Cross-link tendency insoluble

The conversion is generally carried out as a one-pot reaction in two stages. First, the acid is transformed with the CDI to give the imidazolide. The conversion of the alcohol in the first step is also possible for the esterification but yields undesired cross-linking via carbonate formation in case of a polyol (Fig. 29). The imidazolide of the carboxylic acid should always be firstly synthesised. Model reactions and NMR spectroscopy (Fig. 30) with acetic acid confirm that during a treatment at room temperature CDI is consumed completely within 6 h. Thereby, the tendency of cross-linking initiated by unreacted CDI, which would lead to insoluble products, is avoided. 

Cross Linking. Tieing together of the structural units of wood with stable molecular cross-links should greatly reduce its tendency to swell. This is illustrated by the fact that incorporating only small amounts of divinyl benzene in the vinyl benzene used 1n making polystyrene, converts the polymer from a benzene soluble to a benzene insoluble material (21), with single cross-links per several thousand carbon atoms in each polymer chain. 

Humic acids are soluble in weak alkaline solutions and are essentially insoluble in water and mineral acids. They may be precipitated from solution by the action of mineral acids and bivalent or trivalent cations, however, they are fairly resistant to the acid hydrolysis. They are dark spherocol-loids with a cross-linked structure which plays a part in their high sorption capacity. They exhibit different degrees of a tendency to aggregation and very different degrees of dispersion. In comparison with other types of natural organic substances, the humic acids are characterized by their extraordinary stability in the soil. This stability is due to their ability to form organomineraJ complexes, particularly with clay minerals and with aluminium and iron hydroxides. 

A fundamental impediment to the use of solution-state NMR in the analysis of cross-linked materials is the tendency of the polymers to become partially insoluble at the gel dose. As will be discussed below the rapid increase in strength... 

Polymerization of a series of dienes of formula [106] has been studied (/i, 16, 93). The polymers that were obtained consisted of both soluble and cross-linked, insoluble fractions. Quantitative infrared and bromination experiments indicated that the soluble polymers possessed cyclic as well as open-chain structures. The percent of cyclization depended on the size of the incipient cycle and corresponded roughly to what is obtained in other cyclization reactions. The results are summarized in the accompanying table. A similar effect of ring size versus tendency toward cyclopolymerization was observed for monomers of structures [107] (92). 

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