Polymer Chain Ends Research Articles

Dual Stimuli Switching: Interconverting Cationic and Radical Polymerizations with Electricity and Light

Summary Increasing demand for advanced materials essential to emerging technologies calls for synthetic methods that can easily generate polymers with complex structures. Multiblock copolymers display a range of material properties depending on the length and number of polymer blocks. Currently, there remains a lack of polymerization processes that can easily synthesize these multiblock structures. Here, we report the in situ synthesis of multiblock copolymers by controlling monomer incorporation with electrochemical and photochemical stimuli. To achieve this, we developed a cationic polymerization where polymer chain growth is controlled by reversible electrochemical oxidation of the polymer chain end and coupled this with a compatible photocontrolled radical polymerization. This process was used to generate higher-order multiblock copolymers wherein the number of blocks and the length of each segment is controlled by the two stimuli. This method, which lends itself to automation, will aid in accelerating the rate at which next-generation materials are discovered.

Catechol End-Functionalized Polysarcosine for Insitu Synthesis and Stabilization of Silver Nanoparticles

ABSTRACTFunctional polymers were previously employed to minimize the susceptibility of metallic nanoparticles (MNPs) for aggregation. Herein, we intended to conjugate catechol moiety into the polymer chain end considering its anchoring ability to virtually most surfaces. Accordingly, catechol end-functionalized polysarcosine (cat-PSar) was successfully prepared from the ring-opening polymerization (ROP) of sarcosine N-carboxyanhydrides (Sar-NCA) using dopamine hydrochloride initiator. ROP of Sar-NCA was carried out at different monomer to initiator feed ratios. The molecular structure of cat-PSar was confirmed by 1H NMR and MALDITOF. Afterward, the obtained catechol functionalized polymer was used for in-situ synthesis and stabilization of silver nanoparticles (Ag-NPs) in aqueous solution. The observed characteristic absorption peak at λmax of 415 nm indicates the formation of Ag-NPs. Scanning electron microscope (SEM) images also elucidate the formation of Ag-NPs with the relatively small sizes of the nanocomposite at a high concentration of silver nitrate. Hence, biomimetic polymers could play a dual role as reducing and stabilizing agents in the preparation of monodispersed MNPs.

Functionalization of PVDF-based copolymer via photo-induced p-anisaldehyde catalyzed atom transfer radical polymerization

An efficient graft modification strategy was developed based on photo-induced organocatalyzed atom transfer radical polymerization (O-ATRP) for the functionalization of poly(vinylidene fluoride) (PVDF) based copolymer. P-anisaldehyde was served as an organic photoredox catalyst to mediated O-ATRP of methyl methacrylate (MMA) and glycidyl methacrylate (GMA) in the presence of poly(vinylidene fluoride-co-chlorotrifluoroethylene) (P(VDF-co-CTFE)) as the macroinitiator under UV irradiation. Excellent temporal control and low catalyst concentration was achieved. Chain extension confirmed the chlorine atom attached to the graft polymer chain end. Metal-free P(VDF-co-CTFE)-g-PMMA and P(VDF-co-CTFE)-g-PGMA with varied graft contents were obtained, which would find potential applications in dielectric materials.

Photocontrolled Radical Polymerization from Hydridic C-H Bonds.

Given the ubiquity of carbon-hydrogen bonds in biomolecules and polymer backbones, the development of a photocontrolled polymerization selectively grafting from a C-H bond represents a powerful strategy for polymer conjugation. This approach would circumvent the need for complex synthetic pathways currently used to introduce functionality at a polymer chain end. On this basis, we developed a hydrogen-atom abstraction strategy that allows for a controlled polymerization selectively from a hydridic C-H bond using a benzophenone photocatalyst, a trithiocarbonate-derived disulfide, and visible light. We performed the polymerization from a variety of ethers, alkanes, unactivated C-H bonds, and alcohols. Our method lends itself to photocontrol which has important implications for building advanced macromolecular architectures. Finally, we demonstrate that we can graft polymer chains controllably from poly(ethylene glycol) showcasing the potential application of this method for controlled grafting from C-H bonds of commodity polymers.

Taking Advantage of Oxidation to Characterize Thiol-Containing Polymer Chains by MALDI-TOF Mass Spectrometry.

