Telechelic Research Articles

Self-accelerating click reaction for preparing cyclic polymers from unconjugated vinyl monomers

An efficient metal-free bimolecular homodifunctional ring-closure method was developed specifically for preparing well-defined cyclic polymers from unconjugated vinyl monomers. In this approach, well-defined homodifunctional linear polymers with azide terminals were prepared by using the reversible addition chain transfer polymerization/macromolecular design by interchange of xanthates (RAFT/MADIX) to polymerize unconjugated vinyl monomers in the presence of a diazide xanthate chain transfer agent. The self-accelerating double strain-promoted azide-alkyne cycloaddition (DSPAAC) reaction was then applied to ring-close the linear polymer precursors with sym-dibenzo-1,5-cyclooctadiene-3,7-diyne (DBA) as small linkers, leading to the formation of corresponding cyclic polymers. By virtue of the self-accelerating property of DSPAAC ring-closing reaction, this novel method smartly eliminated the requirement of equimolar amounts of the telechelic polymers and small linkers in the traditional bimolecular ring-closure methods for pure cyclic polymers. More importantly, the usage of excess DBA small linkers could significantly enhance the preparation efficiency of cyclic polymers. Moreover, the cyclic polymers resulted from this novel method could be conveniently cleaved back to linear polymers by a mild aminolysis of the S-C(S) bond within the cyclic polymer backbone.

Telechelic polymers from reversible-deactivation radical polymerization for biomedical applications.

The main strategies for the design of telechelic polymers synthesized by reversible-activation radical polymerization for biomedical applications are covered spanning from drug delivery and targeting to theranostics and sensing.

A novel hybrid population balance—Brownian dynamics method for simulating the dynamics of polymer-bridged colloidal latex particle suspensions

We developed a novel hybrid population balance-Brownian dynamics (Pop-BD) simulation technique to investigate the phase behavior and linear viscoelastic response of suspensions of colloids bridged reversibly by telechelic polymers such as latex particles bridged by polyethylene oxide urethanes. The Pop-BD method couples the solution of a set of population balance equations of the bridge-to-loop exchange kinetics of the telechelic polymer chains on the latex particles with Brownian dynamics simulations governing the dynamics of the latex particles. Two relaxation times are identified, corresponding to bridge association/dissociation from the latex particle and relaxation of the transient network of bridged particles. The relaxation modulus computed from the Pop-BD equations matches that for fully explicit BD simulations of both colloids and polymers at time scales equal to or greater than the polymer bridge relaxation time.

Periodic “stick-slip” transition within a continuum model for entangled supramolecular polymers

We investigate shear flow instabilities using a recently derived constitutive law, designed for describing the rheology of entangled telechelic polymers. The continuum model displays shear banding, often accompanied by a strong elastic recoil in the transient response, which appears as an “apparent” wall slip. For certain parameters, the model also displays a novel apparent stick-slip behavior, giving rise to strong nonlinear oscillations in the stress response driven by a repeating cycle of elastic recoil followed by diffusive rehomogenization of the flow. We elucidate the detailed mechanism behind this oscillatory response, demonstrating that it arises from a competition between shear-induced breaking of sticky bonds, which leads to recoil and banding, and a diffusive rehomogenization of the flow. We discuss the relation between the apparent stick-slip in our continuum model and similar behavior in “true” wall slip.

Synthesis of Star Polymers using Organocatalyzed Atom Transfer Radical Polymerization Through a Core-first Approach.

Synthetic routes to higher ordered polymeric architectures are important tools for advanced materials design and realization. In this study, organocatalyzed atom transfer radical polymerization is employed for the synthesis of star polymers through a core-first approach using a visible-light absorbing photocatalyst, 3,7-di(4-biphenyl)-1-naphthalene-10-phenoxazine. Structurally similar multifunctional initiators possessing 2, 3, 4, 6, or 8 initiating sites were used in this study for the synthesis of linear telechelic polymers and star polymers typically possessing dispersities lower than 1.5 while achieving high initiator efficiencies. Furthermore, no evidence of undesirable star-star coupling reactions was observed, even at high monomer conversions and high degrees of polymerization. The utility of this system is further exemplified through the synthesis of well-defined diblock star polymers.

