Anionic Chain Ends Research Articles

Controlled and Regioselective Ring-Opening Polymerization for Poly(disulfide)s by Anion-Binding Catalysis.

Poly(disulfide)s are an emerging class of sulfur-containing polymers with applications in medicine, energy, and functional materials. However, the constituent dynamic covalent S-S bond is highly reactive in the presence of the sulfide (RS-) anion, imposing a persistent challenge to control the polymerization. Here, we report an anion-binding approach to arrest the high reactivity of the RS- chain end to control the synthesis of linear poly(disulfide)s, realizing a rapid, living ring-opening polymerization of 1,2-dithiolanes with narrow dispersity and high regioselectivity (Mw/Mn ∼ 1.1, Ps ∼ 0.85). Mechanistic studies support the formation of a thiourea-base-sulfide ternary complex as the catalytically active species during the chain propagation. Theoretical analyses reveal a synergistic catalytic model where the catalyst preorganizes the protonated base and anionic chain end to establish spatial confinement over the bound monomer, effecting the observed regioselectivity. The catalytic system is amenable to monomers with various functional groups, and semicrystalline polymers are also obtained from lipoic acid derivatives by enhancing the regioselectivity.

Ring-opening polymerization of cyclic oligosiloxanes without producing cyclic oligomers.

A long-standing problem associated with silicone synthesis is contamination of the polymer products with 10 to 15% cyclic oligosiloxanes that results from backbiting reactions at the polymer chain ends. This process, in competition with chain propagation through ring-opening polymerization (ROP) of cyclic monomers, was thought to be unavoidable and routinely leads to a thermodynamically controlled reaction mixture (polymer/cyclic oligosiloxanes = 85/15). Here, we report that simple alcohol coordination to the anionic chain ends prevents the backbiting process and that a well-designed phosphonium cation acts as a self-quenching system in response to loss of coordinating alcohols to stop the reaction before the backbiting process begins. The combination of both effects allows a thermodynamically controlled ROP of the eight-membered siloxane ring D4 without producing undesirable cyclic oligosiloxanes.

Unity Makes Strength: Constructing Polymeric Catalyst for Selective Synthesis of CO 2 /Epoxide Copolymer

Unity Makes Strength: Constructing Polymeric Catalyst for Selective Synthesis of CO ₂ /Epoxide Copolymer

1,1,3,3-Tetramethylguanidine-Mediated Zwitterionic Ring-Opening Polymerization of Sarcosine-Derived N-Thiocarboxyanhydride toward Well-Defined Polysarcosine.

Zwitterionic ring-opening polymerization (ZROP) of sarcosine-derived N-thiocarboxyanhydrides (Me-NNTAs) can be induced by using 1,1,3,3-tetramethylguanidine (TMG) initiators in CH2Cl2 at 25 °C, rapidly producing well-defined polysarcosine polymers with controlled molecular weights (Mn = 1.9–37 kg/mol) and narrow molecular weight distributions (Đ = 1.01–1.12). The reaction exhibits characteristics of a living polymerization, evidenced by pseudo-first-order polymerization kinetics, the linear increase of polymer molecular weight (Mn) with conversion, and the successful chain extension experiments. The polymerization is proposed to proceed via propagating macro-zwitterions bearing a cationic 1,1,3,3-tetramethylguanidinium and an anionic thiocarbamate chain end. The TMG not only initiates the polymerization but also serves to stabilize the thiocarbamate chain end where the monomer addition occurs. Because of the enhanced hydrolytic stability of Me-NNTA, the polymerization can be conducted without the rigorous exclusion of moisture, further enhancing the appeal of the method to access well-defined polysarcosine.

Bifunctional Catalysis Prevents Inhibition in Reversible-Deactivation Ring-Opening Copolymerizations of Epoxides and Cyclic Anhydrides.

