Segmental exchange

Ph- -Ph PhJ -f/Ph M -M Ph b Ph A ring segment exchange reaction was studied by NMR in solutions containing mixtures of 26 and 27, leading to equilibrium mixtures of compounds containing Sn—S—Pb moieties. 157... 

Fig. 12.8 Average difference in deuteration levels of PPARy LBD between ligand bound form and apo form [64], Dark blue indicates the segment exchanges about the same rate with or without the ligand.

Other colors indicate the segment exchanges slower with the ligand. 

Thus, chain transfer to polymer does not influence the number average DP , it may however alter the molecular weight distribution. If the reversible chain transfer to polymer described in Eq. (78) occurred frequently, it would lead to statistical distribution, i.e., MJM = 2. The other consequence is that if the two originally present chains are different, the repetition of reaction sequence will lead to segmental exchange (so called scrambling ). Both effects are clearly detectable, for example, in the cationic polymerization of cyclic acetals as it will be discussed in Section III.B. 

Activation segment exchange a common mechanism of kinase autophosphorylation Trends Biochem. Sci., 32, 351-356. 

Hayashida, H., and Miyata, T., 1983, Unusual evolutionary conservation and frequent DNA segment exchange in class I genes of the major histocompatibility complex, Proc. Natl. Acad. Sci. USA 80 2671-2675. 

Liu E K, Kolb R, Sajita S K and Wu W (1999) Enhanced polymer segment exchange kinetics due to an applied shear field, Macromolecules 32 4741-4744. 

The equilibrium in all these reactions ensures that the equilibrium of the segmental exchange (cf Scheme 1) is reached as well, independently of the rates of these reactions. 

Scheme 1 A simplified scheme of an equilibrium binary RO copolymerization. Only the dyad model of propagation and depropagation reactions, reversible cyclizations, and segmental exchange reactions are shown. Equilibrium constants of the corresponding reactions (denoted with the capital letter /Cwith subscripts) are equal to the ratios of the corresponding rate constants. D, E, F, V, W, and Z denote copolymer units A and/or B.

Intermolecular chain transfer to polymer is well documented in the cationic polymerization of cyclic acetals. In the polymerization of TOX, as will be discussed in Section 4.10.3, chain transfer to polymer is essential for the preparation of thermally stable polyacetal. Intermolecular chain transfer to polymer is detrimental to the synthesis of monofunctional polymers such as macromonomers because segment exchange (scrambling) leads to disproportionation and formation of products having two, one, and none of the functional groups (Scheme 21). " Intermolecular chain transfer to polymer prohibits also the synthesis of block copolymers by sequential polymerization of two cyclic acetals. Addition of DXP to a solution of living polyDXL resulted in further polymerization but the copolymer formed had a nearly statistical distribution of units." ... 

In spite of the living character of the anionic polymerization of PL, it was revealed that in the case of p-substituted p-lactones or medium-size cyclic esters (LA and CL) this process suffers from various side reactions such as chain transfer to monomer, racemization, segment exchange, or formation of macrocydic esters. " Finally, it has been concluded that in the so-called coordination-insertion (pseudoanionic) polymerization, initiated with covalent metal alkoxides (R MtOR c. where Mt is Zn, Al, Sn, Ti, etc.), these side reactions can be kinetically suppressed or even eliminated. Correlation of the selectivity parameters, defined as the ratio of rate constants of propagation and transfer (fep/fetr). with the reactivity of active species (fep) showed that these polymerizing systems conform to the reactivity-selectivity principle. ... 

They undergo transfer all the time, but approximately only once for 100 steps of propagation. A new population of macromolecules that participated in transfer reacts slowly with the populations with Poisson distribution. The Af n of the populations participating in the segmental exchange (i.e., intermolecular transfer) is the same, but differs and this appears on the size-exclusion chromatography (SEC) traces, resulting in bimodal and broader distribution. Finally at equilibrium, the value equal to 2, typical for the most... 

Research on rac-LA polymerization has been much more dynamic and successful, especially in the past decade. Suppression or even elimination of the segmental exchange turned out to be possible by applying metal alkoxide initiators with bulky ligands. Steric hindrance at the active center decreases the transesterification rate due to an increase in entropy of activation, while the propagation rate remains constant. ... 

Stereocontrolled polymerization requires elimination of the intermolecular transfer (segmental exchange) between the homochiral chains containing repeating units of opposite configuration. Otherwise, even if initially stereoselective polymerization proceeds giving the homochiral chains. 

Kp = equilibrium constant of propagation Ke = equilibrium constant of end-to-end cycylization Kbb, Kbb" = equilibrium constants of back-biting Kse = equilibrium constant of segmental exchange... 

Chain-transfer side reactions (see Scheme 1.1) can also cause substantial increases in D-values. Macrocychzation is particularly poor in this respect, leading to a complete equihbrium and an especially broad molar mass distribution (see e.g. Figure 1-4). On the other hand, a reversible polymerization devoid of macro-cyclization, but accompanied by the segmental exchange reactions, can fulfill the criteria of the living process [95-99]. However, in this case the D-value also increases with conversion, reaching at equilibrium a value which is predicted by Equation 1.24 and characteristic of the most probable molar mass distribution. Figure 1.7 illustrates the dependence of M /M determined for LA polymerization initiated with Sn(ll) alkoxide [98]. 

O). The points represent experimental data the lines were computed from the kinetic scheme in which reversible propagation is accompanied by the segmental exchange reactions [Scheme 1.1, reactions (a) and (c)] [98],... 

The kinetic scheme of living, reversible ROP can also encompass-apart from initiation (Equation 1.27a) and propagation (Equation 1.27b)-additional reversible reactions, such as segmental exchange (Equation 1.27c) or temporary termination (Equation 1.27d). 

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