Novel Liquid Crystalline Block Copolymers by ATRP and ROMP

Linear Block Copolymer Synthesis

Block copolymers form the basis of the most ubiquitous materials such as thermoplastic elastomers, bridge interphases in polymer blends, and are fundamental for the development of high-performance materials. The driving force to further advance these materials is the accessibility of block copolymers, which have a wide variety in composition, functional group content, and precision of their structure. To advance and broaden the application of block copolymers will depend on the nature of combined segmented blocks, guided through the combination of polymerization techniques to reach a high versatility in block copolymer architecture and function. This review provides the most comprehensive overview of techniques to prepare linear block copolymers and is intended to serve as a guideline on how polymerization techniques can work together to result in desired block combinations. As the review will give an account of the relevant procedures and access areas, the sections will include orthogonal approaches or sequentially combined polymerization techniques, which increases the synthetic options for these materials.

Intramolecular cyclization of long-chain hyperbranched polymers (HyperMacs) from A2 + Bn step-wise polymerization

We presented a precise topological analysis on intramolecular cyclization for long-chain hyperbranched polymers via the new parameter of the macro-cyclic index (m-CI).

Effects of rigid cores and flexible tails on the phase behaviors of polynorbornene-based mesogen-jacketed liquid crystalline polymers

MJLCPs with a polynorbornene main chain and different side groups have been precisely synthesized for investigating the effect of side-chain structures on the liquid crystalline phase behaviors.

Liquid-crystalline ordering as a concept in materials science: from semiconductors to stimuli-responsive devices

While the unique optical properties of liquid crystals (LCs) are already well exploited for flat-panel displays, their intrinsic ability to self-organize into ordered mesophases, which are intermediate states between crystal and liquid, gives rise to a broad variety of additional applications. The high degree of molecular order, the possibility for large scale orientation, and the structural motif of the aromatic subunits recommend liquid-crystalline materials as organic semiconductors, which are solvent-processable and can easily be deposited on a substrate. The anisotropy of liquid crystals can further cause a stimuli-responsive macroscopic shape change of cross-linked polymer networks, which act as reversibly contracting artificial muscles. After illustrating the concept of liquid-crystalline order in this Review, emphasis will be placed on synthetic strategies for novel classes of LC materials, and the design and fabrication of active devices.

Liquid crystalline elastomers as actuators and sensors

This review collects recent developments in the field of liquid crystalline elastomers (LCEs) with an emphasis on their use for actuator and sensor applications. Several synthetic pathways leading to crosslinked liquid crystalline polymers are discussed and how these materials can be oriented into liquid crystalline monodomains are described. By comparing the actuating properties of different systems, general structure-property relationships for LCEs are obtained. In the final section, how these materials can be turned into usable devices using different interdisciplinary techniques are described.

Monotelechelic Poly(oxa)norbornenes by Ring-Opening Metathesis Polymerization using Direct End-Capping and Cross Metathesis

Two different methodologies for the synthesis of monotelechelic poly(oxa)norbornenes prepared by living ring-opening metathesis polymerization (ROMP) are presented. The first method, termed direct end-capping, is carried out by adding an internal cis-olefin terminating agent (TA) to the reaction mixture immediately after the completion of the living ROMP reaction. The second method relies on cross metathesis (CM) between a methylene-terminated poly(oxa)norbornene and a cis-olefin TA mediated by the ruthenium olefin metathesis catalyst (H(2)IMes)(Cl)(2)Ru(CH-o-OiPrC(6)H(4)) (H(2)IMes = 1,3-dimesitylimidazolidine-2-ylidene). TAs containing various functional groups, including alcohols, acetates, bromides, a-bromoesters, thioacetates, N-hydroxysuccinimidyl esters and Boc-amines, as well as fluorescein and biotin groups, were synthesized and tested. The direct end-capping method typically resulted in >90% end-functionalization efficiency, while the CM method was nearly as effective for TAs without polar functional groups or significant steric bulk. End-functionalization efficiency values were determined by (1)H NMR spectroscopy.

