Sequence-Controlled Copolymers Prepared via Entropy-Driven Ring-Opening Metathesis Polymerization

A new general synthetic approach to sequenced macromolecules was developed and applied to the synthesis of polymers comprising lactic acid (<b>L</b>), glycolic acid (<b>G</b>), and ε-caprolactone (<b>C</b>)-derived monomer units. The new method employs entropy-driven ring-opening metathesis polymerization (ED-ROMP) to prepare copolymers with embedded sequences and controlled molecular weights. Cyclic macromonomer precursors were prepared by ring-closing metathesis of ethylene glycol (<b>Eg</b>)-linked sequenced oligomers bearing terminal olefins. ED-ROMP of the resulting macrocycles using Grubbs’ second generation catalyst yielded <b>poly­(CL-Eg-LC-Oed)</b>, <b>poly­(CLL-Eg-LLC-Oed)</b>, <b>poly­(LGL-Eg-LGL-Oed)</b>, and <b>poly­(LGL-Eg-LGL-Hed)</b> (<b>Oed</b> = octenedioc acid; <b>Hed</b> = hexenedioc acid). Hydrogenation produced the saturated sequenced copolymers. Molecular weight was well-controlled and could be adjusted by varying the monomer-to-catalyst ratio. <i>M</i>_ns of 26–60 kDa were obtained (dispersities = 1.1–1.3). The methodology proved general for three different sequences and two olefinic metathesis groups

Salt Effects on the Phase Behavior and Cocrystallization Kinetics of POCB–Water Mixtures

Mixtures of water with polyoxacyclobutane (POCB) have a unique phase diagram which combines liquid–liquid equilibrium (LLE) at high temperatures and cocrystallization of a POCB-hydrate at low temperatures. Such cocrystal hydrate formation is extremely rare among polymers. We report on the effects of adding NaCl salt on the phase behavior of POCB–water mixtures and the kinetics of hydrate crystallization from such mixtures. Salt loadings of less than 0.1 wt % were found to greatly expand the LLE region. Salt loadings of ∼10 wt % were found to significantly decrease the melting temperature of the hydrate below its ∼37 °C value under salt-free conditions. The hydrate was found to be remarkably tolerant of salt and persists at room temperature even when equilibrated with salt-saturated water. Salt was found to slow down hydrate crystallization, and the degree of slowing was greater than that expected from the salt-induced decrease in undercooling due to melting point depression

Quantitative Assessment of the Connection between Steric Hindrance and Electronic Coupling in 2,5-Bis(alkoxy)benzene-Based Mixed-Valence Dimers

The effect of the bridging ligand on electronic delocalization was examined in a series <i>p</i>-bis­(alkoxy)­benzene dimers relevant to conducting polymers used for organic devices. Using spectroscopic methods, the degree of delocalization for an ethylene-bridged <i>p</i>-bis­(alkoxy)­benzene dimer was determined and compared to the electronic coupling for directly coupled and phenylene-bridged <i>p</i>-bis­(alkoxy)­benzene dimers reported previously. Despite a significant increase in distance (53%) between the redox-active sites, the ethylene-bridged compound exhibited a higher electronic coupling than either of the others previously reported. The increased coupling can be attributed to the lower rotational barrier to planarization for the ethylene-bridged dimer. This result highlights the need to minimize both sterics and distance between redox active sites in molecular systems designed for promoting electron mobility and provides quantitative evidence that an optimal balance between these parameters can be achieved

The Effect of Monomer Order on the Hydrolysis of Biodegradable Poly(lactic-<i>co</i>-glycolic acid) Repeating Sequence Copolymers

The effect of sequence on copolymer properties is rarely studied despite the precedent from Nature that monomer order can create materials of significant diversity. Poly­(lactic-<i>co</i>-glycolic acid) (PLGA), one of the most important biodegradable copolymers, is widely used in an unsequenced, random form for both drug delivery microparticles and tissue engineering matrices. Sequenced PLGA copolymers have been synthesized and fabricated into microparticles to study how their hydrolysis rates compare to those of random copolymers. Sequenced PLGA microparticles were found to degrade at slower, and often more constant, rates than random copolymers with the same lactic to glycolic acid ratios as demonstrated by molecular weight decrease, lactic acid release, and thermal property analyses. The impact of copolymer sequence on <i>in vitro</i> release was studied using PLGA microparticles loaded with model agent rhodamine-B. These assays established that copolymer sequence affects the rate of release and that a more gradual burst release can be achieved using sequenced copolymers compared to a random control

Sequence Effects in Donor–Acceptor Oligomeric Semiconductors Comprising Benzothiadiazole and Phenylenevinylene Monomers

To understand the influence of monomer sequence on the properties and performance of conjugated oligomers, a series of dimers, trimers, and tetramers were prepared from phenylene (<b>P</b>) and benzothiadiazole (<b>B</b>) monomers linked by vinylene groups. Optical and electrochemical studies established the influence of sequence on both the λ_max and redox potentials of this series of structurally related oligomers. For tetramers with bromo end groups (<b>PBBP</b>, <b>BPPB</b>, <b>PBPB</b>, <b>PPBB</b>), the λ_max ranged from 493 to 512 nm (Δ = 19 nm), the electrochemical oxidation potential from 0.65 to 0.82 (Δ = 0.17 V) and the reduction potential from −1.45 to −1.31 (Δ = 0.14 V), all of which are sequence-dependent. The effect of end groups (cyano, bromo, and alkyl) was also demonstrated to be important for the properties of these oligomers. DFT calculations of the tetramers were performed and the energy levels were correlated well with the experimentally determined spectroscopic data. Bulk heterojunction (BHJ) solar cells fabricated with selected tetramers as the donor and PC₆₁BM as the acceptor exhibited power conversion efficiencies that varied by a factor of 3 as a function of sequence (0.47–1.85%). These results suggest that sequence control is important for tuning optoelectronic properties and photovoltaic performance of these structurally related conjugated oligomers