MALDI-TOF mass spectrometry analyses revealed the oxidation of thiol-containing polymer chain-ends during sample preparation using THF as solvent. In these conditions, the extent of oxidation was hardly reproducible, and led to various types of oxidized compounds. Preparing the samples at the last minute using commercial THF stabilized with an antioxidant led to more reproducible results, with the least oxidation. However, it is demonstrated herein that thiol oxidation can be advantageously taken into profit to further ascertain the presence of the thiol at the polymer chain-end. To force thiol oxidation we used THF without any antioxidant stabilizer, thus more prone to form peroxides. Thiol-containing polymer chains can thereby be indirectly evidenced by the formation of oxidation products such as chain-chain disulfide bonds and sulfonic acid chains-ends. More importantly, in these oxidizing conditions and in the negative mode, sulfonic acid-terminated polymer chains can be more sensitively detected than thiol ones (the low pKa of sulfonic acids facilitating their anionization in MALDI source). In conclusion, performing MALDI-TOF mass spectrometry analyses in oxidizing conditions, as complement to regular analyses, was found to be very useful for the chain-end identification of different thiol-containing polymer chains.

Organocatalyzed controlled radical polymerization with alkyl bromide initiator via in situ halogen exchange under thermal condition

Alkyl bromide (R–Br) was employed as a starting compound (precursor) to initiate the bulk controlled radical polymerization of methyl methacrylate (MMA) under thermal condition (60 °C) in the presence of tetra-n-octylammonium iodide (Oct4NI) and 2,2′-azobisisoheptonitrile (V65). Alkyl iodide (R–I) and tetraoctylammonium bromide (Oct4NBr) were in situ generated by the reaction of R–Br with Oct4NI and then R–I was employed as an initiating dormant species for the polymerization. Carbon-iodine (C–I) bonds were cleaved and radicals were generated by the catalyst of Oct4NI and Oct4NBr. The chain end groups of the obtained polymers contain both I and Br, which was proved by the results of 1H nuclear magnetic resonance spectra, and elemental analysis. The reversible halogen exchange of Br with I at the polymer chain end promotes the polymerization. The introducing of V65 not only increased the polymerization speed, but also ensured high monomer conversion. Low-polydispersity (Mw/Mn = 1.1–1.5) polymers were obtained with high conversions (e.g., 70–90%) in the polymerizations of methyl methacrylate, styrene, acrylonitrile, and functional methacrylates. The attractive feature was that alkyl bromide was able to initiate the controlled radical polymerization of methyl methacrylate under thermal condition in the presence of organic catalyst. Besides, this new system presented desirable features of high monomer versatility and the accessibility to a wide range of polymer structural designs for use in a range of applications.

Geometrically Constrained Polymerization of Styrene Over Heterogeneous Catalyst Layer in Silica Nanotube Reactors

Styrene has been polymerized to syndiotactic polystyrene (sPS) over a layer of heterogeneous Cp*Ti(OCH3)3/MAO catalyst immobilized onto the surfaces of silica nanotube reactor (SNTR) arrays of 60–200 nm in diameter. The polymer produced in the SNTR arrays has been found to have the molecular weights much larger than the polymers synthesized by a liquid slurry polymerization over silica‐supported catalysts. A dynamic reactor model that consists of diffusion and reaction terms has been derived and solved to quantify the kinetics of styrene polymerization in a single nanotube reactor. The two‐site kinetic model applied to the silica nanotube reactor model shows that the experimentally observed high polymer molecular weight can be fitted if the chain transfer rate constants for monomer and β‐hydride elimination are reduced significantly. The simulation results suggest that the presence of dense crystalline sPS nanofibrils filling the nanotubes constrain the molecular movements of polymer chain ends in the proximity of catalyst sites to limit the chain transfer reactions. POLYM. ENG. SCI., 60:700–709, 2020. © 2020 Society of Plastics Engineers

Tunable hydantoin and base binary organocatalysts in ring-opening polymerizations

A (thio)hydantoin (HHyd) was deprotonated by a Brønsted base (B) to afford iminolate Hyd1 or Hyd3 that activated polymer chain-end (P), the conjugate acid (B–H+) activated monomer (M).

Tandem Unzipping and Scrambling Reactions for the Synthesis of Alternating Copolymers by the Cationic Ring-Opening Copolymerization of a Cyclic Acetal and a Cyclic Ester.

Cationic copolymerization of different types of monomers, 4-hydroxybutyl vinyl ether (HBVE) and ε-caprolactone (CL), was explored using EtSO3H as an acid catalyst, producing copolymers with a remarkably wide variety of compositions and sequences. In the initial stage of the reaction, HBVE was unexpectedly isomerized to 2-methyl-1,3-dioxepane (MDOP), followed by concurrent copolymerization of MDOP and CL via active chain end and activated monomer mechanisms, respectively. The compositions and sequences of the copolymers were tunable, depending on the initial monomer concentrations. Moreover, a unique method was developed for transforming a copolymer with no CL homosequences into an "alternating" copolymer by removing MDOP from the system using a vacuum pump. This was achieved by the tandem reactions of depolymerization (unzipping) and random transacetalization (scrambling) under thermodynamic control. Specifically, the unzipping of HBVE homosequences proceeded at the oxonium chain end until a nondissociable ester bond emerged next to the chain end, while the scrambling of the main chain via transacetalization transferred midchain HBVE homosequences into the polymer chain end.