Reversible-deactivation chain transfer is a viable strategy to increase the catalytic efficiency of ring-opening polymerizations, such as the alternating copolymerization of epoxides and cyclic anhydrides. In conjunction with the catalyst, protic chain transfer agents (CTAs) initiate polymerization and facilitate rapid proton transfer between active and dormant chains. Functional-group-tolerant Lewis acid catalysts are therefore required to successfully apply protic CTAs in reversible-deactivation ring-opening copolymerizations (RD-ROCOP), yet the predominant binary Lewis acid catalyst/nucleophilic cocatalyst systems suffer lower polymerization rates when used with protic CTAs. New mechanistic insight into the inhibition pathways reveals that the alcohol chain ends compete with epoxide binding to the Lewis acid and hydrogen-bond with anionic chain ends to impede epoxide ring opening. We report that a bifunctional aminocyclopropenium aluminum salen complex maintains excellent activity in the presence of protic functionality, exhibiting resilience against these inhibition pathways, even at high CTA concentrations. We apply reversible-deactivation chain transfer in the bifunctional ROCOP system to demonstrate precise molecular-weight control, CTA functional group scope, and accessible polymer architectures.

Mechanism of Janus Polymerization: A DFT Study

Janus polymerization is featured as a combination of cationic and anionic growing ends in one living polymer chain. In the copolymerization of THF and CL catalyzed by lutetium triflates and initiated by propylene oxide, three stages are identified by kinetic study including (1) fast cationic polymerization with slow anionic one, (2) fast anionic polymerization with dormant cationic one, and (3) reactivation of cationic polymerization with coupling of anionic and cationic chain ends. In this work, density functional theory (DFT) calculation is employed to investigate the reaction details of ionic polymerization and dormancy. A “tripedal crow” configuration is proposed to illustrate the unique high-coordinated ligand exchange configuration in anionic polymerization in different stages. The trigger of dormancy is determined as chain structures rather than concentration of triflate anion according to both calculation and experimental results.

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.

Janus Polymerization

Is it possible to combine cationic and (coordinated) anionic ring-opening polymerizations (CROP and AROP) into a unimolecular chain? In this work, we design polymerizations of ε-caprolactone (CL) and tetrahydrofuran (THF) using a single catalytic system of lutetium triflate/propylene oxide (Lu(OTf)3/PO) in which a cationic copolymerization of THF with CL and an AROP of CL perform at two chain ends of a growing chain. Kinetic and mechanistic studies demonstrate that the two chain-growth ROPs which have no mutual interference up to 90% of CL conversion keep a living manner producing diblock copolymers of PCL-b-P(THF-co-CL) with narrow polydispersities below 1.2. Afterward, the depletion of CL triggers a final step-growth polymerization via intermolecular coupling reaction of cationic and anionic chain ends, developing multiblock polyester–polyether elastomers of [PCL-b-P(THF-co-CL)]m. A concept of “Janus polymerization” is defined as a combination of cationic and anionic polymerizations on the two ends of a single growing chain, involving an initially living chain-growth polymerization and a subsequently self-triggered step-growth polycondensation.

Preparation of pH- and thermo-sensitive chitosan-PNIPAAm core–shell nanoparticles and evaluation as drug carriers

Environmentally sensitive polysaccharide nanoparticles (NPs) were prepared by in situ polymerization of N-isopropylacrylamide (NIPAAm) monomer in the presence of chitosan (CS) micelles. First, CS was found to develop a cationic micelle-like structure in the acetic acid solution when its concentration was increased to above the critical micelle concentration, as evidenced by fluorescence and TEM. When the NIPAAm was polymerized in the CS micelle solution by using potassium persulfate as initiator, the produced PNIPAAm with anionic chain end(s) became hydrophobic, as long as the reaction temperature was above its phase transition temperature; and therefore it would diffuse into the hydrophobic core of the CS micelles, producing CS-PNIPAAm core–shell NPs. Increasing the feeding amount of NIPAAm increased the monomer conversion and therefore the particle size; yet it decreased the surface zeta potential. Moreover, the CS-PNIPAAm NPs were sensitive to both pH value and temperature. For the study of drug release properties, doxycycline hyclate was used as a model drug and loaded into the NPs at pH 4.5 and 25 °C. The result illustrated that these NPs had a continuous drug release behavior up to 1 week, depending on the pH value and temperature. In addition, enzyme or hydrogen peroxide capable of degrading CS shell was added in the solution to facilitate the drug release.

Effects of surfactants on cloud points in aqueous poly(N-isopropylacrylamide) solutions

The cloud point was determined for aqueous solutions of two poly(N-isopropylacrylamide) samples, one having a hydrophobic nonionic chain-end group (M sample) and the other having a hydrophilic anionic chain-end group (R sample), with an addition of two kinds of surfactants, anionic sodium n-dodecyl sulfate (SDS) or cationic n-dodecyltrimethylammonium chloride (DTAC). It is found that behavior of the cloud point as a function of the concentration cs of the surfactants largely depends on the kinds of chain-end group and surfactant.

