Gradient copolymers compositional drift

The limit of detection for this instrument is about 10 pg/ ml for polystyrene in 2-butanone,163 which is close to two orders of magnitude higher than that of the deflection-type DRI. Moreover, the response of the ELSD is linear over only two decades in concentration.163 The ELSD is a useful backup detector when the DRI or UV detectors are not appropriate, e.g., when the UV absorbance or RI change is a function of copolymer composition as well as concentration or in gradient elution systems where changes in solvent composition cause drift in baselines of the UV and DRI detectors. Compounds about as volatile as the solvent are poorly detected by ELSD. 

In ordinary batch copolymerization there is usually a considerable drift in monomer composition because of different reactivities of the two monomers (based on the values of the reactivity ratios). This leads to a copolymer with a broad chemical composition distribution (CCD). In many cases (depending on the specific final product application) a composition drift as low as 3-5% cannot be tolerated, for example, copolymers for optical applications on the other hand, during production of GRIN (gradient index) lenses, a controlled traj ectory of copolymer composition is required. This is partly circumvented in semibatch operation where the composition drift can be minimized (i.e., copolymer composition can be kept constant ) by feeding a mixture of the monomers to the reactor with the same rate by which each of them is consumed in the reactor. 

In most copolymeiization, the monomers are consumed at different rates dictated by the steric and electronic properties of the reactants. Consequendy, both the monomer feed and copolymer composition will drift with conversion. Thus conventional copolymers are generally not homogeneous in composition at the molecular level. In RAFT polymerization processes, where all chains grow throughout the polymerization, the compositional drift is captured within the chain stmcture (Scheme 32). All chains will have similar composition and the copolymers formed have a gradient or tapered structure - poly(monomer A-grad-monomer B). 

It is known that the gradient (tapered) nature of copolymers, which can be synthesized in free-radical polymerization processes, is due to a drift in the free monomer composition during solution polymerization [17]. Such copolymers can be considered as a special type of block copolymers in which the composition of one component varies along the chain. With a decreasing difference in the monomer reactivity rations, the formation of gradient sta-... 

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