Chain termination, modes

Compartmentalization of radicals into polymer particles may yield a unique MWD for the linear chains, as discussed in Sect. 3.1, except when the dominant chain termination mode is the chain transfer reaction. Branched polymer molecules are assemblies of linear polymer chains (called primary chains), and compartmentalization effects on the primary chain length distribution must be properly accounted for. 

The main reason that the decreases as the polymerization temperature increases is the increase in the initiation and termination reactions, which leads to a decrease in the kinetic chain length (Fig. 17). At low temperature, the main termination mechanism is polystyryl radical coupling, but as the temperature increases, radical disproportionation becomes increasingly important. Termination by coupling results in higher PS than any of the other termination modes. 

The mode of chain termination affects the type of block copolymer formed. For example, if a MAI (based essentially on the first monomer A) possessing one central azo bond is decomposed in the presence of monomer B, the growing chain Bn can terminate either by disproportionation or combination, leading to AB and ABA type copolymers, respectively. 

Analysis of radioactively labelled end-groups Absolute single determination wide range of mol. wt. Restricted to certain types of polymer requires knowledge of mode of chain termination 1-2... 

The mechanistic issues to be discussed are the initiation modes of the reaction, the propagation mechanism, the perfect alternation of the polymerisation reaction, chain termination reactions, and the combined result of initiation and termination as a process of chain transfer. Where appropriate, the regio- and stereoselectivity should be discussed as well. A complete mechanistic picture cannot be given without a detailed study of the kinetics. The material published so far on the kinetics comprises only work carried out at temperatures of -82 to 25 °C, which is well below the temperature of the catalytic process. 

This reaction may account in part for the oligomers obtained in the polymerization of pro-pene, 1-butene, and other 1-alkenes where the propagation reaction is not highly favorable (due to the low stability of the propagating carbocation). Unreactive 1-alkenes and 2-alkenes have been used to control polymer molecular weight in cationic polymerization of reactive monomers, presumably by hydride transfer to the unreactive monomer. The importance of hydride ion transfer from monomer is not established for the more reactive monomers. For example, hydride transfer by monomer is less likely a mode of chain termination compared to proton transfer to monomer for isobutylene polymerization since the tertiary carbocation formed by proton transfer is more stable than the allyl carbocation formed by hydride transfer. Similar considerations apply to the polymerizations of other reactive monomers. Hydride transfer is not a possibility for those monomers without easily transferable hydrogens, such as A-vinylcarbazole, styrene, vinyl ethers, and coumarone. 

Fully aromatic polyamides are synthesized by interfacial polycondensation of diamines and dicarboxylic acid dichlorides or by solution condensation at low temperature. For the synthesis of poly(p-benzamide)s the low-temperature polycondensation of 4-aminobenzoyl chloride hydrochloride is applicable in a mixture of N-methylpyrrolidone and calcium chloride as solvent. The rate of the reaction and molecular weight are influenced by many factors, like the purity of monomers and solvents, the mode of monomer addition, temperature, stirring velocity, and chain terminators. Also, the type and amount of the neutralization agents which react with the hydrochloric acid from the condensation reaction, play an important role. Suitable are, e.g., calcium hydroxide or calcium oxide. 

The ratio Mw/Mn (MWD) decreased with increasing PEO-MA fraction in the monomer feed and/or the number of EO units in the macromonomer. Generally, the Mw/Mn in bulk (homogeneous) systems is a function of the termination mode and the chain transfer events and varies between 1 and 2. In the present disperse systems, MWD is much broader (much above 2) as a result of further contributions, such as polymerization in the continuous phase, interface, and polymer particles. The chain transfer to PEO chains decreased the molecular weight, i,e., the Mw of copolymer decreased with increasing macromonomer concentration and PEO chain length. 

NRTIs are nucleoside analogues that become incorporated into the growing viral DNA chain. Once in the oligonucleotide chain, the NRTI prevents further elongation of the chain because the NRTI lacks a functional 3 -OH for attachment of the next nucleotide. Based on this mode of action, NRTIs are sometimes called chain terminators. With the synthesis of DNA severely impeded, the progress of the viral infection is slowed. 

Termination in this polymerization is not completely final since the M +1 species formed in Reaction 8 may participate again via Reaction 7 hence, Reaction 8 represents a type of chain transfer mode of termination. 

The reason why FR polymerizations are not living is that growing polymer radicals interact with each other, resulting in chain termination. There are several modes whereby polystyryl radicals become terminated, including radical coupling, disproportionation, and chain transfer. Total elimination of these bimolecular processes from an FR polymerization is impossible. However, if one can keep the FR concentration very low, the rate of these termination... 

The mono-, di-, and tri-phosphorylates of d4T were analysed by ion-pair LC-MS after lysis of the PBMC cell in Tris/methanol and centrifugation [50]. The supernatant was injected into the LC system with a 150x2.1-mm-ID Cjg column (5 pm) and a mobile-phase gradient of 70 to 35% solvent A (10 mmol/1 DMHA and 3 mmol/1 ammonium formate adjusted at pH 11.5) in solvent B (50% acetonitrile in 20 mmol/1 DMHA and 6 mmol/1 ammonium formate). Negative-ion ESI-MS was performed in SRM mode. The method enabled the direct measurement of the chain terminator ratio (d4T-triphosphate/deoxythymidine-triphosphate). Subsequently, the same group [51] reported modifications of this method, including simplifications of the sample pretreatment, replacement of the LC column for another type, and reduction of the column inner diameter from 2 mm ID to 0.32 mm ID. This improved method was applied to the determination of the phosphorylates of d4T, 3TC, and ddl. The sample throughput is 200 samples per week. The determination of intracellular AZT-triphosphate in PBMC [52], and the validation of the method for the determination of the ddl and d4T triphosphates was reported separately [53]. 

Much effort has been devoted during the last 30 years toward understanding the mechanisms operative in the coordination catalysis of ethylene and a-olefin polymerization using Ziegler-Natta systems (metal halide and aluminum alkyl, sometimes with Lewis base modifiers). Aspects of the complex heterogeneous reactions have been elucidated (jL- ) but the intimate mechanistic detail - for example the role of inhibitors and promoters, kinetics and thermodynamics of chain growth, modes of chain transfer and termination - comes primarily from studies of homogeneous catalysts ... 

It is often observed that the measured molecular, weight of a polymer product made by free-radical chain polymerization is lower than the molecular weights predicted from Eq. (6.102) for termination by either coupling [Eq. (6.103)] or disproportionation [Eq. (6.104)]. Such an effect, when the mode of termination is known to be disproportionation, can be due to a growing polymer chain terminating prematurely by transfer of its radical center to other species, present in the reaction mixture. These are referred to as chain transfer reactions and may be generally written as... 

As we have seen in previous sections, the radical chain polymerization involves several possible modes of chain termination — disproportionation, coupling, and various chain transfer reactions. These contribute to the complexity of molecular... 

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