Combatting ionic aggregation using dielectric forces—combining modeling/simulation and experimental results to explain end-capping of primary amine functionalized polystyrene

Chain-end functionalization of living poly(styryl)lithium using 1-(3-bromopropyl)-2,2,5,5-tetramethyl-1-aza-2,5-disilacyclo-pentane (BTDP) to generate primary amine end-functionalized polystyrene was investigated using high vacuum anionic polymerization techniques. 13C NMR spectroscopy and Matrix Assisted Laser Desorption Ionization Time-of-Flight Mass Spectrometry (MALDI-TOF MS) were used to evaluate polymer end-groups and demonstrated that quantitative amine functionalized polymer was attained under appropriate reaction conditions. In general, the polymerization of styrene was conducted in benzene and the end-capping reaction was performed by adding tetrahydrofuran (THF) to the reaction prior to the addition of BTDP in THF at room temperature. Results indicated that approximately 20% THF by volume is required to obtain 100% end-capping free from side reactions. When too little or no THF was present, side reactions such as lithium halogen exchange followed by Wurtz coupling resulted in unfunctionalized head-to-head dimer as well as other byproducts. Modeling and simulation of the solvent effects using hybrid methods (the so-called QM/MM method) suggest that THF effectively dissociated the anionic chain-end aggregation, thereby resulting in the desired primary amine functionalized polymer. Molecular dynamics (MD) simulations were conducted to develop an understanding of the physics of counterions involved in the end-functionalization process.

One-pot synthesis of primary amino end-functionalized polymers by reaction of living anionic polybutadienes with nitriles

We present a convenient method to synthesize polymers with primary amino groups by end-capping of living anionic chain ends with nitriles followed by reduction in a one-pot process. As an example, the addition of pivalonitrile to the chain ends of oligobutadienyllithium in toluene, followed by the reduction with sodium borohydride quantitatively leads to ω-amino-oligo-1,4-butadiene, as demonstrated by 1H and 13C NMR, thin-layer chromatography and MALDI-ToF mass spectrometry.

Chain‐end modification of living anionic polybutadiene with diphenylethylenes and styrenes

AbstractThe first step in the transformation of poly(butadienyl)lithium into a macromolecular atom transfer radical polymerization initiator or reversible addition–fragmentation chain transfer agent is the modification of the anionic chain end into a suitable leaving/reinitiating group. We have investigated three different modification reactions to obtain a styrenic end group at the chain end of poly(butadienyl)lithium. In all cases, we have looked at the influence of a Lewis base on the progress of the reaction. The first modification reaction with α‐methylstyrene leads to partial functionalization and oligomerization. The second reaction with 1,2‐diphenylethylenes, particularly trans‐stilbene, results in monoaddition to the poly(butadienyl)lithium chain ends. Quantitative functionalization is not obtained, possibly because of a hydrogen abstraction reaction, which causes termination. In the third modification reaction, a small polystyrene block is successfully added to the chain ends, as shown by a detailed matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry analysis of the block copolymers. Nearly quantitative block copolymer formation is achieved, with an average styrene block size of four monomer units and a polydispersity index of 1.19 for the polystyrene block. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 2536–2545, 2005

Anionic polymerization of isocyanates with optical functionalities

Polyisocyanates of 2-[4-(4-nitrophenylazo)- N-ethylphenyl amino]ethoxy carbonyl amino hexyl isocyanate, DR1NCO, and ( s)-(−)-2-methyl-1-butyloxy carbonyl amino hexyl isocyanate, MBI, were synthesized by anionic polymerization using sodium naphthalenide as an initiator in THF at −98°C. In the polymerization reaction, crown ether (15C5) and octyl isocyanate (Oct-NCO) were used to suppress side reactions such as backbiting or chain transfer reaction. In the homopolymerization of DR1NCO, the polymers with high yield (∼94%) were obtained due to suppression of backbiting by 15C5. In the random copolymerization of DR1NCO with MBI, the chain transfer reaction was suppressed due to the low reactivity of anionic chain end induced by octyl isocyanate. The random copolymers showed the optical activity in CD and ORD measurements, and specific rotations increased with increasing the component of MBI.

