Stepwise coupling polymerization

Fig. 7 Stepwise coupling polymerization in the s5mthesis of ladder polysilsesquioxanes...

Polyurethanes are macromolecules in which the constitutional repeating units (CRUs) are coupled with one another through urethane (oxycarbonylamino) groups. They are prepared almost exclusively by stepwise addition polymerization reactions of di- or polyfunctional hydroxy compounds with di- or polyfunctional isocyanates ... 

The second common method of polymer synthesis involves the stepwise coupling of small molecules which are difunctional by virtue of reactive functional groups. A typical example of step-reaction polymerization would be the synthesis of polyamides by reaction of a diamine with a diacid. In these systems the chain is built up slowly by reaction of any pair of functional groups in the system and it is common for the coupling to involve elimination of a small molecule. Conventionally these polymerizations allow more control over the chain structure but difficulties in reaching very high conversions and problems of reagent purity usually lead to much shorter... 

There are two general routes to the synthesis of conjugated polymers addition polymerization of unsaturated monomers and condensation polymerization or stepwise coupling of monomers with difunctional groups. 

Even quadruple-chain ladder polyphenylsiloxane structures are feasible by a supra-molecular template-directed stepwise coupling and polymerization method [35]. 

The control of the coupling of coniferyl alcohol has been attempted. When the metalloenzyme is mixed with coniferyl alcohol, a stepwise polymerization proceeds by dimerization and tetramerization, which result in the uncontrolled couplings. When the monomer is slowly supplied to the enzyme-containing reaction mixture, the radicals attack the terminal end of the polymer chain, because the diluted radical reacts preferentially to form the dimer by the coupling that means that comblike lignin grows on the cell wall. Some attempts have been reported to control the polymerization in the presence of cellulose as the matrix [23,24],... 

Thus, in this method the stepwise synthesis proceeds on the soluble polymer, as in the liquid phase method the only difference is in the coupling step where the carboxyl component is an insoluble polymeric active ester. The use of the solubilizing C-terminal macromolecular protecting group overcomes the limitations encountered in the original application of the polymer reagent method of peptide synthesis. However, due to the steric limitations characteristic of the polymer-polymer interactions, the attainment of quantitative coupling in each step of the synthesis can be difficult in this method. 

Pol I also has a potent 5 3 exonucleolytic activity (Figure 24-7). This activity is directed against a base-paired strand and consists of stepwise removal of nucleotides one by one from the 5 -P terminus. Furthermore, the nucleotide removed can be either of the deoxyribo or the ribo type. The 5 —> 3 exonucleolytic activity also can be coupled to the polymerizing activity. Recall that pol I can add nucleotides to a 3 -OH group at a nick and displace... 

---