Gradient polymer architecture

A number of different types of copolymers are possible with ATRP—statistical (random), gradient, block, and graft copolymers [Matyjaszewski, 2001]. Other polymer architectures are also possible—hyperbranched, star, and brush polymers, and functionalized polymers. Statistical and gradient copolymers are discussed in Chap. 6. Functionalized polymers are discussed in Sec. 3-16b. 

Statistical, gradient, and block copolymers as well as other polymer architectures (graft, star, comb, hyperbranched) can be synthesized by NMP following the approaches described for ATRP (Secs. 3-15b-4, 3-15b-5) [Hawker et al., 2001]. Block copolymers can be synthesized via NMP using the one-pot sequential or isolated macromonomer methods. The order of addition of monomer is often important, such as styrene first for styrene-isoprene, acrylate first for acrylate-styrene and acrylate-isoprene [Benoit et al., 2000a,b Tang et al., 2003]. Different methods are available to produce block copolymers in which the two blocks are formed by different polymerization mechanisms ... 

FIGURE 16.11 Schematic representation of eluent gradient polymer HPLC. Two polymer species A and B are separated. They exhibit different nature and different interactivity with the column packing (e.g., adsorp-tivity) or with the mobile phase (solubility). The linear gradient from the retention promoting mobile phase to the elution promoting mobile phase is applied. The focused peaks—one for each polymer composition/ architecture—are formed in the appropriately chosen systems. Each peak contains species with different molar masses. 

An additional reason for peak broadening under the critical conditions may be the temperature gradient inside of column generated by viscous heat dissipation [174] and, to a much less extent pressure gradient [ 175]. As is known, critical conditions are quite sensitive to temperature [7,38,39,79,125] and, in some systems, to pressure [7,17,92]. Furthermore, another reason for broadening could be differences in polymer architecture, because isomers may manifest itself under critical conditions (Table 1). In any case, the question as to why polymer peaks are not significantly narrower under critical conditions, remains unanswered and requires... 

ATRP was recently reported as a new and powerful route to the synthesis of well-defined (co)polymers of such monomers as styrene, acrylates, methyl methacrylate, acrylonitrile, and isobutene. ATRP is a versatile tool for preparation of random, block, alternating, and gradient copolymers with controlled molecular weight, narrow polydispersities, and desired architectures. 

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