Main-chain modification

This volume brings together most of these critical issues by highlighting recent advances in a number of core areas of peptidomimetic synthesis. Section 9 focuses on side-chain-modified peptides, Section 10 describes the preparation and use of a variety of peptide main-chain modifications. Combined side-chain- and main-chain-modified peptides are covered in Section 11. Finally, Section 12 provides chemistry leading to peptides incorporating secondary structure ((1- and y-turns, helices, p-sheets) mimetics and inducers. 

Sections 9 and 10 deal with side-chain- or main-chain-modified peptides, respectively. In Section 11 combined side-chain- and main-chain-modifications of peptides are discussed. More specifically, two families of peptidomimetics are treated in detail. 

With an accelerated system, a simple network structure with dtalkenyl mono- and disulfide crosslinks and conjugated tnene units as main-chain modifications is obtained ... 

FT-IR results also showed that one new (small) absorption at 1659 cm"1 appeared, which could not be attributed to peroxide decomposition products. This absorption also appeared when the peroxide-curing experiments were carried out using an amorphous EPM, indicating that the absorption did not relate to rearrangement of the third monomer moiety (ENB in this case). It is tentatively concluded that the absorption at 1659 cm 1 is related to EPDM main-chain modifications, resulting from disproportionation reactions of EPDM macroradicals with BHT radical fragments. 

Applications of Fourier transform Raman spectroscopy to the analysis of NR are described. Of particular interest is the observation of main chain modifications during vulcanisation and the ability to observe the conversion of insoluble to soluble sulphur under conditions appropriate to compounding and vulcanisation. The influence of crystallisation, both temperature and strain induced, on the Raman spectmm of NR is also demonstrated. 9 refs. EUROPEAN COMMUNITY UK USA WESTERN EUROPE... 

In fact, the physical properties obtained depend on the types of cross-hnk formed and the extent of main-chain modification by side reactions. This is usually being largely determined by the vulcanization system, although cure time and temperature also have an important effect. Generally, there are three types of typical sulfur vulcanization systems, namely, conventional vulcanization, efficient vulcanization and semi-efficient vulcanization. 

This figure also illustrates the three main regions of rubber vulcanization. The first regime is the induction period, or scorch delay, during which accelerator complex formation occurs. The second time period is the cure period, in which the network or sulfurization structures are formed. The network structures can include crosslinks, cyclics, main chain modification, isomerization, etc. The third regime is the overcure, or reversion regime. 

The changes depicted in Fig. 18.2 may continue to occur after the formal vulcanization process is complete. The reactions may continue whilst the vulcanizate is in service, particularly if elevated temperatures are encountered. Monosulphide cross-links are thermally stable and hence vulcanizates of Type (i) (Fig. 18.2) show relatively little change on aging. On the other hand, polysulphide cross-links are thermally unstable and vulcanizates of Type (ii) undergo reversion (loss of cross-links) and main chain modification with corresponding changes in physical properties. Polysulphide cross-links are also susceptible to interchange reactions, initiated by nucleophilic species such as... 

CORRELATION OF TYPE OF CROSS-LINK TERMINUS WITH RELATIVE MOLAR YIELDS OF MAIN-CHAIN MODIFICATIONS FORMED BY SULPHURATION OF TRANS-2,6-DIMETHYLOCTA-2,6-DIENE... 

The main chain modifications formed by side reactions also appear to affect a selection of properties (Table 3). Some of these are probably a consequence of an increase in polarity and/or an increase in the glass transition temperature of the rubber. Others are a result of interruptions in the stereoregularity of the elastomeric backbone, as a consequence of which the tendency to crystallize is reduced. (It is interesting to note that, on the basis of limited evidence so far available, " accelerator-terminated pendent groups do not appear to hinder the low temperature crystallization of NR significantly.)... 

The extent to which the type of crosslink and type and degree of main chain modification play a role in determining physical properties varies considerably with the nature of the elastomer and other factors such as the presence of fillers and oils. Tables 1-3 have largely been constructed on the... 

Table 3 Influence of Main Chain Modifications on Properties...

Crosslink shortening with additional crosslinking Crosslink destruction with main chain modification S—S bond interchange... 

The conclusions reached in the two previous sections allow the second part of the reaction sequence for sulfur vulcanization outlined in Scheme 1 to be expanded into that shown in Scheme 5 (at least as far as NR and IR are concerned). We now see that the rubber-bound polysulfidic pendent group has not one fate, but three. The two which do not immediately lead to crosslinks both give rise to a loss of crosslinking efficiency (i) the decomposition route by main chain modification and diversion of sulfur, and (ii) the desulfuration route by the removal of accelerator as inactive monosulfidic pendent groups which appear to be reasonably stable except at high temperatures but which, in any case, are not converted into crosslinks. The existence of at least one other route to such pendent groups has been deduced. 

The dependence of the physical properties of sulfur vulcanizates of diene rubbers on network structure is reviewed and the influence of degree of crosslinking, crosslink structure, and main-chain modification are discussed. In polyisoprenes, these are determined, in practice, by the balance between three competing types of reaction the conversion of polysulfidic pendent groups into polysulfidic crosslinks the desulfuration of polysulfidic pendent groups and crosslinks, eventually to the corresponding monosulfides, with recirculation of the removed sulfur into the crosslinking... 

The first category is the desulfurization of polysulfides to di- and monosulfidic crosslinks. This pathway is affected by the Zn-accelerator complex (found in accelerated sulfur vulcanization). The routes in the other category are characterized as thermal decomposition, in which the crosslinks and the sulforation species decompose into conjugated species, cyclic sulfides, shorter sulfur crosslinks, and main-chain modifications. 

The phenomenon of reversion is associated with decrease in crosslink density and formation of main-chain modifications. The consequence of degradation of network and the formation of sulphidic and non-sulphidic modifications during reversion is explained. Emphasis is placed on a chemical, l,3-bis(citraconimidomethyl) benzene (Perkalink 900), which counteracts reversion by a process called crosslink compensation. Not only does Perkalink 900 add crosslinks lost during reversion, it also eliminates dienes and trienes form the network. The mechanism of Perkalink 900 crosslinking is discussed. Based on the above studies, the advantage of using Perkalink 900 in NR-based compounds is examined. Key applications and guidelines for optimal use are outlined. 25 refs. 

Reversion of sulphur-based crosslinks continues to a problem for NR compounders. Reversion is the thermal degradation of polysulphidic crosslinks leading to a reduction of crosslink density and an introduction of main chain modifications. In practice, reversion leads to a decline in compound physical properties. Reversion occurs when compounds are overcured or when vulcanisates are exposed to anaerobic ageing. Ideally, a... 

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