Active growth center

Not all the dislocations outcropped on a crystal surface are active growth centers. In Fig. 6.6, the distribution of growth hillocks on a (111) face of an as-grown Ba(N03)2 crystal is compared with the distribution of dislocations revealed by X-ray... 

Ionic polymerizations are characterized by successive monomer additions to a growing macroion. Here, anionic polymerizations, with macroanions as active growth centers, are distinguished from cationic polymerizations which have macrocations as active growing centers ... 

Termination. By some reaction, generally involving two polymers containing active centers, the growth center is deactivated, resulting in dead polymer ... 

This results in strong polarization of the n bond and dissociation of the Ti—C bond, thus promoting insertion into the activator aluminum-alkyl bond. Repetitive insertions of alkene molecules result in lengthening of the polymer chain. This mechanism is also termed bimetallic after the growth center complex species 44. 

Anionic polymerization of s-caprolactam is used to make cast or RIM polyamide-6. Using a premade lactam chain end and a metal catalyst, it proceeds rapidly at 100-160°C, well below the melting temperature of the polymer, Tm 220°C. The propagation differs from anionic propagation of most unsaturated monomers because the growth center at the chain end is not represented by an anionically activated group but by a neutral N-acy-lated lactam, and the anionically activated species is the incoming monomer (Table 2.26). 

The type of monomer attached to the growth center during the simulation under kinetic control (at large tr values, see Eq. 3) is determined by the conformation and primary structure of the growing chain as a whole, not only by the local concentration of reactive monomers near the active end of the macroradical. As a result of such cooperativity, the formation of sequences with specific LRCs of the Levy-flight type was observed [76-78]. 

There are essentially two mechanisms of chain growth in the cationic polymerization of lactams. Either the cationically activated monomer reacts with neutral growth center or the neutral monomer molecule reacts with cationic active centers located at the end of the growing chain [214) ... 

Preliminary data on MMD of our samples are given in Table IV. It is evident that equimolar concentrations of activator and initiator produce PCL polymers characterized by a regularly decreasing polymolecularity index Q, from ca. 2.6 to 2.0. In Figure 1 the number of polymer molecules formed per acyllactam molecule is plotted as a function of initiator concentration. The actual values should be compared to the theoretical value of 1, which corresponds to the assumption that the number of macromolecules would be equal to the number of acyllactam molecules (26J, as in the ideal case of a step-addition of lactam anions to a constant number of growth centers. 

Phosphorus does not remain fixed in plant tissues but moves about from cell to cell where needed. If the total supply in a plant is limited it will move readily from the older tissues to the actively growing centers. This is in line with its essential role in energy transfer processes that are so much a part of the metabolic processes involved in growth. A marked phosphate deficiency in a plant may produce a marked stunting of growth but few other evidences of lack of phosphorus except possibly a reddish pigmentation in the leaves. 

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