<i>Cis</i>-Selective Metathesis to Enhance the Living Character of Ring-Opening Polymerization: An Approach to Sequenced Copolymers

The hydrolytic behavior and physical properties of a polymer are directly related to its constituent monomer sequence, yet the scalable and controllable synthesis of sequenced copolymers remains scarcely realized. To address this need, an enhanced version of entropy-driven ring-opening metathesis polymerization (ED-ROMP) has been developed. An unprecedented level of control is obtained by exploiting the kinetic and thermodynamic differences in the metathesis activity of <i>cis</i>- and <i>trans</i>-olefins embedded in large, unstrained macrocycles. First-order rate kinetics were observed, and polymer molecular weights were found to be proportional to catalyst loading. Computational analysis suggests that incorporation of a <i>cis-</i>olefin into the monomer backbone both introduces a thermodynamic driving force and increases the population of metathesis-active conformers. This approach offers a generally applicable method for enhancing living character in ED-ROMP

Stimuli-Responsive Iron-Cross-Linked Hydrogels That Undergo Redox-Driven Switching between Hard and Soft States

A unique class of stimuli-responsive hydrogels, termed electroplastic elastomers (EPEs), whose mechanical properties can be reversibly tuned between hard and soft states with the application of an electric potential, is described. Electrochemically reversible cross-links formed within a permanent, covalently cross-linked polymeric hydrogel network are switched between strongly binding Fe³⁺ and weak to nonbinding Fe²⁺, as determined by potentiometric titration. With the incorporation of graphene oxide (GO) into the EPE, a significant enhancement in modulus and toughness was observed, allowing for the preparation of thinner EPE samples, 80–100 μm in thickness, which could be reversibly cycled between soft (Young’s modulus: ∼0.38 MPa) and hard (∼2.3 MPa) states over 30 min. Further characterization of EPE samples by magnetic susceptibility measurements suggests the formation of multinuclear iron clusters within the gel

Sequence Matters: Modulating Electronic and Optical Properties of Conjugated Oligomers via Tailored Sequence

Although sequence must necessarily affect the photophysical properties of oligomers and copolymers prepared from donor and acceptor monomers, little is known about this effect, as nearly all the donor/acceptor materials have an alternating structure. A series of sequenced <i>p</i>-phenylene–vinylene (PV) oligomers was synthesized and investigated both experimentally and computationally. Using Horner–Wadsworth–Emmons (HWE) chemistry, a series of dimers, trimers, tetramers, pentamers, and hexamers were prepared from two building block monomers, a relatively electron-poor unsubstituted <i>p-</i>phenylene–vinylene (A) and an electron-rich dialkoxy-substituted <i>p-</i>phenylene–vinylene (B). UV–vis absorption/emission spectra and cyclic voltammetry demonstrated that the optoelectronic properties of these oligomers depended significantly on sequence. Calculations predicting the HOMO–LUMO gap of the sequenced oligomers correlated well with the experimental properties for the 2- to 4-mers, and the consensus model developed was used to design hexameric sequences with targeted characteristics. Despite the weak acceptor qualities of the “A” monomer employed in the study, HOMO–LUMO gap differences of ∼0.25 eV were found for isomeric, sequenced oligomers. In no case did the alternating structure give the largest or smallest gap. The use of sequence as a strategy represents a new dimension in tailoring properties of π-conjugated polymers

Manipulating Mechanical Properties with Electricity: Electroplastic Elastomer Hydrogels

The dawn of the 21st century has brought with it an increasing interest in emulating the adaptive finesse of natural systems by designing materials with on-demand, tunable properties. The creation of such responsive systems could be expected, based on historical precedent, to lead to completely new engineering design paradigms. Using a bioinspired approach of coupling multiple equilibria that operate on different length scales, a material whose bulk mechanical properties can be manipulated by electrical input has been developed. The new macroscale electroplastic elastomer hydrogels can be reversibly cycled through soft and hard states while maintaining a three-dimensional shape by sequential application of oxidative and reductive potentials. This input changes the cross-linking capacity of iron ions within the gel matrix, between a poorly coordinating +2 and a more strongly binding +3 oxidation state. Inclusion of carbon nanotubes in the hydrogel preparation increases conductivity and decreases transition time

Chemical and Electrochemical Manipulation of Mechanical Properties in Stimuli-Responsive Copper-Cross-Linked Hydrogels

Inspiration for the design of new synthetic polymers can be found in the natural world, where materials often exhibit complex properties that change depending on external stimuli. A new synthetic electroplastic elastomer hydrogel (EPEH) that undergoes changes in mechanical properties in response to both chemical and electrochemical stimuli has been prepared based on these precedents. In addition to having the capability to switch between hard and soft states, the presence of both permanent covalent and dynamic copper-based cross links also allows this stimuli-responsive material to exhibit a striking shape memory capability. The density of temporary cross links and the mechanical properties are controlled by reversible switching between the +1 and +2 oxidation states