a distribution that is more narrow than Poisson, especially at low conversions. 

Certain monomers may be able to act as reversible deactivators by a reversible addition-fragmentation mechanism. The monomers are 1,1-disubstituted and generate radicals that are unable or extremely slow to propagate or undergo combination or disproportionation. For these polymerizations the dormant species is a radical and the persistent species is the 1,1 -disubstituted monomer. 

Although more studies need to be performed to study the scope and generality of this system, the use of amine hydrochloride salts as initiators for controlled NCA polymerizations shows tremendous promise. Fast, reversible deactivation of a reactive species to obtain controlled polymerization is a proven concept in polymer chemistry, and this system can be compared to the persistent radical effect employed in all controlled radical polymerization strategies [37]. Like those systems, success of this method requires a carefully controlled matching of the... 

Termination is formally an irreversible deactivation of growing species. That is, reversible termination is not a real termination process and would be more appropriately labeled reversible deactivation. If this reversible deactivation is sufficiently dynamic, the number of growing species remains constant throughout the polymerization and all chains have the same opportunity to grow, resulting in polymers with narrow molecular weight distributions. This will be discussed in detail in Chapter 4. 

Although more studies need to be performed to study the scope and generality of this system, the use of amine hydrochloride salts as initiators for controlled NCA polymerizations shows tremendous promise. The concept of fast, reversible deactivation of a reactive species to obtain controlled polymerization is a proven concept in polymer chemistry, and this system can be compared to the persistent radical effect employed in all controlled radical polymerization strategies [34]. Like those systems, the success of this method requires a carefully controlled matching of the polymer chain propagation rate constant, the amine/amine hydrochloride equilibriiun constant, and the forward and reverse exchange rate constants between amine and amine hydrochloride salt. This means it is likely that reaction conditions (e.g. temperature, halide counterion, solvent) will need to be optimized to obtain controlled polymerization for each different NCA monomer, as is the case for most vinyl monomers in controlled radical polymerizations. Within these constraints, it is possible that controlled NCA polymerizations utilizing simple amine hydrochloride initiators can be obtained. 

All CRP methods rely on a dynamic equihbration between tiny amounts of propagating radicals and various types of dormant species. The essence of the process is a rapid reversible deactivation of growing radicals. Radicals always terminate and therefore CRP is never living in the pure sense of the living polymerization definition. Indeed, lUPAC recommends to avoid using term Uving for the radical polymerization and suggests to use the term controlled reversible deactivation radical polymerization . 

ATRP was applied to the copolymerization of a monovinyl monomer and a divinyl cross-linker to study the experimental gelation behavior. The fundamental features of ATRP, including fast initiation and reversible deactivation reactions, resulted in a retarded gelation and the formation of a more homogeneous network in the ATRP process compared to gel formation in a conventional radical polymerization. The experimental gel point based on the monomer conversion in the ATRP reaction occurred later than the calculated value based on Flory-Stockmayer s mean-field theory, which was mainly ascribed to intramolecular cyclization reactions. The dependence of the experimental gel points on several parameters was systematically studied, including the ratio of cross-linker to initiator, the concentration of reagents, reactivity of vinyl groups, initiation efficiency of initiators, and polydispersity of primaiy chains. 

Miilhaupt et al. studied the kinetics of propylene oligomerization catalyzed by Cp2ZrCl2/MAO in toluene and subsequently proposed a reversible + irreversible deactivation process kinetic scheme to fit the decay of the polymerization rate as a function of time (eq 49). The reversible deactivation is second-order relative to the zirconium active site concentra-... 

However, the possibility that hydrogen reactivates other types of reversibly deactivated metal species (such as Mt—Mt dimers, Mt—Al complexes), should be always kept in mind. One might consider that all these mechanisms will operate to different extents, depending on the type of catalyst and the polymerization conditions. 

Jenkins, A.D., Jones, R.G., Moad, G., 2010. Terminology for reversible-deactivation radical polymerization previously called controlled radical or living radical polymerization. Pure Appl. Chem. 82 (2), 483 91. 

Fast initiation and rapid reversible deactivation are the key to a successful ATRP, combing of a small number of terminated chains and an uniform chain growth mechanism. Thus, the polymerization rateRp plays an important role and can be defined by the kinetic parameters and reagent concentrations... 

As of 1990, this conventional free radical polymerization is developed to the newer reversible deactivation radical polymerization. More details of each will be... 

It is apparent that, as a result of the extremely rapid propagation, if aU chain ends were ionized and grew simultaneously, monomer would disappear at such a high rate that the polymerization would be uncontrollable. In hving cationic polymerization, therefore, a dynamic equilibrium must exist between a very small amount of active ionic and a large pool of inactive dormant species. The expression of controlled polymerization is sometimes used to describe, perhaps questionably, such polymerizations with reversible deactivation of the chain carriers. 

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