Direct Polymerization of Carbon Dioxide, Diynes, and Alkyl Dihalides under Mild Reaction Conditions

Fixing carbon dioxide (CO2) into useful polymeric materials has attracted broad interest since carbon dioxide is an abundant, inexpensive, nontoxic, and renewable C1 resource. Nevertheless, the polymerization of CO2 and alkynes attracted less attention because the propagation step involving CO2 is a major obstacle. Herein, we overcome this obstacle by developing a facile and efficient Ag2WO4-catalyzed polymerization of CO2, diynes, and alkyl dihalides under mild reaction conditions. Soluble and thermally stable poly(alkynoate)s with high weight-average molecular weights (up to 31 400) were obtained in high yields (up to 95%). Thanks to its unique reaction mechanism, this step-growth polymerization can produce an ethynyl group terminated telechelic polymer that can be used as macromonomer to prepare poly(alkynoate)s with higher molecular weights by either continually adding alkyl dihalide into the reaction solution or mixing the isolated telechelic polymers with alkyl dihalide and catalytic system under a ...

Self-Assembly of Telechelic Tyrosine End-Capped PEO Star Polymers in Aqueous Solution.

We investigate the self-assembly of two telechelic star polymer-peptide conjugates based on poly(ethylene oxide) (PEO) four-arm star polymers capped with oligotyrosine. The conjugates were prepared via N-carboxy anhydride-mediated ring-opening polymerization from PEO star polymer macroinitiators. Self-assembly occurs above a critical aggregation concentration determined via fluorescence probe assays. Peptide conformation was examined using circular dichroism spectroscopy. The structure of self-assembled aggregates was probed using small-angle X-ray scattering and cryogenic transmission electron microscopy. In contrast to previous studies on linear telechelic PEO-oligotyrosine conjugates that show self-assembly into β-sheet fibrils, the star architecture suppresses fibril formation and micelles are generally observed instead, a small population of fibrils only being observed upon pH adjustment. Hydrogelation is also suppressed by the polymer star architecture. These peptide-functionalized star polymer solutions are cytocompatible at sufficiently low concentration. These systems present tyrosine at high density and may be useful in the development of future enzyme or pH-responsive biomaterials.

Mechanical response of transient telechelic networks with many-part stickers.

A central question in soft matter is understanding how several individual, weak bonds act together to produce collective interactions. Here, gel-forming telechelic polymers with multiple stickers at each chain end are studied through Brownian dynamics simulations to understand how collective interaction of the bonds affects mechanical response of the gels. These polymers are modeled as finitely extensible dumbbells using an explicit tau-leap algorithm and the binding energy of these associations was kept constant regardless of the number of stickers. The addition of multiple bonds to the associating ends of telechelic polymers increases or decreases the network relaxation time depending on the relative kinetics of association but increases both shear stress and extensional viscosity. The relationship between the rate of association and the Rouse time of dangling chains results in two different regimes for the equilibrium stress relaxation of associating physical networks. In case I, a dissociated dangling chain is able to fully relax before re-associating to the network, resulting in two characteristic relaxation times and a non-monotonic terminal relaxation time with increasing number of bonds per polymer endgroup. In case II, the dissociated dangling chain is only able to relax a fraction of the way before it re-attaches to the network, and increasing the number of bonds per endgroup monotonically increases the terminal relaxation time. In flow, increasing the number of stickers increases the steady-state shear and extensional viscosities even though the overall bond kinetics and equilibrium constant remain unchanged. Increased dissipation in the simulations is primarily due to higher average chain extension with increasing bond number. These results indicate that toughness and dissipation in physically associating networks can both be increased by breaking single, strong bonds into smaller components.

Decoding the linear viscoelastic properties of model telechelic metallo-supramolecular polymers

This work focuses on the linear viscoelastic properties of entangled telechelic bulk metallo-supramolecular polymers. The latter are based on linear and star poly(n-butyl acrylate)s functionalized with a terpyridine ligand at each chain extremity, in the presence of transition metal ion of varying nature. The systems are investigated both experimentally and theoretically using small amplitude oscillatory shear and a modified version of the tube-based time marching algorithm (TMA), respectively. The experimental data reveal that sample relaxation depends on both disentanglement and association dynamics, with the respective importance of these two processes depending on the nature of the metal ion and on the temperature. A good description of the data is achieved using the modified TMA model, provided that dissociation events of metal-ligand complexes occur via ligand exchange. The model contains two fitting parameters, i.e., the fraction of unassociated stickers and the longest time needed in order to ensu...

Brittle fracture of polymer transient networks

We study the fracture of reversible double transient networks, constituted of water suspensions of entangled surfactant wormlike micelles reversibly linked by various amounts of telechelic polymers. We provide a state diagram that delineates the regime of fracture without necking of the filament from the regime where no fracture or break-up has been observed. We show that filaments fracture when stretched at a rate larger than the inverse of the slowest relaxation time of the networks. We quantitatively demonstrate that dissipation processes are not relevant in our experimental conditions and that, depending on the density of nodes in the networks, fracture occurs in the linear viscoelastic regime or in a non-linear regime. In addition, analysis of the crack opening profiles indicates deviations from a parabolic shape close to the crack tip for weakly connected networks. We demonstrate a direct correlation between the amplitude of the deviation from the parabolic shape and the amount of non linear viscoelasticity.

