Conversion of functional groups

The chemical inertness of the three-membered ring permitted many conversions of functional groups in diazirines. Esterifications, cleavage of esters and acetals, synthesis of acid chlorides, oxidation of hydroxy groups to carboxyl groups as well as Hofmann alkenation all left the three-membered ring intact (79AHC(24)63). 

Figure 1.6 Relationship of degree of polymerization to conversion of functional groups in step-growth polymerizations.

This chapter lists the conversion of functional groups into methyl, ethyl, propyl, etc. as well as methylene (CH2), phenyl, etc. 

Noteworthy that all the above formulated results can be applied to calculate the statistical characteristics of the products of polycondensation of an arbitrary mixture of monomers with kinetically independent groups under any regime of this process. To determine the values of the elements of the probability transition matrix of corresponding Markov chains it will suffice to calculate only the concentrations Q()- of chemical bonds (ij) at different conversions of functional groups. In the case of equilibrium polycondensation the concentrations Qy are controlled by the thermodynamic parameters, whereas under the nonequilibrium regime of this process they depend on kinetic parameters. 

The values of molar fractions A of units S and complete conversion of functional groups can be obtained by solving the following equations ... 

Fig. 1. The dependence of weight fraction of gel on conversion of functional groups A under irreversible polycondensation of monomer SA3 described by the simplified FSSE model with kinetic parameters K k k0 and K2=k2/k0. The curves are depicted proceeding from the results of calculations at values of these parameters equal to kx=1, k2=0.1 (a), and kx=1, 2=10 (b)...

As a synthetic tool, heterogeneous catalytic dehydrogenations offer selective conversions of functional groups. Most popular dehydrogenations involve... 

Presently, the quantitative theory of irreversible polymeranalogous reactions proceeding in a kinetically-controlled regime is well along in development [ 16,17]. Particularly simple results are achieved in the framework of the ideal model, the only kinetic parameter of which is constant k of the rate of elementary reaction A + Z -> B. In this model the sequence distribution in macromolecules will be just the same as that in a random copolymer with parameters P(Mi ) = X =p and P(M2) = X2 = 1 - p where p is the conversion of functional group A that exponentially depends on time t and initial concen-... 

Figure 5.1 Simulated dependence of the weight fraction of the largest molecule, wg, on conversion of functional groups, a. Varying systems size counted in the numbers of monomer molecules used in simulation is indicated 105,106,107, 5x 107. Modified Figure 5b of ref. [3] reprinted with permission...

In ternary systems, amorphous hard clusters can be formed. As explained above, at certain fraction of hard units and a certain conversion of functional groups, percolation threshold of the hard structure is reached. It has been found experimentally by analyzing the ultimate behavior of three- and four-component... 

Thus, to improve upon the physical and mechanical properties of the polymer material, one must not only consider the materials used, but also the conditions under which the polymer was formed. These reaction conditions, along with the type of monomer system chosen, will completely control the conversion of functional groups in the system. More importantly, the conversion will ultimately determine the mechanical, physical, and wear properties of the material. Since most dental materials are crosslinked polymers, characterizing the polymerization reaction becomes even more important since the physical nature of a crosslinked polymer is fixed upon completion of the polymerization. For example, not only is the microstructure (i.e. the degree of crosslinking) largely unalterable after polymerization, but the system is insoluble and fixed macroscopically. Clearly, to produce crosslinked networks with the desired material properties, one must ascertain the appropriate reaction conditions and the effects of the reaction conditions on the network structure. 

In the glassy state relaxation times become very large, making the continuation of the polymerization reaction difficult. Moreover, once in the glassy state, small advances in the conversion of functional groups produce a further increase in the already large relaxation times. Thus, the reaction becomes autoretarded and rapidly ceases for practical purposes. Only an increase in the cure temperature can restart the polymerization reaction. 

Therefore, for particular values of the conversion of functional groups and temperature, the rate of chainwise polymerizations depends on the concentration of active species which, in turn, depends on the particular thermal history. Thus, phenomenological equations derived from Eq. (5.1), or isoconversional methods of kinetic analysis, should not be applied for this case. 

When the polymer network is formed by a single type of reaction, conversion of functional groups has a unique meaning. This leads to a direct... 

Figure 8.5 Oxidative and reductive conversions of functional groups.

Two different approaches have been adopted here in describing fluorination reactions the production of highly fluorinated systems is discussed in this chapter on the basis of a comparison of methods, whereas selective fluorinations are described in Chapter 3 in terms of the conversion of functional groups. If we wish to produce highly fluorinated systems, then the starting materials are usually hydrocarbons, polychloro compounds or, of course, highly fluorinated building-blocks for conversion to other compounds. 

As will be seen, it can be related in a relatively simple fashion to the fractional conversion of functional groups [18], provided cyclization is insignificant. 

Gel point. The Carothers equation can also be used to estimate the conversion needed to reach the so-called gel point in condensation polymerization involving monomers with functionalities higher than 2. The gel point is defined as the state of conversion at which gel formation caused by crosslinking begins to become apparent. With the assumption that this occurs when practically all molecules of the limiting monomer have reacted, the requisite fractional conversion of functional groups can be estimated with a rearranged form of the Carothers equation ... 

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