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Zwitterionic copolymerization: synthesis of cyclic gradient copolymers.

studies indicate that linear gradient copolymers exhibit distinct physical properties from random and block copolymers as a consequence of their sequence distribution. Because cyclic polymers have no ends, we were intrigued by the challenge of devising an expedient synthesis of cyclic gradient copolymers as these materials would contain both a sharp comonomer interface and a gradient interface in the same molecule. The topological differences between cyclic and linear macromolecules influence their behavior in ways that remain poorly understood. Cyclic polymers are more compact and entangle differently as a consequence of the chemical bond that connects the chain ends. 4] Cyclic block copolymers exhibit smaller microdomains and, in some cases, different morphologies than linear diblocks as a consequence of topological constraints on phase separation. These studies highlight the need for synthetic strategies to prepare cyclic polymers with defined sequences to enable studies on the influence of topology and sequence distribution on the selforganization and properties of macromolecules. We have previously described a synthetic strategy for generating cyclic polyesters by the zwitterionic ring-opening of lactones with nucleophilic N-heterocyclic carbenes. Kinetic studies revealed that the active zwitterions exhibit lifetimes commensurate with that of the growing chains, but attempts to generate block-copolymers were frustrated by reinitiation from unactivated carbenes upon addition of a second batch of monomer. We report here that a one-step gradient batch copolymerization provides a facile method for generating cyclic gradient block copolymers. The synthesis of gradient copolymers in a batch copolymerization requires that all chains grow under similar conditions and the composition of the monomers in the polymerization medium must change continually as the chains grow. The first criterion is generally met by living polymerization methods, but it was our hypothesis that the kinetic lifetime of the growing zwitterions might be sufficient to satisfy this first criterion for a gradient copolymerization. The second criterion requires that the reactivity of the two monomers be sufficiently different such that one monomer is consumed more rapidly than the other. The reactivity of different lactone monomers in ring-opening polymerization of lactones with metal alkoxides is typically not very different, limiting the gradients that could be generated. As previous studies had suggested that the copolymerization selectivities for organic catalysts are different than those of metal alkoxides, the reactivity ratios for the copolymerization of e-caprolactone (CL) and d-valerolactone (VL) were determined using benzyl alcohol and 1,3,4,5-tetramethylimidazol-2-ylidene (IMe4) carbene in THF to generate linear copolymers. The reactivity ratios for VL and for CL were determined to be: rVL = 9.0 and rCL = 0.24, by the Fineman– Ross method (see Supporting Information). The large difference in reactivity ratios between VL and CL suggested that this system would be competent for a gradient copolymerization. In addition, these data illustrate the significant influence of the organocatalysts on the comonomer reactivities, as the copolymerization of CL and VL with the carbene IMe4 at 25 8C exhibits a very different copolymerization behavior than Sn(Oct)2 catalysts at 160 8C, for which rVL = 0.49 and rCL = 0.25. [9] As the IMe4 catalyst was fast and difficult to control, we investigated the less active carbene 1,3-diethyl-4,5-dimethylimidiazol-2-ylidene (Me2IEt) for the generation of cyclic VL/ CL copolymers in toluene solution (Figure 2). Copolymerizations of e-caprolactone (CL) and d-valerolactone (VL) ([Mtot]0 = 1.0m, Mtot/I = 100) (Mtot = total monomer; I = iniFigure 1. Schematic diagrams of linear and cyclic gradient copolymers.