Regioselective Termination Reagents for Ring-Opening Alkyne Metathesis Polymerization.

Alkyne cross-metathesis of molybdenum carbyne complex [TolC≡Mo(OCCH3(CF3)2)3]·DME with 2 equiv of functional ynamines or ynamides yields the primary cross-metathesis product with high regioselectivity (>98%) along with a molybdenum metallacyclobutadiene complex. NMR and X-ray crystal structure analysis reveals that ynamides derived from 1-(phenylethynyl)pyrrolidin-2-one selectively cleave the propagating molybdenum species in the ring-opening alkyne metathesis polymerization (ROAMP) of ring-strained 3,8-dihexyloxy-5,6-dihydro-11,12-didehydrodibenzo[a,e][8]annulene and irreversibly deactivate the diamagnetic molybdenum metallacyclobutadiene complex through a multidentate chelate binding mode. The chain termination of living ROAMP with substituted ethynylpyrrolidin-2-ones selectively transfers a functional end-group to the polymer chain, giving access to telechelic polymers. This regioselective carbyne transfer strategy gives access to amphiphilic block copolymers through synthetic cascades of ROAMP followed by ring-opening polymerization of strained ε-caprolactone.

Embedding Well-Defined Responsive Hydrogels with Nanocontainers: Tunable Materials from Telechelic Polymers and Cyclodextrins.

Design, synthesis, and application of cyclodextrin (CD) containing thermoresponsive hydrogels fabricated from thiol-reactive telechelic polymers are reported. Hydrophilic polymers containing 2-hydroxyethyl methacrylate and/or di(ethylene glycol)methylether methacrylate monomers as side chains and thiol-reactive groups at chain ends were synthesized. A series of hydrogels was fabricated using thiol–ene conjugation of these thiol-reactive polymers with multivalent thiol-containing CDs as crosslinkers. Clear and transparent hydrogels were obtained with good conversion (79–89%) by utilizing the “nucleophilic” and “radical” thiol–ene “click” reactions. Analysis of the amount of residual thiol groups in these hydrogels using Ellman’s reagent suggested that gels with a moderately well-defined network structure were obtained. Hydrogels fabricated using different telechelic polymers were examined for their properties such as morphology, equilibrium water uptake, and rheological characteristics. Cytocompatibility of these hydrogels was ascertained by a cell viability assay that demonstrated low toxicity toward fibroblast cells. Thereafter, the CD-containing hydrogels were evaluated for the loading and controlled release of puerarin, an antiglaucoma drug. Utilization of thermoresponsive polymers as the matrix for these hydrogels allows use of temperature as a stimulus to modulate the drug release. A slower and more sustained drug release was observed at physiological temperatures compared to ambient conditions. The effect of temperature on the elasticity of the hydrogel was investigated rheologically to demonstrate that the collapse of the network structure occurs near physiological temperatures. The increased hydrophobicity and compactness of the gel matrix at higher temperatures results in a slower drug release. The strategy employed here demonstrates that tuning the matrix composition of hydrogels with well-defined network structures through appropriate choice of responsive copolymers allows design of materials with control of their physical properties and drug-release behavior.

Chemoselective polymerization platform for flow synthesis of functional polymers and nanoparticles

A chemoselective polymerization platform was designed for flow synthesis of telechelic thiol-functionalized polymers, amphiphilic block copolymers and noble metal nanoparticles. By assembling Novozyme435 packed tubular reactor, α-thiol-ω-hydroxyl telechelic poly(ε-caprolactone) were efficiently obtained via metal-free and protecting-group-free ring-opening polymerizations. Fast polymerization, high end group fidelity, controlled molecular weight and distribution were achieved under the flow conditions. The resultant mercapto-polymer solution derived from the enzymatic process was directly pumped into the subsequent microreactor system (isolation-step-free) to prepare amphiphilic poly(N-vinylpyrrolidone)-block-poly(ε-caprolactone). Relative narrower molecular weight distribution was observed in contrast to the batch mode. Moreover, silver nanoparticles were facilely fabricated in flow fashion by using mercapto-polymer solution as the stabilizer without isolation. The surface plasma resonance band absorption of nanoparticles could be adjusted by regulating the flow rates. This chemoselective polymerization platform would provide a powerful toolbox for the novel polymer and nanomaterial preparations.

