Stepwise polymerisation

The stepwise polymerisation of activated amino acids leads to the formation of activated dimers, which very often cyclise to diketopiperazines and are thus removed from the chain elongation process (Orgel, 1989). 

Neglect of the formation of polymeric rings, however, is sometimes too crude an approximation. It may happen that the cyclisation constant C2 of the linear dimer is larger than the cyclisation constant Cl of the monomer. It may also happen that the concentration of the monomer is comparable to Ct but smaller than C2. When this is the case, the open chain dimer, once formed, will show a higher tendency to cyclise than to react with the monomer to give the linear trimer. Under the above conditions the stepwise polymerisation is truncated after the first step, and the system is described to a useful approximation by scheme (6). 

They are obtained by stepwise polymerisation of diisocyanates with diamines ... 

Only the catalysts which exhibited Fischer-Tropsch activity consumed CO in TPC runs. This suggests that carbiding and FT polymerisation proceed via a conmon intermediate formed by dissociative CO chemisorption. All active FT catalysts examined promoted dissociative CO chemisorption below 200°C but none of these catalysts showed FT activity below 250°C. This is consistent with a CHx stepwise polymerisation being the rate determining step, as advocated by many workers (ref. 9). 

Polymerisation may occur as a result of dehydration of these compounds to methylene and dimethylene urea or more probably by a stepwise loss of water between the molecules of methylol and dimethylol-urea. 

There is no indication as to whether these compounds are formed by hot or thermal reactions. Many of the products e.g. the vinyl compounds and the polymers) are explainable as resulting from free radical reactions. The virtual disappearance of the parent compound at high radiation doses is attributable to the interception of the stepwise reformation by competing radical reactions. The decrease in vinyl compounds is explained as being due to increased polymerisation. 

The mode of polymerisation and crosslinking for coatings is very similar to that of bulk polymers. An important requirement is that premature polymerisation should not take place before application. In the presence of activators, e.g., cobalt naphthenate, many paints on exposure to air polymerise by radical oxidation resulting in crosslinked structures. Stepwise growth polymerisation, e.g., urethanes, is promoted by heat therefore storage at high temperatures (>50°C) should be avoided. 

The process of condensation takes place in a stepwise manner, ultimately resulting in the formation of polymer. This process is also known as step growth polymerisation. 

Stepwise addition polymerisation (polyaddition) or condensation polymerisation (polycondensation) are possible polyreactions for the first step. The two latter combinations attained interest in the technical synthesis of polyimides and polybenzimidazoles. 

This method involves a combination of stepwise swelling of preformed seed particles followed by polymerisation (Fig. 6). 

Another approach to monolithic imprinted polymer capillary columns has been reported. The polymerisation was performed at 60°C and thus thermally initiated [70,71]. The result is a dense polymer inside the whole capillary. The resulting MIP capillary then has to be connected to an electrolyte-filled open capillary via a teflon tube and a detection window is prepared on the open capillary to facilitate detection (as described above. Fig. 16.2). Also, ammonium acetate (1-2 mM) is added as a conducting agent to the pre-polymerisation mixture. This is done to allow exchange of the solvent of polymerisation for electrolyte, which can be achieved electrophoretically by stepwise increase of the electric field until a stable base-line is obtained. 

After an efficient system has been found, the main open question is how to solidify the foams. One route could be to utilise water-soluble polymers or/and polymerise water-soluble monomers. Another route could be to switch to polymeric microemulsions. One would stepwisely replace the water by the preferred polymerisable hydrophilic monomer/polymer. By doing so one can be sure that the nanostructure of microemulsion remains unaffected. The generation of the nanofoam depends on the used monomer. A fixation via irradiation with fight would be preferable, when photoactive cross-linkers are... 

One of the most common technologies for the synthesis of polymer polyols by a radical mechanism is based on the stepwise addition of a mixture of vinylic monomers (polyether polyol, initiator, transfer agent (mixture I)) to a second mixture (mixture II) of polyether polyol (identical with the polyether used for mixture I) and NAD (macromer or nonreactive NAD) under a nitrogen protective atmosphere, in the polymerisation reactor at 115-... 

Ozol-Kalnin and co-workers [25] derived the kinetic equation that describes the polymerisation on assumption that the reaction proceeds via stepwise addition of the monomer of one kind. To describe the formation of a macroscopic network, the criterion for gelation, snggested previonsly for free radical polymerisation of nnsaturated componnds ... 

Diblock copolymers were synthesised by two stepwise anionic polymerisation methods. One method produced diblock copolymer plus 30% of poly(2-vinylpyridine) homopolymer. The copolymers were dissolved in O.IM hydrochloric acid. When the pH was increased by the dropwise addition of 0.1 M sodium hydroxide, micelles with well-defined hydrodynamic diameters formed spontaneously at around pH 5. Further basification produced stable micelle structures and reacidification produced the mirror image of this titration curve. Blue swirls were observed when sodium hydroxide was added at pH4 or pH5. The micelle sizes were measured by quasielastic light scattering. It is shown that (1) it is possible to control micelUsation by pH and (2) formation of well-behaved micelles of variable hydrodynamic diameter is possible by titration of different ratios and different total polymer concentrations of poly(2-vinylpyridine/poly(2-vinylpyridine-block-PEO). Relevance to drug release systems that can remain intact and circulate for long periods within the vascular system is suggested. 17 refs. 

The polymerisation then follows a schedule that raises the temperature of the reaction mixture stepwise towards nearly total conversion of the monomer into the polymer. Pentane, a mixture of normal and isopentane, is added as a blowing agent during the course of the polymerisation. 

BLOCK COPOLYMERS BY STEPWISE SYNTHESIS, NEW MONOMERS, AND POST POLYMERISATION TECHNIQUES... 

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