Zhenbin Niu

Supramolecular AA−BB-Type Linear Polymers with Relatively High Molecular Weights via the Self-Assembly of Bis(m-phenylene)-32-Crown-10 Cryptands and a Bisparaquat Derivative

Two novel bis(m-phenylene)-32-crown-10-based cryptands, one bearing covalent linkages and the other metal-complex linkages, were designed and prepared. By self-assembly of these biscryptands, which can be viewed as AA monomers, and a bisparaquat, which can be viewed as a BB monomer, AA-BB-type linear supramolecular polymers with relatively high molecular weights were successfully prepared.

Stress-Responsive Polymers Containing Cyclobutane Core Mechanophores: Reactivity and Mechanistic Insights

A primary goal of covalent mechanochemistry is to develop polymer bound mechanophores that undergo constructive transformations in response to otherwise destructive forces. The [2 + 2] cycloreversion of cyclobutane mechanophores has emerged as a versatile framework to develop a wide range of stress-activated functionality. Herein, we report the development of a class of cyclobutane bearing bicyclo[4.2.0]octane mechanophores. Using carbodiimide polyesterification, these stress-responsive units were incorporated into high molecular weight polymers containing up to 700 mechanophores per polymer chain. Under exposure to the otherwise destructive elongational forces of pulsed ultrasound, these mechanophores unravel by ∼7 Å per monomer unit to form α,β-unsaturated esters that react constructively via thiol-ene conjugate addition to form sulfide functionalized copolymers and cross-linked polymer networks. To probe the dynamics of the mechanochemical ring opening, a series of bicyclo[4.2.0]octane derivatives that varied in stereochemistry, substitution, and symmetry were synthesized and activated. Reactivity and product stereochemistry was analyzed by (1)H NMR, which allowed us to interrogate the mechanism of the mechanochemical [2 + 2] cycloreversion. These results support that the ring opening is not concerted but proceeds via a 1,4 diradical intermediate. The bicyclo[4.2.0]octanes hold promise as active functional groups in new classes of stress-responsive polymeric materials.

Pseudocryptand-Type [2]Pseudorotaxanes Based on Bis(meta-phenylene)-32-Crown-10 Derivatives and Paraquats with Remarkably Improved Association Constants

The first dual component pseudocryptand-type [2]pseudorotaxanes were designed and prepared via the self-assembly of synthetically easily accessible bis(meta-phenylene)-32-crown-10 pyridyl, quinolyl, and naphthyridyl derivatives with paraquat. The formation of the pseudocryptand structures in the complexes remarkably improved the association constant by forming the third pseudobridge via H-bonding with the guest and π-stacking of the heterocyclic units.

Pseudocryptand-Type [3]Pseudorotaxane and “Hook-Ring” Polypseudo[2]catenane Based on a Bis(m-phenylene)-32-crown-10 Derivative and Bisparaquat Derivatives

The first pseudocryptand-type supramolecular [3]pseudorotaxane was designed and prepared via the self-assembly of a bispicolinate BMP32C10 derivative and a bisparaquat. The complexation behavior was cooperative. In addition, the complex comprised of the BMP32C10 derivative and a cyclic bisparaquat demonstrated strong binding; interestingly, a poly[2]pseudocatenane structure was formed in the solid state for the first time.

The Mechanical Strength of a Mechanical Bond: Sonochemical Polymer Mechanochemistry of Poly(catenane) Copolymers

Topological molecular connections and structures, including physical entanglements in polymer networks, knots along polymer chains, and rotaxanes in sliding ring gels, have important consequences for the physical properties of polymeric materials. Often these topologies contribute through their ability to bear mechanical stress, but experimental measures of their relative mechanical strength are rare. Here, we use sonochemical polymer mechanochemistry to assess the relative mechanical strength of a multicatenane copolymer relative to copolymers of cyclic and linear analogs. The relative mechanical strengths are obtained by comparing the limiting molecular weights (Mlim ) and contour lengths (Llim ) of the polymers under pulsed ultrasound of their dilute solutions. The values of Mlim and Llim , and thus the inferred mechanical strengths of the polymers, are effectively identical. The mechanical bonds of the catenanes are therefore as strong, or stronger, mechanically as the covalent bonds along the polymer backbone.