Termination modes

This contrasts with a limiting ratio of 2 for the case of termination by disproportionation. Since and can be measured, this difference is potentially a method for determining the mode of termination in a polymer system. In most instances, however, termination occurs by some proportion of both modes. Although general expressions exist for the various averages and their ratio when both modes of termination are operative, molecular weight data are generally not sufficiently precise to allow the proportions of termination modes to be determined in this way. 

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. 

Many researchers have considered models for possible intermediates in the nitrogenase reaction. Two possible dinitrogen attachments to the FeMoco factor of MoFe-protein have been put forward. Symmetric, edge- or side-on modes discussed by Dance48 would lead to a reaction sequence such as is shown in reaction 6.11. In contrast, the asymmetric end-on terminal mode discussed in the work of Nicolai Lehnert50 may be favored thermodynamically and by molecular orbital calculations. Reaction sequence 6.13 below illustrates one scenario for the asymmetric model. 

Alternately, the termination mode may change from the normal bimolecular termination between propagating radicals to primary termination, which involves propagating radicals reacting with primary radicals [Berger et al., 1977 David et al., 2001 Ito, 1980] ... 

The kinetics of redox-initiated polymerizations generally fall into two categories depending on the termination mode. Many of these polymerizations proceed in the same manner as other polymerizations in terms of the propagation and termination steps the only difference is the source of radicals for the initiation step. For these polymerizations where termination is by bimolecular reaction of propagating radicals, the initiation and polymerization rates will be given by appropriate expressions that are very similar to those developed previously ... 

The structures of 204-207 have been established in the solid state by single crystal X-ray diffraction. Clusters 205 and 206 are based on the Ru6C octahedral skeleton. The [2.2]paracyclophane ligand in 205 coordinates over a triangular metal face, and in 206 one [2.2]paracyclophane adopts a similar face-capping bonding mode and the other coordinates in a terminal mode to an apical Ru atom. The metal polyhedron in 204 is relatively open in comparison to the octahedron having only nine Ru-Ru bonds. A carbide atom occupies the central cavity and interacts with five of the six Ru atoms. 

Cellobiose producer. A decrystallizing enzyme (42) with terminal mode of attack (2,48). Cellobiose is competitive inhibitor (45, 48) Glucose is not an inhibitor (48). Needed for hydrolysis on highly ordered substrates (41) ... 

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. 

Polymers containing benzoin terminal groups can act as photochemical macroinitiators and are effective in photogenerating polymeric electron donor radicals. The initiation of polymerization by means of azo-benzoin initiators yields polymers with one or two benzoin end-groups according to the termination mode of the particular monomer involved [72-74], The general synthetic procedure is depicted below as illustrated for the case styrene polymerization (Scheme 18). 

The opinions of various authors on the mode of termination therefore do not always agree. Bamford et al. have found that polymethacrylate radicals mostly disproportionate while those of polyacrylonitrile mostly combine at 363 K [15]. More detailed measurements by Schulz et al., who also considered transfer in methyl methacrylate polymerization, enabled them to express the ratio of the two termination modes quantitatively [16]... 

According to eqns. (105)—(107), the dependence of the decay of active centres on time is linear, exponential, and hyperbolic, respectively. The best approximation is obtained by a comparison of the observed course with the rate of monomer decay calculated for the assumed termination mode. This is, of course, a very rough method, which often does not reveal the effect of other components on termination, for example that of the monomer (its wrong addition etc.)... 

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. 

DEP afforded the synthesis of telechelic oligomers [29]. With the telomerization process, DEP appeared to be the first free-radical polymerization leading to telechelic oligomers. Tobolsky [30] first stated the conditions of DEP, i.e., the half life of the growing species has to be equivalent to that of the initiator. These specific conditions result in an unusual high rate of termination, which allows for the synthesis of oligomers. The telechelic structure will be obtained by combining a termination mode exclusively by recombination with the use of a difunctional initiator. 

The case of alumina is more complicated than that of Ti02/ since most forms contain 6-coordinated as well as 4-coordinated Al3+. Figure lb represents the most commonly accepted model for the surface of YAI2Q3 (an improved version has been published recently ). This oxide has a spinelle-related structure and is known to chiefly expose its (110), (100) and (111) planes. We illustrate the (110) plane, or rather one of its two termination modes (plane C). Note that the termination indicated does not insure electroneutrality however, the condition is satisfied by adding OH groups to each cus Al3+ (lower part of the figure). In the present case, no relevant data can be provided by monocrystals studies since the latter are concerned with a different form, aAl2Q3 (corundum). 

The RR spectral motif is altered once again when the cubanelike [Fe4S4 (Cys)4] chromophores are considered, as illustrated in Fig. 22. The dominant band is the breathing mode of the cube at around 336 cm", and other modes of the cube range up to 400 cm" and down to 240 cm". The terminal modes are found at approximately 360 (72 ) and 390 (Aj ) cm". It is interesting that the terminal mode symmetry order is inverted from that found in [Fe(Cys)4]" complexes (Fig. 19), reflecting the kinematic interactions with the cube modes. 

Zammit, M. D., Davis, T. R, and Haddleton, D. M., Determination of the Propagation Rate Coefficient (kp) and Termination Mode in the Free-Radical Polymerization of Methyl Methacrylate, Employing Matrix-Assisted Laser Desorption Ionization Time-of-Flight Mass Spectrometry for Molecular Weight Distribution Anal5 is, Macromolecules, 29, 492, 1996. 

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