Alternating copolymer, definition

Note An alternating copolymer may be considered as a homopolymer derived from an implicit or hypothetical monomer, see Note 1 to Definition 2.4. 

The excellent agreement obtained for )5-PLA would indicate that this force field should be transferable to other APRS polypeptides, of course taking due accoimt of side-chain differences. This has been done for Ca-poly(L-glutamate), Ca-PLG [92], which X-ray and electron diffraction data had indicated to have an APPS structure but which were not extensive enough to provide a definitive conclusion. A proposed model [93] was used as the basis for the vibrational analysis, and the good prediction of the observed bands [92] supports both the model as well as the viability of the force field. The subtle differences between the -PLA and fi-Ca-PVG spectra are accounted for by the normal mode calculation, and emphasize the influence of the side chain on the main-chain modes, particularly on amide III. A similar successful vibrational analysis has been done on an alternating copolymer, APPS poly(L-alanylglycine) [19]. 

In the extreme case where rjrj =0 because both rj and i2 equal zero, the copolymer adds monomers with perfect alternation. This is apparent from the definition of r, which compares the addition of the same monomer to the other monomer for a particular radical. If both r s are zero, there is no tendency for a radical to add a monomer of the same kind as the growing end, whichever species is the terminal unit. When only one of the r s is zero, say rj, then alternation occurs whenever the radical ends with an Mj unit. There is thus a tendency toward alternation in this case, although it is less pronounced than in the case where both r s are zero. Accordingly, we find increasing tendency toward alternation as rj 0 and rj 0, or, more succinctly, as the product X1X2 0. 

While the percentage composition of copolymers (i.e., the ratio of comonomers) is not given, copolymers with architecture other than random or statistical are identified as alternating, block, graft, etc. Random or statistical copolymers are not so identified in the CA index. Oligomers with definite structure are noted as dimer, trimer, tetramer, etc. 

So far in our discussion of micros tincture, we have considered homopolymers. To some degree, however, there is an element of semantics involved in our definition. Is a branched polyethylene a true homopolymer or should it be considered a copolymer of ethylene and whatever units comprise the branches Here our concern is real copolymers, those synthesized from two (or more) distinct monomers. The simplest possible arrangements are shown in Figure 2-22 and are self-explanatory. But, as we will see, real life is more complex. True random copolymers are rare and in most cases there are tendencies to blockiness or alternating arrangements. There are also graft copolymers, but we will discuss all this in more detail when we consider copolymerization. 

So far, activation of mechanophores in the cyclic chain has not been reported. One important issue is the location of the mechanophore in the cyclic polymer. If only one mechanophore is incorporated into a ring chain (Fig. 18a), it is unlikely to experience the maximum hydrodynamic force (red dots) because the ring has no definitive midpoint in the flow field. Even if the ring chain breaks, the positimi of the mechanophore is unlikely to locate just at the midpoint of the linear product To improve the chance of activation, it is better to have multiple mechanophores incorporated into the cyclic chain, such as random, alternative, or block cyclic copolymers. For example, in Fig. 18b the mechanophores are randomly dispersed into a cyclic macromolecule to increase the activation probability. If the cyclic chain breaks, the mechanophores still have the chance to be located near the midpoint of the linear chain. The linear fragment then undergoes CST and activates the mechanophores. 

---