L-menthol-based xanthate mediator for RAFT polymerization of vinyl acetate

A xanthate RAFT agent 2-isopropyl-5-methylcyclohexyl 2-(ethoxycarbonothioyl)thio) propanoate has been synthesized via the reactions of L-menthol with 2-bromo-propionyl bromide in THF in the presence of triethylamine leading to the formation of 2-isopropyl-5-methylcyclohexyl 2-bromopropanoate followed by its reaction with potassium-O-ethyl xanthate in ethanol at room temperature and characterized by 1H, 13C NMR and FTIR. RAFT polymerization of vinyl acetate (VAc) using it has resulted in the formation of well-defined polyVAc in controlled manner. This is evidenced by (i) the observation of the pseudo first order kinetics upto ∼90% conversion, (ii) linear increment of molecular weight (unimodal GPC chromatogram) of the polymer keeping its PDI low with the progress of the reaction upto ∼90% conversion, (iii) the linear increment of the molecular weight of the polymers with low PDI on increasing monomer loading, (iv) chain-end analysis of the polymer using 1H NMR confirming the presence of the RAFT agent fragment at the polymer chain-ends, and (v) successful homo- and hetero-chain extension with VAc and NVP, respectively. L-Menthyl-capped polyvinyl acetate has shown higher (i) light transmission properties of its dichloromethane solution at ∼277 nm, (ii) thermal stability, and (iii) glass transition temperature in comparison to its non-L-Menthyl-capped homolog.

Engineering Surface Hydrophilicity via Polymer Chain-End Segregation in Coatings Formed by Photopolymerization

The manipulation of surface chemistry, and more precisely the hydrophilic/hydrophobic nature of an interfacial material, is essential for many applications in coatings, biomaterials, and microfluid...

From ultra-high molecular weight polydimethylsiloxane to super-soft elastomer

A polydimethylsiloxane-α,ω-diol (UHMW-PDMS) with unusually high molecular weight (Mw = 1,395,000 g/mol) and narrow polydispersity (PDI = 1.49), free of monomer or catalyst traces, was obtained in high yield by a scalable process, easy to achieve (bulk anionic ring-opening polymerization of octamethylcyclotetrasiloxane catalyzed by tetramethylammonium hydroxide). Spectral and thermogravimetric analysis confirmed the theoretical structure, purity and thermal stability of the polymer. Individual or aggregated macromolecules were assessed by dynamic light scattering (DLS) and visualized by microscopic methods (TEM, AFM). The mechanical, dielectric, and thermal performances of application interest for this polymer were evaluated on samples of chemically defined elastomer cUHMW-PDMS prepared by condensation crosslinking through the polymer chain ends with tetraethylorthosilicate. The results were compared with those obtained on a lower molecular weight (Mw = 720,100 g/mol, PDI = 1.87) homologue (PDMS) prepared and processed in identical conditions, but also with a commercial silicone elastomer. UHMW-PDMS-based elastomer shows superior results on elastic recovery and stress decay at high elongation, as compared with reference samples.

Phase Transition Behaviors and Nanoscale Film Morphologies of Poly(δ-valerolactone) Axles Bearing Movable and Fixed Rotaxane Wheels.

In this study, poly(δ-valerolactone) (PVL) axles bearing movable and fixed dibenzo-24-crown-8-ether wheels (rot-M and rot-F) are investigated for the first time in the terms of phase transition and nanoscale film morphology: PVL-rot-M and PVL-rot-F. Interestingly, the PVL axles reveal a strong tendency to form a horizontal lamellar structure with three different rotational crystal lattice domains in nanoscale films. The morphological structural parameters are discernibly varied by the movable and fixed rotaxane wheels. In particular, the rot-M wheel tends to be populated in both the interfacial and amorphous layers. The rot-M wheel is found to significantly influence the phase transition characteristics of the PVL axle because of its movability along the polymer backbone chain. In contrast, the rot-F wheel tends to be more localized in the interfacial layer rather than in the amorphous layer because of its immovability constrained at the polymer chain end. The rot-F wheel causes severe thermal instability in the PVL axle, which can be attributed mainly to the presence of its counter anion (PF6 - ).

