Depolymerization/polymerization techniques

ADMET represents a versatile tool for both polymerization and depolymerization.To stay within the thematic limits of this review, only ADMET polymerization will be discussed in this section. Combinations of ADMET with other polymerization techniques as well as further applications are presented in following sections if applicable. 

These techniques help in providing the following information specific heat, enthalpy changes, heat of transformation, crystallinity, melting behavior, evaporation, sublimation, glass transition, thermal decomposition, depolymerization, thermal stability, content analysis, chemical reactions/polymerization linear expansion, coefficient, and Young s modulus, etc. 

Two-shot techniques for acyclic diene metathesis, 435-445 for polyamides, 149-164 for polyimides, 287-300 for polyurethanes, 241-246 for transition metal coupling, 483-490 Anionic deactivation, 360 Anionic polymerization, 149, 174 of lactam, 177-178 Apolar solvents, 90 Aprotic polar solvents, 185, 338 Aprotic solvents, low-temperature condensation in, 302 Aqueous coating formulations, 235 Aqueous polyoxymethylene glycol, depolymerization of, 377 Aqueous systems, 206 Ardel, 20, 22... 

Fluorescein-labeled proteins are also used to measure the translational mobility of proteins and lipids by the Fluorescence Recovery After Photo-bleaching technique [54-59]. The uniformly labeled fluorescent sample is flashed with an intense light source to bleach a spot, thus producing a concentration gradient. The rate of recovery of fluorescence in that bleached area is measured and used to calculate the diffusion coefficient of the probe dye into the bleached zone. Such diffusion coefficient measurements have been used to determine the association constants of proteins in cells [60], to measure the exchange of tubulin between the cytoplasm and the microtubules [61,62], to study the polymerization-depolymerization process of actin [63-65] and to monitor the changes that occur upon cell maturation [66,67]. 

Only up to 58% of natural rubber can be practically depolymerized to isoprene during pyrolysis. A random chain scission may also take place along the polymeric chain. The result is the formation of molecules of lower molecular weight. However, in order to be volatile enough to be analyzed by typical analytical techniques associated with analytical pyrolysis, these fragments have to be relatively small. The formation of monomers as a final step in the random chain scission is not uncommon, and sometimes it is difficult to decide if a depolymerization or a random chain scission was the first step in pyrolysis. 

This latter depolymerization mechanism which takes place at the chain ends was proposed by Verkhotin158 and Aleksandrova154 and their respective coworkers. The most complete study is that of Grassie and MacFarlane151 who took into account the numerous data accumulated over the years, the importance of residual catalysts and the need of analysis by several techniques (TG, TV A, IR, GLC, GC-MS, NMR and Osmometry). They showed the necessity of a precise and reproducible method of polymerization and of precise control of the conditions under which depolymerization must be carried out in order to clearly establish mechanisms of reactions. 

Chemical components characteristic of specific nticroorganisms or nticrobial groups are usually enclosed in or part of a polymeric cellular matrix. Such nonvolatile and intractable biological samples present some difficulties for their direct characterization by GC, MS, or GC/MS. For these techniques to be useful, chemical components characteristic of the microbial sample must be released intact or a related compound must be generated before GC or MS analysis. Depolymerization is usually performed off-line by acid hydrolysis, methanolysis, sapoifification, or other reactions. One or more chemical derivatization steps might then be employed to produce volatile and thermally stable derivatives suitable for GC or MS. 

In headspace analysis, the plastic is placed in a vial (at a raised temperature) and the volatiles formed are stripped by a flow of carrier gas. The stripped volatiles are trapped in a suitable sorbent (e.g., using a solid-phase microextraction device) and subsequently thermally desorbed into a gas chromatograph. Process gas chromatographs are used in industrial analysis of volatiles in plastics. An example of this technique is the determination of residual vinyl chloride monomer in plastics in the range of 5-50 g per kg. With direct injection of a polymer solution, there is a danger of side-effects (a loss of reactive monomers due to polymerization in the injection port or an increase in its content due to depolymerization at a high injection temperature). 

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