Functional groups multifunctional

For a fixed extent of reaction, the presence of multifunctional monomers in an equimolar mixture of reactive groups increases the degree of polymerization. Conversely, for the same mixture a lesser extent of reaction is needed to reach a specified with multifunctional reactants than without them. Remember that this entire approach is developed for the case of stoichiometric balance. If the numbers of functional groups are unequal, this effect works in opposition to the multifunctional groups. 

FIGURE 25.9 Fatty add synthase in animals contains all the functional groups and enzyme activities on a single multifunctional subunit. The active enzyme Is a head-to-tall dimer of Identical subunits. (Adapted from Wakit, S. J., Stoops,... 

To control the formation of nanoparticles with desired size, composition, structure, dispersion, and stability, a multifunction nanoagent is used. The active metals (Pd and Pt) react with the functional groups of the nanoagent, i.e., a pol5mier template. The polymer template determines the size, monodisperity, composition, and morphology of the particles (which is somewhat reminiscent of the reversed micelles technique mentioned above). 

Improve adhesion of dissimilar materials such as polymers to inorganic substrates. Also called primers. Primers generally contain a multifunctional chemically reactive species capable of acting as a chemical bridge. In theory, any polar functional group in a compound may contribute to improved bonding to mineral surfaces. However, only a few organofunc-tional silanes have the balance of characteristics required... 

Attainment of a maximum double bond conversion is typical in multifunctional monomer polymerizations and results from the severe restriction on bulk mobility of reacting species in highly crosslinked networks [26]. In particular, radicals become trapped or shielded within densely crosslinked regions known as microgels, and the rate of polymerization becomes diffusion limited. Further double bond conversion is almost impossible at this point, and the polymerization stops prior to 100% functional group conversion. In polymeric dental composites, which use multifunctional methacrylate monomers, final double bond conversions have been reported ranging anywhere from 55-75% [22,27-29]. 

The mantiosdcctivity, expressed as enantiomeric excess (ee, %) of a catalyst should be >99% for pharmaceuticals if no purification is possible. This case is quite rare, and ee-values >90% are often acceptable. Chemosdectivity (or functional group tolerance) will be very important when multifunctional substrates are involved. The catalyst productivity, given as turnover number (TON mol product per mol catalyst) or as substrate catalyst ratio (SCR), determines catalyst costs. For hydrogenation reactions, TONs should be >1000 for high-value products and >50000 for large-scale or less-expensive products (catalyst re-use increases the productivity). 

In stepwise reactions, all functional groups take part in bond formation. Their reactivity can be considered independent of the size and shape of the molecules or substructures they are bound to (Flory principle). If such a dependence exists, it is mainly due to steric hindrance. In chain reactions only activated sites participate in bond formation if propagation is fast relative to initiation, transfer and termination, long multifunctional chains are already formed at the beginning of the reaction and they remain dissolved in the monomer. Free-radical copolymerization of mono- and polyunsaturated monomers can serve as an example. The primary chains can carry a number of pendant C=C double bonds... 

This simple DPn equation is generally valid for mono- or multifunctional inifers however with multifunctional inifers ktr> j and kt in Equation 21 correspond to one inifer/polymer molecule, i.e., the parameters are not normalized to one functional group. 

We are currently investigating the synthesis and characterization of such molecules for numerous different applications such as surfactants, cosmetics, and toner resins. A point of interest in the preparation and characterization of multifunctional hyperbranched molecules is the compositional heterogeneity (how many and what type of functional groups are on a particular hyperbranched po-lyesteramide molecule) and the positional heterogeneity (spatial distribution of the functional groups on a particular hyperbranched polyesteramide molecule). 

To date, only a few iridium catalysts have been applied to industrially relevant targets, especially on the larger scale. It is likely that several types of Ir catalyst are, in principle, feasible for technical applications in the pharmaceutical and agrochemical industries. At present, the most important problems are the relatively low catalytic activities of many highly selective systems and the fact, that relatively few catalysts have been applied to multifunctional substrates. For this reason, the scope and limitations of most catalysts known today have not yet been explored. For those in academic research, the lesson might be to employ new catalysts not only with monofunctional model compounds but also to test functional group tolerance and-as has already been done in some cases-to apply the catalysts to the total synthesis of relevant target molecules. 

Koshland (1962) has calculated, however, that such a propinquity effect will not explain the large rate enhancements observed with enzymes unless there are more than two functional groups involved with utilization of five functional groups (2 substrates and 3 catalytic groups) a rate increase of 10 would be possible. Such multifunctional catalysis would, of course, be impossible to demonstrate... 

Another fundamental idea that has been invoked to explain enzymatic catalysis is that such reactions utilize bifunctional or multifunctional catalysis that is, several functional groups in the active site are properly aligned with the substrate so that concerted catalysis may occur. Mutarotation of tetramethyl glucose is frequently cited as an example of bifunctional catalysis. Lowry and... 

---