Cobalt(II) phenoxy‐imine complexes in radical polymerization of vinyl acetate: The interplay of catalytic chain transfer and controlled/living radical polymerization

ABSTRACTA series of cobalt(II) phenoxy‐imine complexes (CoII(FI)2) have been synthesized to mediate the radical polymerization of vinyl acetate (VAc) and methyl acrylate (MA) to evaluate the influence of chelating atoms and configuration to the control of polymerization. The VAc polymerizations showed the properties of controlled/living radical polymerization (C/LRP) with complexes 1a and 3a, but the catalytic chain transfer (CCT) behaviors with complexes 2a, 1b, 2b, and 3b. The control of VAc polymerization mediated by complex 1a could be improved by decreasing the reaction temperature to approach the molecular weights that not only linearly increased with conversions but also matched the theoretical values and relatively narrow molecular weight distributions. The catalytic chain transfer polymerizations (CCTP) mediated by complexes 2a, 1b, 2b, and 3b were characterized by Mayo plots and the polymer chain end double bonds were observed by 1H NMR spectra. The tendency toward C/LRP or CCTP in VAc polymerization mediated by CoII(FI)2 could be determined by the ligand structure. Cobalt complex coordinated by the ligand with more steric hindered and less electron‐donating substituents favored the controlled/living radical polymerization. In contrast, the efficiency of CCT process could be enhanced by less steric hindered, more electron‐donating ligands. The controlled/living radical polymerization of MA, however, could not be achieved by the mediation of these cobalt(II) phenoxy‐imine complexes. Associated with the results of polymerization mediated by other cobalt complexes, this study implied that the configuration and spin state of cobalt complexes were more critical than the chelating atoms to the control behavior of radical polymerization. © 2019 Wiley Periodicals, Inc. J. Polym. Sci. 2020, 58, 101–113

Fast and selective organocatalytic ring-opening polymerization by fluorinated alcohol without a cocatalyst

Organocatalysis is an important branch of catalysis for various organic transformations and materials preparation. Polymerizations promoted by organic catalysts can produce polymeric materials without any metallic residues, providing charming materials for high-value and sensitive domains such as biomedical applications, microelectronic devices and food packaging. Herein, we describe a fluorinated alcohol based catalytic system for polypeptide synthesis via catalytic ring-opening polymerization (ROP) of α-amino acid N-carboxyanhydride (NCA), fulfilling cocatalyst free, metal free, high rate and high selectivity. During polymerization, the fluorinated alcohol catalyst forms multiple dynamic hydrogen bonds with the initiator, monomer and propagating polymer chain. These cooperative hydrogen bonding interactions activate the NCA monomers and simultaneously protect the overactive initiator/propagating polymer chain-ends, which offers the whole polymerization with high activity and selectivity. Mechanistic studies indicate a monocomponent-multifunctional catalytic mode of fluorinated alcohol. This finding provides a metal free and fast approach to access well-defined polypeptides.

Surface-Induced ARGET ATRP for Silicon Nanoparticles with Fluorescent Polymer Brushes.

Well-defined polymer brushes attached to nanoparticles offer an elegant opportunity for surface modification because of their excellent mechanical stability, functional versatility, high graft density as well as controllability of surface properties. This study aimed to prepare hybrid materials with good dispersion in different solvents, and to endow this material with certain fluorescence characteristics. Well-defined diblock copolymers poly (styrene)-b-poly (hydroxyethyl methyl acrylate)–co-poly (hydroxyethyl methyl acrylate- rhodamine B) grafted silica nanoparticles (SNPs-g-PS-b-PHEMA-co-PHEMA-RhB) hybrid materials were synthesized via surface-initiated activators regenerated by electron transfer atom transfer radical polymerization (SI-ARGET ATRP). The SNPs surfaces were modified by 3-aminopropyltriethoxysilane (KH-550) firstly, then the initiators 2-Bromoisobutyryl bromide (BIBB) was attached to SNPs surfaces through the esterification of acyl bromide groups and amidogen groups. The synthetic initiators (SNPs-Br) were further used for the SI-ARGET ATRP of styrene (St), hydroxyethyl methyl acrylate (HEMA) and hydroxyethyl methyl acrylate-rhodamine B (HEMA-RhB). The results indicated that the SI-ARGET ATRP initiator had been immobilized onto SNPs surfaces, the Br atom have located at the end of the main polymer chains, and the polymerization process possessed the characteristic of controlled/“living” polymerization. The SNPs-g-PS-b-PHEMA-co-PHEMA-RhB hybrid materials show good fluorescence performance and good dispersion in water and EtOH but aggregated in THF. This study demonstrates that the SI-ARGET ATRP provided a unique way to tune the polymer brushes structure on silica nanoparticles surface and further broaden the application of SI-ARGET ATRP.

