Pendant chains/bonds

The structural versatility of pseudopoly (amino acids) can be increased further by considering dipeptides as monomeric starting materials as well. In this case polymerizations can be designed that involve one of the amino acid side chains and the C terminus, one of the amino acid side chains and the N terminus, or both of the amino acid side chains as reactive groups. The use of dipeptides as monomers in the manner described above results in the formation of copolymers in which amide bonds and nonamide linkages strictly alternate (Fig. 3). It is noteworthy that these polymers have both an amino function and a carboxylic acid function as pendant chains. This feature should facilitate the attachment of drug molecules or crosslinkers,... 

FIGURE 3 Schematic representation of a pseudopoly (amino acid) derived from the side chain polymerization of a dipeptide carrying protecting groups X and Y. The wavy line symbolizes a nonamide bond. In this polymer, the amino acid side chains are an integral part of the polymer backbone while the termini have become pendant chains. In the backbone, amide and nonamide bonds strictly alternate. 

Recently, PHAs based on fatty acids have been crosslinked. By varying the type of fatty acid in the fermentation process, the type and amount of double bonds in the pendant chains was easily adjusted (Table 6). PHA based on coconut... 

By using an olefin embedded into the parent molecule Stoltz developed the oxidative annulation of indoles. The optimal catalyst consisted of palladium acetate and ethyl nicotinate, and molecular oxygen was used as the oxidant in the process. The reaction proceeded equally well irrespective of the attachment point of the alkyl chain bearing the pendant olefin bond on the five membered ring, and the formation of five and six membered rings were both effective (6.95.),127... 

Taking into consideration the Si—O bonds within the glass surface, the difference between a strongly reacted layer and a highly polymerized network is difficult to define. However, with the above model the siloxane layer also contains partially polymerized structural units and/or hydrolysed remnants of the three-dimensional layer which would be expected from the random deposition of the hydrolysed APS. Consequently, some fragments may arise from pendant chains. Thus, the actual struture of the deposit will consist of a poly-siloxane probably chemically bonded to the glass surface every third silicon atom. 

In recent work by Arkles el al. [4, 5], it has been proposed that, in comparison with monomeric silanes, polymeric silanes may react with substrates more efficiently. A typical polymeric silane is shown in Fig. la, in which pendant chains of siloxanes are attached through methylene chain spacers to a polyethyleneimine backbone. The film-forming polymeric silane thus provides a more continuous reactive surface to the polymer matrix in the composite. In this case, the recurring amino groups on the polymeric silane backbone can react with an epoxy resin matrix through chemical bond formation. 

At the very beginning of polymerization, there is a very small concentration of macromolecules dissolved in the monomers. For this reason, the probability of the active center meeting a pendant double bond of another primary chain is close to zero and most of the pendant double bonds participate in intramolecular reactions. Spatial correlations increase the effective reactivity of pendant double bonds and explain the occurrence of intramolecular cyclization at the beginning of the reaction. 

The competition between these opposite effects (increase and decrease of pendant double bond reactivity) depends on the concentration of multifunctional monomers, the length and flexibility of the primary chains, and the quality of thermodynamic interactions between monomers and macromolecules. As a rule, cyclization is more effective at the beginning of polymerization, whereas the steric excluded-volume effects are more effective at the later stages. 

Grafting is achieved by chain transfer of the growing polymer chains to the rubber molecules. For unsaturated rubbers such as polybutadiene grafting probably occurs at allylic carbons or at pendant double bonds. 

Soper et al. have investigated the copolymerization of St with DVB [21] the intrinsic viscosity [ii] of the resulting polymer was measured in order to explore the occurrence of intramolecular cyclization leading to the formation of loop structure and thus, resulting in the reduced [r ] of the polymer. As a matter of course, this intramolecular cyclization is influenced by the flexibility of the polymer chain, the content of pendant double bonds, and the primary chain length. 

Even if this does not reflect the swollen state porosity, it would lead to increased diffusional limitations and a larger specific surface area. The photopolymers probably have a more open pore structure in the swollen state giving the template more rapid access to the sites, which are in this case confined to a smaller surface area. The difference in the conversion of pendant double bonds, and thereby the difference in cross-linking densities between the two types of materials, is probably also a factor that comes into play. An increase in chain flexibility at the sites is likely to cause an increase in the template adsorption-desorption rate coefficients. In this context it is interesting to note that increased rate enhancements were observed upon controlled hydrolysis of the polymer backbone of an imprinted esterase model [73]. 

Thus, an interaction across the space might represent a possible reaction path, but it would not explain, alone, the preferential addition of the active chain end on the pendant double bond. 

The connector chain is PS, whereas the side branches are PEO. Using anionic polymerization techniques and potassium naphthalenide as initiator, a triblock copolymer, poly(B-fr-S-fr-PB) with short polybutadiene chains, was prepared. The PB blocks were subjected to hydroboration-oxidation reaction for the addition of H2O to the pendant double-bonds of the 1,2-PB units, according to the reactions (Scheme 98). 

For poly(styryl)lithium chains, the rate of crossover to DVB is comparable to the rate of DVB homopolymerization and both of these rates are faster than the rate of the linking reaction of poly(styryl)lithium with the pendant double bonds in the poly(vinylstyrene) block formed from DVB. 

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