Self-Assembly of Mixtures of Telechelic and Monofunctional Amphiphilic Polymers in Water: From Clusters to Flowerlike Micelles

We study the self-assembly of mixtures of n-alkyl mono- and difunctionalized poly(ethylene oxide) (PEO) chains in the dilute concentration regime. The monofunctional PEOs were prepared by living anionic polymerization with varying n-alkyl length (n = 14, 16, 22, 28) and constant PEO molecular weight of 5 kg/mol. The difunctional materials were obtained through end-to-end coupling of two of the monofunctionalized PEOs via their terminal hydroxyl groups. The chosen synthetic pathway yields well-defined model compounds with narrow molecular weight distribution and complete end-group functionalization. By using both small-angle neutron scattering (SANS) and dynamic light scattering (DLS) combined with theoretical data modeling, we have systematically investigated both the global and inner structure of the self-assembled micellar structures. For short n-alkyl chain-ends, we find a formation of clustered micelles with a finite size whereas, intriguingly, at longer n-alkyls, we observe a crossover to flowerlike ...

Supramolecular Multiblock Copolymers Featuring Complex Secondary Structures.

This contribution introduces main-chain supramolecular ABC and ABB'A block copolymers sustained by orthogonal metal coordination and hydrogen bonding between telechelic polymers that feature distinct secondary structure motifs. Controlled polymerization techniques in combination with supramolecular assembly are used to engineer heterotelechelic π-sheets that undergo high-fidelity association with both helical and coil-forming synthetic polymers. Our design features multiple advances to achieve our targeted structures, in particular, those emulating sheet-like structural aspects using poly(p-phenylenevinylene)s (PPVs). To engineer heterotelechelic PPVs in a sheet-like design, we engineer an iterative one-pot cross metathesis-ring-opening metathesis polymerization (CM-ROMP) strategy that affords functionalized Grubbs-II initiators that subsequently polymerize a paracyclophanediene. Supramolecular assembly of two heterotelechelic PPVs is used to realize a parallel π-sheet, wherein further orthogonal assembly with helical motifs is possible. We also construct an antiparallel π-sheet, wherein terminal PPV blocks are adjacent to a flexible coil-like poly(norbornene) (PNB). The PNB is designed, through supramolecular chain collapse, to expose benzene and perfluorobenzene motifs that promote a hairpin turn via charge-transfer-aided folding. We demonstrate that targeted helix-(π-sheet)-helix and helix-(π-sheet)-coil assemblies occur without compromising intrinsic helicity, while both parallel and antiparallel β-sheet-like structures are realized. Our main-chain orthogonal assembly approach allows the engineering of multiblock copolymer scaffolds featuring diverse secondary structures via the directional assembly of telechelic building blocks. The targeted assemblies, a mix of sequence-defined helix-sheet-coil and helix-sheet-helix architectures, are Nature-inspired synthetic mimics that expose α/β and α+β protein classes via de novo design and cooperative assembly strategies.

Heterofunctional RAFT‐derived PNIPAM via cascade trithiocarbonate removal and thiol‐yne coupling click reaction

ABSTRACTAn efficient one‐pot process to functionalize the α‐ and ω‐positions of RAFT‐derived poly(N‐isopropylacrylamide) (PNIPAM) by two inherently different mechanistic pathways is reported. The method relies on the RAFT polymerization of NIPAM using a new alkyne‐based RAFT agent, namely 2‐cyano‐5‐oxo‐5‐(prop‐2‐yn‐1‐ylamino)pentan‐2‐yl dodecyltrithiocarbonate (COPYDC) and the combination of thiol‐yne click chemistry and thiocarbonylthio chain‐end removal reactions. COPYDC was prepared in good yield and used as an efficient chain transfer agent during the RAFT polymerization of NIPAM. Well‐defined polymers with controlled molar masses ( = 7500–14,700 g.mol−1) and narrow dispersities (Đ = 1.18–1.26) are thus obtained. Cascade thiol‐yne click reaction at the alkyne α‐chain end and trithiocarbonate removal at the ω‐chain end are successfully achieved using benzyl mercaptan and excess AIBN. The reported method provides a facile and mild route to heterofunctional telechelic RAFT polymers with predictable molar masses and low dispersities. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017, 55, 3597–3606

