Chain extension, removable

Ketone (4) looks very difficult to disconnect. With a chain extension in mind, disconnection to acid (5) and removal of CHg gives an easily made cyclopropane acid (6). 

In the single-step system, the entire reaction takes place when all three basic components plus catalysts and pigments are mixed together. Careful use of catalysts enables the preparation of the prepolymer and the subsequent chain extension to take place in one step. As all the reactions are exothermic, heat is liberated and must be removed otherwise, part distortion will take place. 

A major problem is the exotherm that is given out in the reaction. The heat generated must be removed in the molding. The total amount of heat given out is the sum of the heat from the initial prepolymer reaction and the final chain extension stage. This leads the process to be used in relatively thin-walled products. 

The polyurethane (PU) can be considered an environment-friendly material because the urethane bond resembles the amide bond, which implies possible biodegradability. It can be used in various elastomer formulations, paints, adhesives for polymers and glass, and artificial leather as well as in biomedical and cosmetic fields. Polyurethane spheres were prepared from 20/40% of PU prepolymer solution in xylene [91]. PU droplets were formed in water with the SPG membrane of different pore size (1.5-9.5 pm) and then polymerized to form the final microspheres. Finally, spherical and solid PU particles of 5 pm were obtained after the removal of the solvent. In another study, Ma et al. reported the formation of uniform polyurethane-vinylpolymer (PUU-VP) hybrid microspheres of about 20 pm, prepared using SPG membranes and a subsequent radical suspension polymerization process [92], The prepolymers were solubilized in xylene and pressed through the SPG membrane into the continuous phase containing a stabilizer to form uniform droplets. The droplets were left for chain extension at room temperature for some hours with di- and triamines by suspension polymerization at 70 °C for 24h. Solid and spherical PU-VP hybrid particles with a smooth surface and a higher destructive strength were obtained. 

Thus, the O-benzylated glucosamine derivative 60 was condensed with the spacer-linked L-rhamnoside 61, followed by the removal of the base-labile protecting group at the site of the chain extension to afford the disaccharide acceptor 62. The trisaccharide block 67 was prepared in a similar approach. Thus, condensation of the glucosamine derivative 63 with rhamnoside 64 afforded the disaccharide... 

Fig. 2.5. Priming with a long Hae HI restriction fragment on single-stranded circular DNA. After the chain extension step digestion with Hae III removes the unlabelled primer from the heterogeneous set of labelled extension sequences.

After peptide chain extension and disulfide bond formation by oxidative cyclization, the phenylacetyl group could be removed enzymatically in 74% yield. The technique has been applied further in the synthesis of even larger peptides, for example in the deprotection of tris(phenylacetamido)porcine insulin.P l Since PGA is commercially available and devoid of any peptidase activity, this method appears to be generally useful for the construction of lysine-containing oligopeptides. 

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