Living mechanism

The peroxide 179 dissociates in the presence of a monomer giving rise to alkoxyl (CO-) and borinate (BO-) radicals, but the latter are believed to be too stable to initiate polymerization. It should be mentioned that the molecular weight continuously increases throughout the process implying the pseudo-living mechanism for chain growth. After the completion of the process borane residue is completely oxidized into diol <2004MM6260>. Thus, the 8-boraindane molecule not only initiates the polymerization, but also is a precursor to two functionalities in the polymer chain. 

A series of bis(phenoxide) aluminum alkoxides have also been reported as lactone ROP initiators. Complexes (264)-(266) all initiate the well-controlled ROP of CL, NVL.806,807 and L-LA.808 Block copolymers have been prepared by sequential monomer addition, and resumption experiments (addition of a second aliquot of monomer to a living chain) support a living mechanism. The polymerizations are characterized by narrow polydispersities (1.20) and molecular weights close to calculated values. However, other researchers using closely related (267) have reported Mw/Mn values of 1.50 and proposed that an equilibrium between dimeric and monomeric initiator molecules was responsible for an efficiency of 0.36.809 In addition, the polymerization of LA using (268) only achieved a conversion of 15% after 5 days at 80 °C (Mn = 21,070, Mn calc 2,010, Mw/Mn = 1.46).810... 

The reaction is often described as a living polymerization (Matejka et al, 1983), but neither the distribution of molar masses nor the observed gel conversion corresponds to a pure living mechanism (Mauri et al., 1997). Experimental observations may be explained by assuming the presence of a chain transfer step that regenerates the active initiator. This step deter-... 

Kricheldorf et al. reported an anionic polymerization of y-D,L-butyro-lactone or D,L-lactide with cyclic dibutyltin initiators, such as 2,2-dibutyl-2-stanna-l,3-dioxepane, to give cyclic polymers [ 138-140]. Figure 41 shows the ring expansion polymerization of lactone for synthesizing a cyclic polymer as an example. They also synthesized the cyclic polymer with a living mechanism in the polymerization of e-caprolactone [141]. 

The reduced probability of side reactions enables some heterocycle polymerizations to proceed by a living mechanism. 

This is a very important polymerization mode. Usually these polymerizations can be made to proceed by the living mechanism, even cationic. Important polymers are produced in this way (some polyamides, polysiloxanes, polyacetals, polyethers, polyimines, etc.). 

Our picture of the transitions between centres is very incomplete so far, based on studies of distribution curve shapes in the products. When a monomer is polymerized by a living mechanism on two or more centres of widely differing reactivity, chains of characteristic legth are produced on each centre type. In a strictly living medium where centres of one type are not transformed to another, a product with a bi- or multimodal distribution curve of degrees of polymerization is formed. When the various centre types are in a dynamic equilibrium where the centre type changes in the course of propagation, the distribution curve of the product will be broader than the width of either of the peaks in the previous case, but narrower than the overall... 

Uncontrolled terminations are undesirable they prevent polymerizations being carried out by a living mechanism. On the other hand, a suitable method of intentional termination is necessary for the deactivation of centres which would otherwise induce unwanted reactions during processing and application of the polymer. 

These are much slower than to the preceding group of monomers, evidently because of the lower reactivity of oxonium, sulphonium, ammonium, phos-phonium and siloxonium, ions. Moreover, monomers with these heteroatoms are strongly basic, and therefore cations are preferentially solvated by the monomers. This reduces the probability of other kinds of transfer to solvent, impurities, etc. Many heterocycles, e. g. A-substituted aziridines, thiethanes [62], tetrahydrofuran [63], under suitable conditions polymerize by a living mechanism, i. e. without transfer. In situations where transfer does occur, it is assumed to proceed by the mechanism disscussed previously, for example by transfer to the counter-ion. With regard to transfer intensity, vinyl ethers can be ordered between the hydrocarbon monomers and the heterocycles. The mechanism of transfer in their polymerization has yet to be studied. 

Any deviation from the living mechanism broadens the distribution. Transfers and termination caused by residual impurities and insufficient mixing are the main causes of improvable broadening of distribution curves. Internal reasons are inherent to two factors, namely to reversibility of propagation and to propagation on centres with various reactivity. 

This living mechanism consists in the reversible combination of the growing radical chains, Rn, and the so-called persistent radical species , X (the nitroxide radical group), to form dormant polymer chains, R -X ... 

Therefore, the main difference from the previous two systems is that this living mechanism does not form new radicals and a conventional initiator is needed to start and sustain the reaction. The initial amount of this species has to be properly selected. In fact, since the living reaction (3) does not affect the radical concentration, the final concentration of the chains terminated by bimolecular combination will be half of the initial concentration of the initiator. Therefore, the initial concentration of the species carrying the iodine group (in the following simply called the transfer agent ) determines the final DP of the polymer provided that the initiator concentration is small compared with that of the transfer agent. 

Whatever the living mechanism, an essential requirement for a successful LRP is the minimization of the fraction of dead chains. In a bulk or solution reaction, the final amount of dead chains is a function of the radical concentration only large polymerization rates correspond to high dead chain concentrations. 

Table 6.4 Examples of living processes carried out in ab initio emulsion (E) or miniemulsion (mE) polymerization using different living mechanisms...

Carbohydrates and Atherosclerosis Food Choices in Everydav Living Mechanism of the Influence of Dietary Patty Adds on LDL- Cholesterol Effect of Drugs... 

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