Cross-linking reactions, difficulty

The reactions of intramolecular cross-linking is a rather poorly investigated area in the field of macro-molecular reactions. However, the problems of regularities of such processes are related to such important problems of polymer chemistry as chemical modification of polymers, networks formation, sorption of low molecular reagents by polymers, intramolecular catalysis, conformational transitions and so on. In spite of the great importance of the study of regularities of cross-linking reactions, the experimental and theoretical analysis of such processes is complicated by many difficulties. ... 

The reaction of formaldehyde with unsubstituted phenols leads to either soluble or cross-linked resins since condensation occurs at either ortho or para positions. Monosubstituted [35] (ortho or para) phenols give cross-linking with difficulty but phenols doubly substituted in ortho or para positions yield only low molecular weight products. If only one ortho or para position is available on the phenol then the phenol cannot produce resins and reacts with difficulty with aldehydes [33]. Sometimes cresols and phenol are blended together to obtain fully cured resins. In addition to phenol, the other important phenols that are used to give phenolic resins are o-cresol, mixed cresols, / -/e/ r-butylphenol (from isobutylene and phenol), p-phenylphenol (by-product from phenol manufacture), resorcinol, and cardanol (from cashew nutshell liquid). 

We can, however, note that an adhesive intended for thermal-structural applications has a prepolymer with a thermostable structure as a base component, which will carry chemical functions that will allow its cross-linking (as widely as possible) at a high temperature. The main difficulty involved relates in fact to the small difference in temperature ( processability range) between the softening of the prepolymers and the cross-linking reaction typically 30° between 130-160°C and 160-190°C. 

As mentioned previously, the main drawbacks of the thermal route to poly-borylborazine are (1) the presence of both direct intercyclic bonds and three-atom bridges between the rings, and (2) a difficulty in controlling the polycondensation rate. One solution we investigated to address these drawbacks is a route based on the room temperature reaction of /i-chloroborazine with trialkylaminoborane.31 32 We used 2-methylamino-4,6-dichloroborazine instead of 2,4,6-trichloroborazine to prepare a two-point polymer (scheme 4), which is theoretically less cross-linked. 

The polymerization reaction (Figure I) is markedly influenced by the presence of trace impurities which was one of the difficulties encountered in earlier investigations. The conventional route is a melt polymerization of highly purified trimer (NPC1 ), or a mixture of trimer and a small amount of tetramer (NFCl.), sealed under vacuum in glass ampoules, at approximately 250°C. Proper selection of time and temperature is necessary to obtain II and avoid the formation of cross-linked matrix (III). 

Most of the polymeric reagents which have been developed so far make use of an insoluble cross-linked polymer as the backbone. Investigations on the reaction rates and kinetic course in solid-phase synthesis revealed that the reaction sites within the polymeric matrix are chemically and kinetically not equivalent, making quantitative conversions almost impossible. Furthermore, the difficulty in the preparation and accessibility of insoluble polymeric reagents appears to limit a more... 

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