전도성 복합재료 개발을 위한 텔레킬릭 고분자: 메타크릴레이트-끝 씌어진 폴리피롤 합성

일반적인 전도성고분자-폴리피롤(polypyrrole, PPy)의 경우, 화학구조상 화학결합을 할 수 있는 작용기가 없어 그 응용성에 많은 제한이 따른다. 이에 피롤-단량체에 새로운 고분자 중합이 가능한 비닐(vinyl) 작용기를 한쪽 끝에 화학적으로 도입하여, 필요에 따라 선택적으로 고분자 중합을 할 수 있는 이중 작용기 피롤-단량체 유도체, 메타크릴레이트-끝 씌어진 피롤을 합성하였다. 새롭게 합성된 이 단량체를 전기화학적인 방법으로 고분자 중합을 선택적으로 실시하여, 비닐작용기를 포함하는 새로운 전도성고분자, 메타크릴레이트-끝 씌어진 폴리피롤을 성공적으로 합성하였다. 이는 전도성 복합재료 개발을 위한 텔레킬릭 고분자로써 다른 비닐작용기를 포함하는 범용 플라스틱 단량체와 전기화학 이외의 방법으로 다양한 그래프트 공중합체(graft copolymer) 합성이 가능하다.

Efficient Homodifunctional Bimolecular Ring-Closure Method for Cyclic Polymers by Combining RAFT and Self-Accelerating Click Reaction.

An efficient metal-free homodifunctional bimolecular ring-closure method is developed for the formation of cyclic polymers by combining reversible addition-fragmentation chain transfer (RAFT) polymerization and self-accelerating click reaction. In this approach, α,ω-homodifunctional linear polymers with azide terminals are prepared by RAFT polymerization and postmodification of polymer chain end groups. By virtue of sym-dibenzo-1,5-cyclooctadiene-3,7-diyne (DBA) as small linkers, well-defined cyclic polymers are then prepared using the self-accelerating double strain-promoted azide-alkyne click (DSPAAC) reaction to ring-close the azide end-functionalized homodifunctional linear polymer precursors. Due to the self-accelerating property of DSPAAC ring-closing reaction, this novel method eliminates the requirement of equimolar amounts of telechelic polymers and small linkers in traditional bimolecular ring-closure methods. It facilitates this method to efficiently and conveniently produce varied pure cyclic polymers by employing an excess molar amount of DBA small linkers.

Regioselective Carbyne Transfer to Ring-Opening Alkyne Metathesis Initiators Gives Access to Telechelic Polymers.

Regioselective carbyne-transfer reagents derived from (3,3,3-trifluoroprop-1-yn-1-yl)benzene give access to functionalized ring-opening alkyne metathesis polymerization (ROAMP) initiators [R-C6H4C≡Mo(OC(CH3)(CF3)2)3] featuring electron-donating or -withdrawing substituents on the benzylidyne. Kinetic studies and linear free-energy relationships reveal that the initiation step of the ring-opening alkyne metathesis polymerization of 5,6,11,12-tetradehydrobenzo[a,e][8]annulene exhibits a moderate positive Hammett reaction constant (ρ = +0.36). ROAMP catalysts featuring electron-withdrawing benzylidynes not only selectively increase the rate of initiation (ki) over the rate of propagation (kp) but also prevent undesired intra- and intermolecular chain-transfer processes, giving access to linear poly-(o-phenylene ethynylene) with narrow molecular weight distribution. The regioselective carbyne transfer methodology and the detailed mechanistic insight enabled the design of a bifunctional ROAMP-reversible addition-fragmentation chain-transfer (RAFT) initiator complex. ROAMP followed by RAFT polymerization yields hybrid poly-(o-phenylene ethynylene)-block-poly-(methyl acrylate) block copolymers.

Highly active and selective telechelic antimicrobial poly(2-oxazoline) copolymers

The satellite group effect (SG) of telechelic antimicrobial polymers allows to increase or diminish the antimicrobial activity of the biocidal dodecyltrimethylammonium (DDA) group at the terminal of poly(2-methyl-2-oxazoline). Such polymers show good antimicrobial activities with MIC values as low as 10 μg mL−1 against the pathogenic bacterium Staphylococcus aureus and moderate selectivity with respect to blood cells (HC50/MIC: 10–120). Here, we show that the systematic variation of the nature and structure of the polymer backbone of such macromolecules results in biocidal polymers that rank among the highest antimicrobial activity against Staphylococcus aureus (MICS.aureus = 1–1.6 μg mL−1) and the best blood cell compatibilities (HC50/MICS.aureus = 1500–1700) known for amphiphilic antimicrobial polymers. Further, the polymers show a very high selectivity for Gram-positive bacteria (MICE.coli./MICS.aureus up to 750), which is due to the satellite group effect in combination with optimized hydrophilic/hydrophobic balance of the polymer backbone.