Glass Transition Behaviors of Poly (Vinyl Pyridine)/Poly (Vinyl Phenol) Revisited.

We examined the composition and molecular weight dependence of the glass transition temperature in detail for two types of hydrogen bonding miscible blends: poly (2-vinyl pyridine)/poly (vinyl phenol) (2VPy/VPh) and poly (4-vinyl pyridine)/poly (vinyl phenol) (4VPy/VPh). Regarding the functional form of the glass transition temperature, Tg, as a function of the weight fraction, we found a weak deviation from the Kwei equation for 2VPy/VPh blends. In contrast, such a deviation was not observed for the 4VPy/VPh blend. By relating the difference in the functional forms of Tg between the two blend systems to the difference in hydrogen bonding ability, we proposed a modified version of the Kwei equation. As for the interaction parameter, q in the Kwei equation, clear molecular weight dependence was observed for 2VPy/VPh blends: the lower the VPh molecular weight in the oligomer level, the higher the q values, suggesting the higher hydrogen bonding formability near the polymer chain ends than the middle part of a polymer chain.

Protein-functionalized nanoparticles derived from end-functional polymers and polymer prodrugs for crossing the blood-brain barrier

Nanoparticles may provide a viable way for neuroprotective drugs to cross the blood–brain barrier (BBB), which limits the passage of most drugs from the peripheral circulation to the brain. Heterotelechelic polymer prodrugs comprising a neuroprotective model drug (adenosine) and a maleimide functionality were synthesized by the “drug-initiated” approach and subsequent nitroxide exchange reaction. Nanoparticles were obtained by nanoprecipitation and exhibited high colloidal stability with diameters in the 162–185 nm range and narrow size distributions. Nanoparticles were then covalently surface-conjugated to different proteins (albumin, α2-macroglobulin and fetuin A) to test their capability of enhancing BBB translocation. Their performances in terms of endothelial permeability and cellular uptake in an in vitro BBB model were compared to that of similar nanoparticles with surface-adsorbed proteins, functionalized or not with the drug. It was shown that bare NPs (i.e., NPs not surface-functionalized with proteins) without the drug exhibited significant permeability and cellular uptake, which were further enhanced by NP surface functionalization with α2-macroglobulin. However, the presence of the drug at the polymer chain-end prevented efficient passage of all types of NPs through the BBB model, likely due to adecrease in the hydrophobicity of the nanoparticle surface and alteration of the protein binding/coupling, respectively. These results established a new and facile synthetic approach for the surface-functionalization of polymer nanoparticles for brain delivery purposes.

New insight into the cold crystallization of natural rubber: The role of linked and free fatty chains

Natural Rubber (NR) is a polyisoprene (PI) from renewable resources that exhibits mechanical and thermal properties not accessible with synthetic polyisoprenes (IR). In this article, it was developed a simple route to synthesize new models of NR (PI with a pure microstructure (100% 1,4-cis) and two fatty esters linked to the polymer chain-end) in order to clarify the role played by lipids on the cold (CCr) of NR. DSC analysis of this new hybrid polymer revealed that the lipids linked to the backbone were able to crystallize despite of the amorphous matrix, the crystallization temperature being dependent on the length of the linked fatty chain. Furthermore, it was observed that the linked fatty chains prevented PI to crystallize at low temperature (-25°C). Finally, the addition of free fatty acids to the hybrid polymer enhanced the crystallization of the PI chain, more particularly when stearic acid and methyl linoleate were used simultaneously as additives. A 1

Blending the Effectiveness of Anionic Polymerization with the Versatility of RAFT by Use of the Atom Transfer Radical Addition–Fragmentation Technique

AbstractThe efficient conversion of a living anionic polymer chain end to a macrochain transfer agent suitable for reversible addition–fragmentation chain transfer (RAFT) polymerization is reported. Conversion to the macrochain transfer agent efficiency reached as high as 97% by capping the polyanion with alcohol, coupling with α‐bromoisobutryl bromide, followed by atom‐transfer radical addition–fragmentation with bis(thiobenzyl)disulfide to add the chain transfer agent functionality. To our knowledge, this is the most efficient marriage of anionic and reversible addition–fragmentation transfer polymerization reported to date, and can be carried out under common industrial processing conditions. The combination of these polymerization methods broadens the scope of commercially accessible block copolymers by exploiting the advantages of both RAFT and anionic polymerization chemistries.