Monofunctional reagents

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 ... 

Nylon resins are made by numerous methods (53) ranging from ester amidation (54) to the Schotten-Baumann synthesis (55). The most commonly used method for making nylon-6,6 and related resins is the heat-induced condensation of monomeric salt complexes (56). In this process, stoichiometric amounts of diacid and diamine react in water to form salts. Water is removed and further heating converts the carboxylate functions to amide linkages. Chain lengths are controlled by small amounts of monofunctional reagents. The molten finished nylon resin can be dkectly extmded to pellets. 

Reactions with monofunctional reagents are for example carried out in order to increase the thermal and/or chemical stability of the end groups (Poly-oxymethylenes, Example 5-7). Reactions with bifunctional reagents can be used to enlarge the degree of polymerization or to synthesize block copolymers (see Sect. 4.2.1). 

It is interesting that all of the cross-links found in the above studies appear to be sterically permitted on the basis of the X-ray structure, although some movement of the lysine side chains from their reported positions is required. The reason for almost total loss of activity when monofunctional reagents react at Lys 41 and the possible maintenance of substantial activity when the Lys 7-41 dinitrophenylene cross-link is formed is not clear. Preliminary evidence from the X-ray structure study of the DNP Lys-41-RNase-S (120) indicates small movements of many parts of the molecule in comparison with RNase-S, but these motions have not yet been analyzed in detail. They are substantially reversed by 3 -CMP binding. If they should be responsible for the activity loss, then the cross-link may prevent or reverse these changes. 

Table 10.1. Theoretical product ratios of the reaction of a difunctional reagent A with a monofunctional reagent B (A+ B—>A + AB + AB2).

As mentioned above, the statistically predicted amount of monofunctionalized product can only be obtained if both reactants are thoroughly mixed. If the difunctional reagent is poorly soluble under the reaction conditions chosen, its concentration might be lower than that of the monofunctional reagent. This can lead to large... 

Figure 3.11 Schematic illustration of the reaction of a silica surface with a monofunctional reagent (dimethyl-alkyl-ethoxysilane). Figure taken from ref. [317]. Reprinted with permission.

The 1,3-dilithio-compounds exhibit "normal" reactivity with monofunctional reagents (Scheme 5). 1,1- and 1,2-dilithio-compounds do not always react in the expected way of a alkyllithium-compoimd [6]. 1,3-Disilacyclobutanes can be synthesized by the reaction of 18 with dichlorosilanes (e.g., 21, Scheme 5), illustrating the synthetic potential of these new building blocks for the preparation of silacyclobutanes. 

Monofunctional reagents such as alkyldimethylmonochlorosilanes yield monomeric packings (first reaction in Fig. 1.8A). Such bonded-phase materials are well defined as one silanol group reacts with one silane molecule and exhibit high efficiency because of low mass-transfer resistance due to fast diffusion of molecules into the flexible fur - or brush -like structure of the alkyl chains on the silica surface. 

Some difunctional compounds, for example. A -(ethoxycarbonyl)thiocar-boxamides, may behave as monofunctional reagents in a reaction with a diamine. In this example, an ethoxycarbonyl group is eliminated as ethyl carbamate and the thioamide completes the imidazole ring [2899]. Diamines usually react with 3-oxo esters to give a fused dia pine (p. 423), but when the reactants are heated without a solvent or catalysts at or above 100 °C, a benzimidazole is obtained in high yield [2371]. 

It Is Interesting to note that the majority of manufacturers are using monofunctional reagents for the octyl phases. The particular batches of LlChrosorb shown were the only phases found to use dlfunctlonal bonding. The GC method also correctly Identified the trlfunctlonal silane chemistries used In two commercial phases. 

Interfacial polymerization will tolerate the presence of impurities in the reactants that simply dilute the material and thereby produce nonequivalence of reactants. These diluents might be water or inert contaminants in the acid chloride. Reactive monofunctional species are harmful in either phase. To maximize molecular weight, it is essential to use high purity monomers. Molecular weight control can be achieved, if desired, with appropriate use of monofunctional reagents. Examples of impurities interfering with the interfacial polyamidation of MPDI are half hydrolyzed acid chloride, monoamide, partially oxidized amines, and reactive surfactants. 

The degree of substitution, DS, which cannot be experimentally reliably measured, on the other hand, gives the mean number of substituted hydroxyl groups. It is DS = 3 for the glucose residue on the left and DS = 2 for the one on the right. Of course, DS = DR for monofunctional reagents. 

Applications where the NCO group is reacted with a monofunctional reagent first to make a new monomer which can be later copolymerized. 

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