Monomer addition strategies

The knowledge of monomer conversion not only allows delicate adjustment of the process conditions but also intelligoit monomer addition strategies in copolymerizations strongly rely on the monitoring of monomer conversions. 

NHS)-ester of lysine dihydrochloride in the presence of a polyfiinctional core, using the slow monomer addition strategy. A self-evident method for preparing hh poly(lysine) by direct self-condensation was reported more recendy by Klok and co-workers. ... 

Raman spectroscopy was also used for the control of monomer compositions in solution polymerizations. Van den Brink et al. [145] studied the solution polymerization of n-butyl acrylate in dioxane and monitored the monomer concentration by online Raman spectroscopy. It was shown that it is possible to control the monomer to solvent ratio with the help of a feedback monomer addition strategy, while the spectfoscopic data are analyzed in real time. The proposed conttol stfategy was validated by disturbing a semicontinuous polymerization process on purpose and verifying that the process could be operated without abrupt changes in the monomer to solvent ratio and the monomer flow rate. 

Semi-continuous emulsion copolymerisation processes can be performed by applying various monomer addition strategies. 

Optimal addition profile. Arzamendi etal. (1989) developed a so-called optimal monomer addition strategy. By using this method they demonstrated that within a relatively short period of time homogeneous vinyl acetate (VAc)-methyl acrylate (MA) emulsion copolymers can be prepared in spite of the large difference between the reactivity ratios. The reactor was initially charged with all of the less reactive monomer (viz., VAc) plus the amoimt of the more reactive monomer (viz., MA) needed to initially form a copolymer of the desired composition. Subsequently, the more reactive monomer (MA) was added at a computed (time variable) flow rate (optimal addition profile) in such a way as to ensure the formation of a homogeneous copolymer. 

The copolymer composition must be known to monitor and control copolymerization reactions. Because various monomer-addition strategies allow various types of copolymers to be formed (random, gradient, block copolymers), we also must be able to characterize such products. 

Di- and trinucleotides may be used as units instead of the monomers. This convergent synthetic strategy simplifies the purification of products, since they are differentiated by a much higher jump in molecular mass and functionality from the educls than in monomer additions, and it raises the yield. We can illustrate the latter effect with an imaginary sequence of seven synthetic steps, c.g. nucleotide condensations, where the yield is 80% in each step. In a converging seven-step synthesis an octanucleotide would be obtained in 0.8 x 100 = 51% yield, compared with a 0.8 x 100 = 21% yield in a linear synthesis. 

Due to the pronounced tolerance of the Suzuki reaction towards additional functional groups in the monomers, precursor strategies as well as so called direct routes can be applied for polyelectrolyte synthesis. However, the latter possibility, where the ionic functionalities are already present in the monomers, was rejected. The reason is too difficult determination of molecular information by means of ionic polymers. Therefore the decision was to apply precursor strategies (Scheme 1). Here, the Pd-catalyzed polycondensation process of monomers A leads to a non-ionic PPP precursor B which can be readily characterized. Then, using sufficiently efficient and selective macro-molecular substitution reactions, precursor B can be transformed into well-defined PPP polyelectrolytes D, if appropriate via an activated intermediate C. 

Another general approach is to use so-called optimum monomer addition rate profiles, which involves addition of monomers at rates wluch vary with time in a manner calculated to maintain the comonomer composition constant. This approach requires a quantitative model for the particular emulsion copolymerization which is to be controlled and hence requires thorough knowledge of partitioning, reactivity ratios, rate coefficients, etc. Two strategies have been employed [28, 75-80] ... 

Figure 1 Schematic representation of synthetic strategies toward AB dibiock copolymers (a) by sequential monomer addition, (b) by dual initiator, (c) by site transformation technique, and (d) by coupiing of m-functionai poiymers. i, initiator F, functionalization agent , active site.

In addition, two synthesis strategies were studied (Figure 6.11). In route 1, the clay was first reacted with an MMAO heptane solution and then, after saturation with the monomer, the polymerization initiated by the addition of Fe precatalyst. Instead, in route 2, the precatalyst was activated by using the clay-immobilized cocatalyst and the polymerization initiated by monomer addition. The catalytic activities with the different montmorillonites and precatalysts used, following polymerization routes 1 and 2, are summarized in Figure 6.12. 

CEM a route of syndiotactic polymerization that is based on the chain-end control mechanism, where the last stereocenter of the growing polymer dominates the stereochemical outcome during monomer addition process SCM a recent strategy related to application of Cs-symmetric catalysts, where regularly alternating monomer insertion on enantiotopic coordination sites forms syndiotactic polymers depending on the site control mechanism. 

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