Activated monomer kinetics

Improved control was observed, however, upon addition of benzyl alcohol to the dinuclear complexes.887 X-ray crystallography revealed that whereas (296) simply binds the alcohol, (297) reacts to form a trinuclear species bearing four terminal alkoxides. The resultant cluster, (298), polymerizes rac-LA in a relatively controlled manner (Mw/Mn=1.15) up to 70% conversion thereafter GPC traces become bimodal as transesterification becomes increasingly prevalent. NMR spectroscopy demonstrates that the PLA bears BnO end-groups and the number of active sites was determined to be 2.5 0.2. When CL is initiated by (298) only 1.5 alkoxides are active and kinetic analysis suggests that the propagation mechanisms for the two monomers are different, the rate law being first order in LA, but zero order in CL. 

Grossman, S.H., Pyle, J., and Steiner, R.J. (1981) Kinetic evidence for active monomers during the reassembly of denatured creatine kinase. Biochemistry 21, 6122. 

The autocatalytic region could be associated with the buildup of a steady-state concentration of an active monomer 2 a from the dimer 2. The spectroscopic evidence indicates that the concentration of 2a remains low, that is, Zc i > klt Writing Ki = ki/k-i yields the rate law shown in Equation 9 for the region of maximum activity where RuT refers to the total ruthenium calculated as monomer. The observed kinetics are con-... 

Later, other authors utilized the differences found in the optical activity of monomer and polymer to carry out kinetic investigations on the free-radical polymerisation (70,72,120) and copolymerization (71), and tried to achieve the steric control of the propagation step of free-radical polymerization and copolymerization (13, 14, 39, 73, 98) using optically active monomers and initiators. 

Again, we have made use of 0n = which holds for Gaussian chains. Equation (E.4) is formally identical to the equation for linear polycondensates. It differs, however, essentially in the meaning of a that is no longer the overall extent of reaction, which would be proportional to the monomer consumption, but is instead a conditional probability that an activated monomer has formed a bond with another monomer. This probability is only weakly dependent on the monomer conversion and cannot be determined by titration but has to be determined from kinetic measurements107. ... 

Anionic polymerization of 67 by the activated monomer mechanism should occur with the selective cleavage of the CO—NH bond of the monomer to give a polyamide composed of kinetically controlled cis units (68c). However, the cis units isomerize to the thermodynamically more stable trans units (68t) through the proton abstraction from the methine group adjacent to the carbonyl group. This was ascertained by the isomerization experiment in which a polymer consisting of 92% cis unit and 8% trans unit was converted to one containing 40% cis unit and 60% trans unit when heated in dimethyl sulfoxide at 80 °C for 6 hours in the presence of 15 mol% potassium pyrrolidonate. 

The ratio of linear and cyclic macromolecules will be sharply affected by a change in the kinetics and mechanism of propagation, e. g. by transition from growth on activated chain ends to polymerization with activated monomer. An inactive macromolecule cannot produce macrocycles, neither by back- nor by end-biting reactions [339],... 

In this sequence of reactions, it is the monomer that forms oxonium ion [thus Activated Monomer (AM) mechanism] and the growing chain end is neutral. As shown in the series of papers [107-115], if the conditions are created, when AM mechanism predominates, by keeping the low instantaneous ratio of [monomer]/[HO-] (slow addition of monomer to reaction mixture), back-biting is effectively eliminated. Linear polymers, free of cyclic fraction are obtained under these conditions. The mechanism and kinetics of AM polymerization of oxiranes is discussed in detail in recent monograph [6]. 

As discussed already for cationic polymerization of oxiranes, cycliza-tion can be eliminated if polymerization is performed under the conditions at which the activated monomer mechanism operates. This approach was used for cationic polymerization of e-caprolactone and other higher lactones [191]. Thus, in the polymerization of e-caprolactone in the presence of ethylene glycol (EG) and (C2Hs)30 +, PF6- catalyst, linear increase of molecular weight with conversion was observed up to M 3000 and polymers with DP = [M]o/[EG]0 and relatively narrow molecular weight distribution (MJM 1.3) were obtained. No cyclic oligomers were detected in reaction products. Similar results were obtained for polymerization of 5-valerolactone and j8-butyrolactone. Kinetic studies of the AM polymerization of lactones have been reported [192]. 

Kinetics. Kinetic analysis of the reaction of the dimer 8A with propene confirms that dissociation to the active monomer 7 is necessary and that the rate-determining step is insertion of propene into the LU-CH3 bond of 7A (reactions 1 and 2 in Scheme 2). At IS C K = 4.1x10"3 M" and k2 = 1.22x10-1... 

Although the orthoester method has not been investigated kinetically, it probably can be considered as an activated monomer synthesis, a classification introduced by Bamford (131) and elaborated by Szwarc (132). Some implications of Szwarc s analysis may be pertinent to the orthoester polymerization. Szwarc has pointed out regarding a related polymerization of Leuch s anhydrides Increasing the concentration of initiator has a dual effect on the rate of such a polymerization. It increases the stationary concentration rrf growing species — a trivial effect expected in... 

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