Termination depolymerization

The simultaneously occurring transacetalization then provides for a random distribution of ethylene oxide residues in the copolymers. The ethylene oxide groups terminate depolymerization of the oxymethylene chains if the depolymerizations are adventitiously started by chain scission. 

Imide-terminated telechelics are also synthesized by metathesis depolymerization, and it is found that phthalimide-substituted olefins allow for productive depolymerization when only one methylene spacer separates the nitrogen atom and the olefin (Fig. 8.21). This combination of steric hindrance around the nitrogen lone pair and decreased electron donation from resonance prevents the negative neighboring group effect. However, secondary acyclic amines are unable to produce telechelics through metathesis depolymerization because of unfavorable catalyst-amine interactions. 

Since the depolymerization process is the opposite of the polymerization process, the kinetic treatment of the degradation process is, in general, the opposite of that for polymerization. Additional considerations result from the way in which radicals interact with a polymer chain. In addition to the previously described initiation, propagation, branching and termination steps, and their associated rate constants, the kinetic treatment requires that chain transfer processes be included. To do this, a term is added to the mathematical rate function. This term describes the probability of a transfer event as a function of how likely initiation is. Also, since a polymer s chain length will affect the kinetics of its degradation, a kinetic chain length is also included in the model. 

For phloroglucinolysis, a solution of 0.1 N HCl in MeOH, containing 50 g/L phloroglucinol and 10 g/L ascorbic acid, is prepared. The PA of interest is reacted in this solution at 50°C for 20 min and then combined with 5 volumes of 40 mM aqueous sodium acetate to stop the reaction. After acid-catalyzed cleavage in the presence of phloroglucinol, the fraction is depolymerized and the terminal subunits released as flavan-3-ol monomers and the extension subunits released as phloroglucinol adducts of flavan-3-ol intermediates. These products are then separated and quantified by HPLC [25]. 

From these results, it seems likely that PPG might be metabolized and depolymerized via the same mechanism as that for PEG oxidation of terminal alcohol groups leading to the cleavage of the ether linkage. 

A measure of the relative importance of these processes may be obtained by dividing the number of depolymerization steps occurring by the number of chain transfer and termination steps. This parameter is the zip length and gives a measure of the tendency of the polymer to break down into a monomer. If the zip length is low, there will be little monomer in the volatile products with an increased probability for a higher char yield. 

Hydroxy-terminated natural rubber (HTNR) It is a honey-colored viscous liquid and is prepared by the depolymerization of masticated natural rubber (NR) in the presence of H202 in toluene. Similar to other polyols, it is curable with TDI/MDI. Some important attributes of HTNR are ... 

In the course of the depolymerization primary and tertiary terminal macroradicals are formed. A difference in the reactivity these radicals is explained by assuming that the /3-scission depends on the rotational energy barrier the terminal C-C- bond in the primary and tertiary terminal radicals (52). 

Each tubulin dimer binds one molecule of GTP strongly in the a subunit and a second molecule of GTP or GDP more loosely in the P subunit. In this respect, tubulin resembles actin, whose subunits are about the same size. However, there is little sequence similarity. Labile microtubules of cytoplasm can be formed or disassembled very rapidly. GTP is essential for the fast growth of these microtubules and is hydrolyzed to GDP in the process.320 However, nonhydro-lyzable analogs of GTP, such as the one containing the linkage P-CH2-P between the terminal and central phosphorus atoms of the GTP, also support polymerization.321 Since microtubules have a distinct polarity, the two ends have different tubulin surfaces exposed, and polymerization and depolymerization can occur at different rates at the two ends. As a consequence, microtubules often grow at one end and disassemble... 

---