Switchable Polymer Materials Controlled by Rotaxane Macromolecular Switches

Abstract

The synthesis and dynamic nature of macromolecular systems controlled by rotaxane macromolecular switches are introduced to discuss the significance of rotaxane linking of polymer chains and its topological switching. Macromolecular switches have been synthesized from macromolecular [2]rotaxanes (M2Rs) using sec-ammonium salt/crown ether couples. The successful synthesis of M2Rs possessing a single polymer axle and one crown ether wheel, constituting a key component of the macromolecular switch, has allowed us to develop various unique applications such as the development of topology-transformable polymers. Polymer topological transformations (e.g., linear-star and linear-cyclic) are achieved using rotaxane-linked polymers and rotaxane macromolecular switches. The pronounced dynamic nature of these polymer systems is sufficiently interesting to design sophisticated stimuli-responsive molecules, polymers, and materials.

Figure 1

Rotaxane molecular switch driven by two different stimuli.

Figure 2

Schematic illustration of a rotaxane macromolecular switch with two stations on the axle ends for the wheel of the macromolecular [2]rotaxane.

Figure 3

Stimuli-responsive dynamic macromolecular systems: (a) linear–star and (b) linear–cyclic topological transformations controlled by a rotaxane macromolecular switch.

Figure 4

Two-station-type rotaxane molecular switch and its application to the molecular elevator as reported by Stoddart et al.

Scheme 1. Synthesis of a Rotaxane via the Formation of the sec-Ammonium/Crown Ether Complex and Subsequent OH Acylative End Capping

Scheme 2. Rotaxane Molecular Switch Driven by the Nitrogen Protection/Deprotection Protocol

Scheme 3. Rotaxane Molecular Switch Using (a) tert-Ammonium/Benzyl Methylene and (b) tert-Ammonium/Urethane Interactions

Scheme 4. Rotaxane Molecular Switch Capable of Switching under Solvent-Free or Solid-State Conditions (without Accumulation of Any Byproducts by Repeated Switching)

Figure 5

Synthetic strategies for macromolecular [2]rotaxanes (M2Rs): (a) the rotaxane end-cap method and (b) the rotaxane-from method.

Figure 6

Effect of the component mobility of the macromolecular [2]rotaxane (M2R) on the crystallinity of the polymer chain depending on the length of the polymer chain. For the M2R, the axle polymer is amorphous when the wheel component is not fixed (i.e., when it is freely movable) on the axle component, and the degree of polymerization (DP) is approximately 20 (M2R_M); however, for M2R_F the wheel component is fixed at the end of the axle.

Scheme 5. Synthesis of a sec-Ammonium Salt/Crown Ether-Based Macromolecular [2]Rotaxane (M2R) by the Rotaxane-From Method

Scheme 6. Synthesis of a Macromolecular [2]Rotaxane with Movable Components (M2R_M) by Removal of the Attractive Interaction between the Two Components of M2R_F(61)

Scheme 7. Rotaxane Macromolecular Switch Consisting of a Polyrotaxane with Two Ammonium-Type Stations Driven by the Protection/Deprotection Protocol

Figure 7

Topology transition of RLBC from the linear form to the branched form to the linear form during the relative positional change of the wheel component from the α end to the ω end of the axle polymer.

Scheme 8. Syntheses of RLBCs via the (a) Grafting-Onto and (b) Grafting-From Pathways

Scheme 9. Synthesis of 2-PVL_M by N-Acetylation of 2-PVL_F(65)

Figure 8

GPC profiles of (a) 2-PVL_F, (b) 2-PVL_M, and (c) a model polymer.

Scheme 10. (top) Topological Transformation of the Star Polymer 3-PVL_A to the Linear Polymer 3-PVL-U upon N-Acetylation; (bottom) Decomposition of the Star Polymer 3-PVL_OH without the End-Cap Group for Each Polymer Chain End to the Axle and Wheel Components upon N-Acetylation

Figure 9

GPC profiles of (a) a product mixture without end capping, (b) 3-PVL_A, and (c) 3-PVL_U.

Figure 10

Huggins plots of (a) 3-PVL_A and the model star polymer and (b) 3-PVL_U and model linear polymer (solvent: CHCl₃).

Scheme 11. Synthesis of the Rotaxane-Linked ABC Star Triblock Copolymer PEO₄₅-b-PVL₃₃-rot-PS₄₉_A and Its Topology Conversion to Linear Topology (PEO₄₅-b-PVL₃₃-rot-PS₄₉_U)

Figure 11

GPC profiles of (a) the rotaxane-linked ABC star triblock copolymer PEO₄₅-b-PVL₃₃-rot-PS₄₉_A and (b) the linear triblock copolymer PEO₄₅-b-PVL₃₃-rot-PS₄₉_U.

Figure 12

Synthetic strategy for producing a cyclic polymer by the “RMS protocol” via the topological transformation of a macromolecular [1]rotaxane as a linear polymer.

Scheme 12. Synthesis of Cyclic Polymer M1R_C via the Topological Transformation of Linear Polymer M1R_L(56)

Figure 13

GPC profiles of the model linear polymer (M2R_U) and the cyclic polymer (M1R_C)

Scheme 13. Reversible Topological Transformation between Cyclic and Linear Polymers Using an Acid/Base-Responsive RMS

Scheme 14. Reversible Linear–Cyclic Polymer Topological Transformation Using an M1R with Two Amine/Ammonium Stations

Figure 14

Polymer-structure-dependent GPC profiles of similar-molecular-weight poly(tetrahydrofuran)s with different topologies (linear, tadpole or lariat, and cyclic), showing the hydrodynamic volume difference.

Scheme 15. Topological Transformation of the Linear Block Copolymer M1R_CL/HL_A to the Cyclic One M1R_CL/HL_U Using the RMS Protocol

Figure 15

Synthetic strategy for producing cyclic polymers using a rotaxane dimer formed in situ using the RMS protocol.

Scheme 16. Synthetic Pathway to Cyclic Polymers by the Rotaxane Protocol Using a Pseudo[2]rotaxane Dimer (D-OH_A) as a Bifunctional Initiator for the Living Ring-Opening Polymerization of CL