Reversible stepwise condensation

Polymers that have been synthesized by a stepwise condensation route (Section 1.2.1) that involves the evolution of water are susceptible to degradation by hydrolysis since this involves the reversal of the polymerization reaction. The conditions of high temperature and the presence of tmces of an acidic or basic catalyst are easily met in an extruder. The reaction is often auto-catalytic since the product of hydrolysis will be an acid end group. This shown in Scheme 1.73 for the acid-catalysed hydrolysis of poly(ethylene terephthalate) (PET). 

The principle of in vitro selection is governed by a number of the same principles that apply to the Darwinian theory of evolution, as shown in Figure 2. First, the random sequence DNA is prepared by automated solid-phase synthesis. A mixture of four types of nucleotide is added in a stepwise condensation reaction process. When necessary, this DNA library may be converted to an RNA library by in vitro transcription or to a peptide library by in vitro translation. Second, the prepared DNA, RNA, or peptide library is subjected to affinity selection, and the molecules that bind to a target molecule are selected. Because only a very small part of the library is selected in each selection, the selected fraction is then amplified by a polymerase chain reaction (PCR) or a reverse transcription PCR (RT-PCR) technique. Successive selection and amplification cycles bring about an exponential increase in the abundance of the targeting DNA, RNA, or peptide until it dominates the population. 

The resulting amino acid then condenses in a stepwise manner to form the growing polymer chain. As in direct polymerization, cycHc oligomers are also formed hence, caprolactam (qv) can be formed in the reverse of the reaction just shown above. 

The majority of physisorption isotherms (Fig. 1.14 Type I-VI) and hysteresis loops (Fig. 1.14 H1-H4) are classified by lUPAC [21]. Reversible Type 1 isotherms are given by microporous (see below) solids having relatively small external surface areas (e.g. activated carbon or zeolites). The sharp and steep initial rise is associated with capillary condensation in micropores which follow a different mechanism compared with mesopores. Reversible Type II isotherms are typical for non-porous or macroporous (see below) materials and represent unrestricted monolayer-multilayer adsorption. Point B indicates the stage at which multilayer adsorption starts and lies at the beginning of the almost linear middle section. Reversible Type III isotherms are not very common. They have an indistinct point B, since the adsorbent-adsorbate interactions are weak. An example for such a system is nitrogen on polyethylene. Type IV isotherms are very common and show characteristic hysteresis loops which arise from different adsorption and desorption mechanisms in mesopores (see below). Type V and Type VI isotherms are uncommon, and their interpretation is difficult. A Type VI isotherm can arise with stepwise multilayer adsorption on a uniform nonporous surface. 

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