Rate, of activation constant

There are several guidelines that should be followed in order to increase the chemoselectivity of the monoadduct. Firstly, radical concentration must be low in order to suppress radical termination reactions (rate constant of activation [fcal and fca2] < < rate constant of deactivation kd t andfcd2]). Secondly, further activation of the monoadduct should be avoided ( al> >kd2). Lastly, formation of oligomers should be suppressed, indicating that the rate of deactivation (kd 2[Cu"LmX]) should be much larger than the rate of propagation ( [alkene]). Alkyl halides for copper-catalyzed ATRA are typically chosen such that if addition occurs, then the newly... 

Fig. 9 Rate constant of activation (ka in M 1 s ) for various ligands with ethyl-2-bromoisobu-tyrate in the presence of Cu Br in CH3CN at 35 C [130]...

Other methods for the deactivation of carbenium ions are based on the use of nucleophiles for example, sulfides act as deactivators in the polymerization of IBVE initiated by triflic acid [37,38]. In this case, the sulfides react with the carbenium ions to form sulfonium ions. Dynamic NMR measurements were used to estimate the rate of the exchange process and demonstrated that exchange occurs predominantly by dissociation of the sulfonium ions to carbenium ions rather than bimolecularly via exchange of the sulfonium ion with the sulfide [21], The rate constant of activation was kacl = 60 sec-1 and the rate constant of deactivation is d<=aci = 106 mol- -L-sec-1 at -30° C in CH2CI2. 

C ] = concentration of active site at lower monomer concentration kP] — chain propagation rate constant of active site 1, including effects of... 

Scheme 12.2 Illustration of well-defined azo BCs prepared by ATRP. Kact and Kdeact are rate constants of activation and deactivation steps, respectively. Kp and Kt are rate constants of propagation and termination of generated free radicals. X is a halogen atom. P represents a polymer chain.

Fig. 3.5 Nitrogen-based ATRP ligands, a ATRP rate constants of activation in M for different...

In this expression, is Avogadro s number, is the partial pressure, and is the molecular weight of the feth species. Assuming an ideal gas, the rate constant of (activated) adsorption at zero coverage is expressed by... 

CH2 k 5 cm s. The rate constants of activation by various gases have... 

The penultimate unit effect may play a very important role in ATRR The rate constants of activation of monomeric and dimeric alkyl bromides with a CuBr-bpy (bpy=2,2 -bipyridine) complex as activator were determined. The ATRP relies on the reversible activation of a dormant alkyl halide through halogen abstraction by a transition metal complex to form a radical that participates in the classical free-radical polymerization figure (Fig. 2) prior to deactivation. In this equiUbrium, the alkyl radical (Pm ) is formed in an activated process, with a rate constant kact> by the homolytic cleavage of an alkyl halogen bond (Pm-Z) catalyzed by a transition metal complex in its lower oxidation state (Cu ). The relative values of fcact of the alkyl bromides were determined for CuBr/bpy catalyst systems in acetonitrile at 35°C. These systems followed the order EBriB (30) MBrP (3)>iBBrP (1) for monomeric initia-tors and MMA-MMA-Br (100) MA-MMA-Br (20) > MMA-MA-Br (5) > MA-MA-Br (1) for dimeric initiators. ... 

Fig.2 summarizes the general friction (or viscosity) dependence expected for the rate constant of activated barrier crossing reactions. 

Fig. 2 Reversible and dynamic equilibrium between active radical growing species and dormant species (K, means rate constant of activation Kj means rate constant of deactivation Rp means rate constant of propargation M means monomer, P-X represents dormant polymer species represents reactive polymer radical species)...

The rate constants of activation feaa and deactivation fedeact given in the general scheme (Scheme 1) are defined as a pseudo-first-order constant in the unit of s . Every dormant chain is activated once every feact s and deactivated back to the dormant state after a transient lifetime of fedeaa on average. In typical successful LRPs, feaa" = 10-10 s and... 

The ATRP process is characterized by the ATRP equilibrium constant, which can be defined as either the ratio of the rate constants of activation and deactivation or as the ratio of concentrations of all involved species. 

Logically, this section should discuss the other rate constant characterizing the ATRP process, namely the deactivation rate constant, kdeact- However, the values of kdeact are typically rather large and difficult to determine experimentally. They are often calculated as the ratio kdeact = kact/KAXRP of the much easier to determine rate constant of activation and equilibrium constant of ATRP. This is why this section is dedicated to the experimental determination of Katrp as well as to the factors (initiator and catalyst structure, solvent, etc.) that influence its values. As seen from eqn (2), the rate of polymerization under classical ATRP conditions depends on the value of the equilibrium constant. 

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