Ideal copolymerisation

The value of the reachvity rahos is crihcal in determining the composition of the copolymer. If the reactivity raho is greater than 1, the radical prefers to react with chains having the same kind of terminal unit, e.g. A- with A. On the other hand, if the reactivity ratio is less than 1, the monomer prefers to react with chains which end in the other kind of monomer. In the special case that r r2 = 1, the reaction is described as ideal copolymerisation because it results in a truly random copolymer whose composition is the same as the composition of the reaction mixture from which polymerisation took place. 

Polyanhydrides, because of their surface erosion properties, can be ideal materials for a constant rate release profile (a zero order). Furthermore, these polyanhydrides are very hydrophobic. Their hydrolytic degradation may take a relatively long time, which is not suitable for pulsatile release. Hence, to achieve a tuneable erosion kinetics, a two-component polyanhydride made of SA precursor and CPP precursor when copolymerised with polyethylene glycol (PEG) was found to retain the surface erosion of two-component polyanhydride, while increasing the erosion rate due to increased hydrophilicity by PEG functionality. Relatively faster erosion rates can be achieved by adjusting the PEG precursor content (Torres et al., 2007). 

During a copolymerisation, it is important to be able to predict how copolymer composition varies as a function of comonomer reactivity and concentration at any time. The reactivity ratios are defined as tj = kjj/kj2 and t2 = k22/k2i where r and r2 are reactivity ratios of monomer 1 and 2, respectively. The copolymer is random or ideal when rj = r2 = 1, alternating when rj = t2 = 0. 

A major point of criticism for aliphatic-aromatic copolyesters is the final degradability of the aromatic sequences in the polymers. In such statistical copolyesters there are domains in the polymer chains, where several aromatic dicarboxylic acids are linked with the alcohol component, without being interrupted by an aliphatic dicarboxylic acid. The distribution of sequence lengths depends on the ratio of aliphatic and aromatic dicarboxylic acids, and can be calculated for an ideal random copolymerisation using (Equation 10.1) ... 

Water is the ideal solvent from the cost and pollution viewpoints, but it is a non-solvent for many surface coating polymers. It will ssolve a small number of homopolymers, notably those derived from acrylamide, acrylic acid, itaconic acid, vinyl methyl ether, vinyl pyrrolidone and vinyl sulphonic acid, but none of these homopolymers forms flexible films of use in the coatings industry. While copolymers of acrylic or methacrylic acids with acrylate esters are generally insoluble in water, their salts are soluble when the acid content is over 5% (for hydrophilic monomers) and 12% (for hydrophobic monomers). Such polymers can be prepared in solution, or in emulsion, but not in aqueous solution. This is because the acrylate esters are insoluble in water. The acid is copolymerised in the un-ionised form because the ion is unreactive to free radicals. In emulsion polymerisation, care has to be taken to avoid homopolymerisation of the acrylic or methacrylic acid in the water phase. Suppression of homopolymerisation requires a low concentration of acid throughout the polymerisation process. This can be achieved by using a long reaction period and slow addition of monomer mixture, or by careful pH buffer selection. 

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