Thermodynamically Controlled Polycondensations

More recent experiments proving the presence of very large rings in step reactions and a discussion of the role of ring formation in step polymerization are summarized by Kricheldorf HR (2003) The Role of Ring-ring Equilibria in Thermodynamically Controlled Polycondensation, Macromol Symp 199 15-22 see also other papers in the same issue and the introduction What Does Polycondensation Mean Ibid pp 1-13. 

Kricheldorf, H.R. (2003) Macrocycles. 21. Role of ring-ring equilibria in thermodynamically controlled polycondensations. Macromolecules, 36,2302-2308. 

Shorter chain dienes have an increased propensity to form stable five-, six-, and seven-membered rings. This thermodynamically controlled phenomenon is known as the Thorpe-Ingold effect.15 Since ADMET polymerization is performed over extended time periods under equilibrium conditions, it is ultimately thermodynamics rather than kinetics that determine the choice between a selected diene monomer undergoing either polycondensation or cyclization. 

Measured ring concentrations for thermodynamically controlled (reversible) polycondensations in bulk are usually only a few percent, so this is not a major factor in those systems. 

Interestingly, Frey et al. [119] reached exactly the same conclusions, although their polycondensations were kinetically controlled, whereas Feast et al. had worked on a thermodynamically controlled system involving cyclization by back-biting . Frey et al. reinvestigated the proton-catalyzed polycondensation of 2,2-bishydroxymethyl propionic acid ((a) in Formula 11.4). It was found that Hult s polyesters (inch the commercial Perstorf products) possess extremely low... 

Noteworthy that all the above formulated results can be applied to calculate the statistical characteristics of the products of polycondensation of an arbitrary mixture of monomers with kinetically independent groups under any regime of this process. To determine the values of the elements of the probability transition matrix of corresponding Markov chains it will suffice to calculate only the concentrations Q()- of chemical bonds (ij) at different conversions of functional groups. In the case of equilibrium polycondensation the concentrations Qy are controlled by the thermodynamic parameters, whereas under the nonequilibrium regime of this process they depend on kinetic parameters. 

Referring to the ADMET mechanism discussed previously in this chapter, it is evident that both intramolecular complexation as well as intermolecular re-bond formation can occur with respect to the metal carbene present on the monomer unit. If intramolecular complexation is favored, then a chelated complex, 12, can be formed that serves as a thermodynamic well in this reaction process. If this complex is sufficiently stable, then no further reaction occurs, and ADMET polymer condensation chemistry is obviated. If in fact the chelate complex is present in equilibrium with re complexation leading to a polycondensation route, then the net result is a reduction in the rate of polymerization as will be discussed later in this chapter. Finally, if 12 is not kinetically favored because of the distant nature of the metathesizing olefin bond, then its effect is minimal, and condensation polymerization proceeds efficiently. Keeping this in perspective, it becomes evident that a wide variety of functionalized polyolefins can be synthesized by using controlled monomer design, some of which are illustrated in Fig. 2. 

A further example concerns the isolation of cyclic trimers of siloxanes." These cyclic trimers are usually short-lived kinetic products generated during the first phase of the polycondensation of trisilanols. In a bulk, they usually further condense to higher oligomers which are thermodynamically more stable and therefore, these trimers cannot be isolated. However, when the condensation of phenyltrimethoxysilane PhSi(OMe)3 is performed in the nanocage 12, only the cyclic trimer 14 is obtained under stereochemistry control giving exclusively the all-cw isomer (Scheme 10.6). 

The authors [91] proposed description of organic phase influence on limiting characteristics of polyurethanearylates (PUAr) interfacial polycondensation. As it is known [55], one from the methods of polymer solubility parameter 5 experimental determination is plotting of the dependence of intrinsic viscosity [t ], measured in several solvents, on this solvents solubility parameter 5 value. The smaller difference 6p-5J or the better solvent thermodynamical quality in respect of polymer is, the larger [q] is. The dependences [q](5 ) have usually belllike shape and such dependence maximum corresponds to 5 [55]. In Fig. 23 the dependence of on 5 of solvents, used as organic phase at PUAr interfacial polycondensation is adduced. The dependence q /S ) bell-like shape is obtained again and its maximum corresponds to 5 10 (cal/cm ), that is a reasonable estimation for PUAr [36, 55]. Let us note that all q values were determined in one solvent, which was not used at synthesis, namely, in mixture phenol-simm-tetrachloroethane. The dependence qj 4(5 ), adduced in Fig. 23, allows to make two conclusions. Firstly, the value q, reached in PUAr interfacial polycondensation process, is controlled by solvent thermodynamical qnality and the greatest... 

Hence, the stated above results assume, that the main limiting characteristics of PUAr interfacial polycondensation at other equal conditions are controlled by thermodynamic quahty of organic phase, in which synthesis is reahzed, with regard to polymer. This factor changes macromolecular coil structure, described within the fiamewoiks of fiactal analysis. Fixed in synthesis process coil stmcture... 

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