Artificial muscles based on liquid crystal elastomers

This paper presents our results on liquid crystal (LC) elastomers as artificial muscle, based on the ideas proposed by de Gennes. In the theoretical model, the material consists of a repeated series of main-chain nematic LC polymer blocks, N, and conventional rubber blocks, R, based on the lamellar phase of a triblock copolymer RNR. The motor for the contraction is the reversible macromolecular shape change of the chain, from stretched to spherical, that occurs at the nematic-to-isotropic phase transition in the main-chain nematic LC polymers. We first developed a new kind of muscle-like material based on a network of side-on nematic LC homopolymers. Side-on LC polymers were used instead of main-chain LC polymers for synthetic reasons. The first example of these materials was thermo-responsive, with a typical contraction of around 35-45% and a generated force of around 210 kPa. Subsequently, a photo-responsive material was developed, with a fast photochemically induced contraction of around 20%, triggered by UV light. We then succeeded in preparing a thermo-responsive artificial muscle, RNR, with lamellar structure, using a side-on nematic LC polymer as N block.Micrometre-sized artificial muscles were also prepared. This paper illustrates the bottom-up design of stimuli-responsive materials, in which the overall material response reflects the individual macromolecular response, using LC polymer as building block.

Formation of polymer vesicles by liquid crystal amphiphilic block copolymers

We report the formation of polymer vesicles (or polymersomes) by a new class of amphiphilic block copolymers in which the hydrophobic block is a side-on nematic liquid crystal polymer. Two series of these block copolymers, named PEG-b-PA444 and PEG-b-PMAazo444, with different hydrophilic/hydrophobic ratios were synthesized and characterized in detail. Polymersomes and nanotubes were formed by adding water into a solution of copolymers in dioxane. Polymersomes in water were finally obtained by dialyzing the resulting mixture against water. These self-assemblies have been studied by classical TEM and cryo-TEM. For the PEG-b-PA444 series, polymersomes were observed for hydrophilic/hydrophobic ratios ranging from 40/60 to 19/81. For PEG-b-PMAazo444 series, polymersomes were observed for hydrophilic/hydrophobic ratios ranging from 26/74 to 18/82. For a PEG-b-PA444 sample with hydrophilic/hydrophobic ratio equal to 25/75, a tubular morphology with tube diameter of typically 100 nm and tube length of up to 10 mum was also observed together with polymersomes during addition of water into the polymer solution in dioxane.

Synthesis and Hydrolysis Behavior of Side-Chain Functionalized Norbornenes

[reaction: see text] The stabilities of various functionalized norbornenes that are monomers for the ring-opening metathesis polymerization (ROMP) in aqueous solution were evaluated toward hydrolysis under a range of temperatures (37, 60, and 80 degrees C) and pH values (3-9). All monomers contain hydrolyzable linkages to pendant functional groups, and conclusions were drawn relating to how the chemical diversity of these pendant functional groups, in accordance with the pH and temperature variations, affect hydrolysis of the aforementioned linkages. The hydrolysis was monitored by reverse phase HPLC analysis, and/or NMR spectroscopy. As expected, monomers containing ester linkages were fairly labile at higher pH values, while acetal-based linkers were cleaved at lower pH values. Beta-amino ester groups experienced a significant increase in hydrolysis rate, while carboxylic acid-containing monomers did not follow any clear trend. Saccharide-containing monomers exhibited unique behaviors for various pH values and temperature ranges.

Polymer vesicles formed by amphiphilic diblock copolymers containing a thermotropic liquid crystalline polymer block

New amphiphilic diblock copolymers composed of poly(ethylene glycol) and a thermotropic liquid crystalline polymer have been synthesized and demonstrated to form well-defined unilamellar vesicles in water by cryo-electron microscopy.