Monomer structure and reactivity

Differences in monomer structure and reactivity are reflected in the values of rA and tb for particular binary copolymerizations and give rise to different types of copolymerization behaviour. Plots of Equation (1.33) are shown in Figure 1.2 for different ta, kb pairs which represent the main types of copolymerization behaviour. These are discussed below. 

Monomer Structure and Reactivity Eiectronic and Entropic Effects. 

When discussing various methods for the synthesis of protein-like HP-copolymers from the monomeric precursors (Sect. 2.1), we pointed to the possibility of implementation of both polymerization and polycondensation processes. The studies of the potentials of the latter approach in the creation of protein-like macromolecular systems have already been started. The first published results show that using true selected reactions of the polycondensation type and appropriate synthetic conditions (structure and reactivity of comonomers, solvent, temperature, reagent concentration and comonomer ratio, the order of the reagents introduction into the feed, etc.) one has a chance to produce the polymer chains with a desirable set of monomer sequences. 

This very efficient, time-honoured pathway suffers however a very severe limitation, i.e. the relative reactivity of the living centre C must be adapted to the structure and reactivity of monomer M2, a requirement which is not very often met. 

This is not a trivial problem, and has important implications for the mechanism of the reaction. However, the bulk of the evidence is for centrosymmetric rings, which would be in keeping with our experience in small-molecule systems. For the present purposes we assume this to be the case. On this basis DSP is one of a class of monomers of crystal structural type 100 that polymerize to polymers 101. Note that, as is typical of topochemical reactions, there are cases of polymorphism of the monomers, in which only those of structure 100 are reactive. Also small changes in the substitution of this molecule frequently result in changes in crystal structure and reactivity. 

The monomer reactivity ratios for many of the most common monomers in radical copolymerization are shown in Table 6-2. These data are useful for a study of the relation between structure and reactivity in radical addition reactions. The reactivity of a monomer toward a radical depends on the reactivities of both the monomer and the radical. The relative reactivities of monomers and their corresponding radicals can be obtained from an analysis of the monomer reactivity ratios [Walling, 1957]. The reactivity of a monomer can be seen by considering the inverse of the monomer reactivity ratio (1 jf). The inverse of the monomer reactivity ratio gives the ratio of the rate of reaction of a radical with another monomer to its rate of reaction with its own monomer... 

In general, an appropriate initiator is a species which has approximately the same structure and reactivity as the propagating anionic species, ie, the pK of the conjugate acid of the propagating anion should correspond closely to the ipKa of the conjugate acid of the initiating species. If the initiator is too reactive, side reactions between the initiator and monomer can occur if the initiator is not reactive enough, then the initiation reaction may be slow or inefficient. 

Our own studies of the copolymerization of tetrachloroethylene with ethylene showed that besides telomerization, copolymerization occurs simultaneously. The influence of the experimental variables (pressure, ratio of the monomers, structure and concentration of the initiator, temperature, and time) were studied. As catalysts azoisobutyrodinitrile, fert-butylperoxyisopropyl carbonate, benzoyl peroxide, cyclohexylperoxy carbonate and tert-h xty peroctoate were used. The reactive ratio rx... 

DePue, J. S. Collum, D. B. Structure and reactivity of lithium diphenylamide. Role of aggregates, mixed aggregates, monomers, and free ions on the rates and selectivities of N-alkyla-tion and E2 elimination. /. Am. Chem. Soc. 1988, 220, 5524-5533. 

Thus, it must be remembered, that in the polymerization of heterocycles, species originally formed by interaction of initiator with monomer, may differ significantly (both in structure and reactivity) from the propagating active species. Initiation may involve the sequence of at least two reactions and the second one may be the slow, thus rate-determining step in initiation. 

Tsuruta, T. Structure and reactivity of vinyl monomers. In Structure and mechanism in vinyl ptdymerization. (Tsuruta, T., O Driscoll, D. F., eds.). New York Marcel Dekker 1969, p. 27... 

To have a useful preceramic polymer, considerations of structure and reactivity are of paramount importance. Not every inorganic or organometallic polymer will be a useful preceramic polymer. Although preceramic polymers are potentially high-value products if the desired properties result from their use, the more generally useful and practical systems will be those based on commercially available and relatively cheap starting monomers. 

It appears that the basic mechanisms involved in polymerization and, in particular, in photopolymerization of diacetylenes are understood. Nevertheless, to date it is not possible to design a diacetylene monomer on the basis of desired reaction behavior and/or product properties. To this end more quantitative information on the reaction kinetics of diacetylenes including quantum-mechanical aspects is needed which could form the basis for developing a quantitative relationship between structure and reactivity. 

The similarities outlined, if valid, should mean that the rates of the many reactions indicated should be governed by some balance of the same list of structural influences relating monomer structure to reactivity, and the effect of the nature of the reaction medium. The following guiding principles are outlined briefly. More detailed discussions of factors affecting the rates of carbonyl additions, as well as mechanistic details, are given in several references [13—15] and in later sections of this chapter. 

The active center in this reaction is presumably a carbonium ion ion pair, as shown above, which can vary in structure and reactivity from a free carbonium ion at one extreme to a contact ion pair (or even a readily dissociated covalent compound) at the other. The initiator, which consists of the catalyst shown in the equation and generally a cocatalyst, has a controlling effect on the structure of the ion pair because it provides the counterion, Y, for the active center. Hence, small changes in the composition of the initiator as well as in monomer structure, reaction solvent, and temperature can cause profound changes in both the rates of the propagation and termination reactions and in the structure of the polymer formed. For this reason, polymerization reactions have been referred to as "chemical amplifiers" in that the polymer molecule is formed by hundreds or thousands of propagation reactions followed by one termination reaction. 

The present state of knowledge about the true mechanism of these polymerization reactions is not sufficiently advanced to permit a satisfactory rationalization of these effects. It should be remembered that the growing chains in these systems have been convincingly demonstrated (]J) to be associated in pairs at the site of the carbon-lithium bond. Hence it appears that the incoming monomer must react with the associated complex, which apparently can affect the mode of entry. This undoubtedly can explain the greater extent of cis-1,4 addition in the case of isoprene compared to butadiene. Furthermore, such factors as lithium concentration and presence of different solvents can be assumed to have an effect on the structure and reactivity of the associated carbon-lithium bond at the active chain end. This would certainly be expected for the highly polar carbon-lithium... 

Correlations of structures and reactivities for anionic and cationic ring-opening polymerization are reviewed. The following topics are discussed chemical structure of active species and their isomerism, determination of active centers concentration, covalent vs ionic growth and correlations between structures of active centers or monomers and their reactivities. 

Correlations of structures and reactivities require for the ring-opening polymerization as well as for other ionic polymerizations approaches differing from these in radical polymerization of the unsaturated monomers. This is because in radical polymerization free radicals are the unique chemical structure of the growing species and double bonds are the only chemical groups involved in polymerization (1), C ). ... 

Chain copolymerization is important both from academic and technological viewpoints. Thus much of our knowledge of the reactivities of monomers, free radicals, carbocations, and carboanions in chain polymerization comes from copolymerization studies. The behavior of monomers in copolymerization reactions is especially useful for studying the relation between chemical structure and reactivity of monomers. From the technological viewpoint. 

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