B Group

It is important in the body as, except for methionine, it is the only substance known to take part in methylating reactions. Sometimes regarded as a member of the vitamin B group. 

Fig 1. (a) Phase (b) group velocity dispersion curves for aluminium. Circles show minimum dispersion points. Diamonds show excitation positions for transducer designed for X/d = 2.4 where d is the plate thickness. 

Figure 5.1 The reaction of A and B groups at the ends of two different chains. Note that rotations around only a few bonds will bring A and B into the same cage of neighboring groups, indicated by the broken line enclosure.

We define the problem by assuming the polymerization involves AA and BB monomers and that the B groups are present in excess. We define and to be the numbers of A and B functional groups, respectively. The number of either of these quantities in the initial reaction mixture is indicated by a superscript 0 the numbers at various stages of reaction have no superscript. The stoichiometric imbalance is defined by the ratio r, where... 

As with other problems with stoichiometry, it is the less abundant reactant that limits the product. Accordingly, we define the extent of reaction p to be the fraction of A groups that have reacted at any point. Since A and B groups... 

Equation (5.40) also applies to the case when some of the excess B groups present are in the form of monofunctional reagents. In this latter situation the definition of r is modified somewhat (and labeled with a prime) to allow for the fact that some of the B groups are in the BB-type monomers (unprimed) and some are in the monofunctional (primed) molecules ... 

The parameter r continues to measure the ratio of the number of A and B groups the factor 2 enters since the monofunctional reagent has the same effect on the degree of polymerization as a difunctional molecule with two B groups and, hence, is doubly effective compared to the latter. With this modification taken into account, Eq. (5.40) enables us to quantitatively evaluate the effect of stoichiometric imbalance or monofunctional reagents, whether these are intentionally introduced to regulate or whether they arise from impurities or side reactions. 

The monofunctional reagent B is the acetic acid in this case and the number of monofunctional carboxyl groups is 2(0.010) = 0.020 = The number of B groups in BB monomers is 1.980 = We use Eq. (5.41) to define i for this situation, assuming the number of hydroxyl groups equals 2.00 mol ... 

What we seek next is a quantitative relationship between the extent of the polymerization reaction, the composition of the monomer mixture, and the point of gelation. We shall base our discussion on the system described by reaction (5.U) other cases are derived by similar methods. To further specify the system we assume that A groups limit the reaction and that B groups are present in excess. Two parameters are necessary to characterize the reaction mixture ... 

The ratio of the initial number of A to B groups as defined by the factor r. given by Eq. (5.34). The total number of A groups from both AA and A are included in this application of r. 

The terminal B group reacts with an A group from AA rather than Af- ... 

The fraction of unreacted B groups is rp, so this gives the probability of reaction for B. Since p is the fraction of A groups on multifunctional monomers, rp must be multiplied by 1 - p to give the probability of B reacting with an AA monomer. The total probability for the chain shown is the product of the probabilities considered until now ... 

Step-growth polymerizations can be schematically represented by one of the individual reaction steps VA + B V —> Vab V with the realization that the species so connected can be any molecules containing A and B groups. Chain-growth polymerization, by contrast, requires at least three distinctly different kinds of reactions to describe the mechanism. These three types of reactions will be discussed in the following sections in considerable detail. For now our purpose is to introduce some vocabulary rather than develop any of these beyond mere definitions. The principal steps in the chain growth mechanism are the following ... 

Hunter L,a,b and Other Color Spaces. The CIELAB and CIELUV color spaces were the outgrowth of a large and complex group of interrelated early systems and have replaced essentially all of them except for the 1942 Hunter Y,a,b group of color spaces (3). This was the earliest practical opponent-based system which is still widely used. In this system, for illuminant C and the 2° standard observer ... 

The material known as bouncing putty is also a silicone polymer with the occasional Si—O—B group in the chain, in this case with 1 boron atom to about every 3-100 silicon atoms. The material flows on storage, and on slow extension shows viscous flow. However, small pieces dropped onto a hard surface show a high elastic rebound, whilst on sudden striking they may shatter. The material had some use in electrical equipment, as a children s novelty and as a useful teaching aid, but is now difficult to obtain. 

Structure (39) conforms to the predicted (2632) topology (p. 158) and shows that the 2 added electrons have relieved the electron deficiency to the extent that the 2 B-H -B groups have been... 

Refractive index A-group solvent > nonsolvent, B-group solvent < nonsolvent. Mark for piezochromism O positive, x negative. 

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