1,3-Cyclohexadiene Polymers. 2. Near-Monodisperse Star and Star-Block Polymers Based on Poly(1,3-cyclohexadiene)

The synthesis of well-defined, near-monodisperse, three-arm star and star-block copolymers based on poly(1,3-cyclohexadiene) (PCHD) is described. The stars were prepared by the reaction of living anionic chain ends of PCHD, poly(styrene−1,3-cyclohexadiene) (PS-PCHD), and poly(1,3-cyclohexadiene−styrene) (PCHD-PS) with CH3SiCl3 in benzene. 1,4-Diazabicyclo[2.2.2]octane (DABCO) was used to control the anionic polymerization of 1,3-CHD; it also acted as a promoter during the linking reaction. The star polymers were fractionated to remove excess arm material and thoroughly characterized by size exclusion chromatography, matrix-assisted laser desorption ionization time-of-flight mass spectrometry, and light scattering.

Anionic block copolymerization of vinyl functionalized triphenylphosphines with styrene

The anionic polymerization of vinyl monomers with electron donating substituents was investigated. The 4- and 3-styryldiphenylphosphines were synthesized and homopolymerized at –85°C in tetrahydrofuran using sec-butyllithium as initiator. Block copolymerization studies were carried out using styrene as a comonomer. 4-Styryldiphenylphosphine (I) forms a much more stabilized anionic chain end than styrene (III). Thus only the sequence III/I is effective in block copolymerization. In contrast, 3-styryldiphenylphosphine (II) can be combined in both ways to block copolymers.

Anionic block copolymerization of vinyl functionalized triphenylphosphines with styrene

The anionic polymerization of vinyl monomers with electron donating substituents was investigated. The 4- and 3-styryldiphenylphosphines were synthesized and homopolymerized at –85°C in tetrahydrofuran using sec-butyllithium as initiator. Block copolymerization studies were carried out using styrene as a comonomer. 4-Styryldiphenylphosphine (I) forms a much more stabilized anionic chain end than styrene (III). Thus only the sequence III/I is effective in block copolymerization. In contrast, 3-styryldiphenylphosphine (II) can be combined in both ways to block copolymers.

Analysis of livingtert-butyl methacrylate oligoanions in tetrahydrofuran by NMR spectroscopy, viscosity and light scattering

Previous kinetic studies concluded on the aggregation of the anionic chain ends of living tert-butyl methacrylate oligomers [PtBMA - , Li + ] in THF at low temperature. This aggregation has been confirmed by viscosimetry, light scattering and gradient field NMR techniques in a range of temperature from 253 K to 298 K. These techniques have been systematically used for the analysis of both the living oligomers and the same chains recovered after hydrolysis of the anionic end groups. The structure of the growing anionic species was analyzed by means of 13 C and 7 Li NMR spectroscopy. 13 C NMR required the selective labeling of the last monomer units by 13 C atoms.

Investigation of the Reaction of Poly(styryl)lithium with Propylene Oxide

The room-temperature functionalization reaction between poly(styryl)lithium and propylene oxide in benzene was studied. The functionalized polymer was characterized by 1H NMR, 13C NMR, and APT-13C NMR. 13C-methyl-labeled propylene oxide-functionalized polystyrene was also synthesized and characterized. Hydroxylated polystyrene was obtained in only 93% yield, but with high regioselectivity (97% of addition of the anionic chain-end to the least substituted carbon of the propylene oxide ring) and no oligomerization. The lower than quantitative yield was ascribed to the proton-transfer reaction of poly(styryl)lithium from the methyl group of propylene oxide, which formed unfunctionalized polystyrene.

On the Anionic Polymerization of (Dialkylamino)isoprenes. 2. Influence of the Tertiary Amino Group on the Polymer Microstructure

The anionic polymerization of 5-(N,N-dialkylamino)isoprenes carrying various alkyl substituents was studied in nonpolar (benzene, hexane) and polar solvents (tetrahydrofuran, dioxane, triethylamine). The microstructure is found to be strongly dependent on the bulkiness of the tertiary amino group. All polymerizations in nonpolar solvents occur by a 4,1-addition of the monomer to the anionic chain end. In THF no polymerization occurs. The results are discussed with respect to the stereoelectronic situation at the growing chain end. For monomers with the most bully branched side groups (isopropyl, isobutyl, and 2,6-cis-dimethylpiperidyl) the addition is not only regioselective but also stereoselective, giving rise to the formation of stereoregular polymers with cis-4,1-repeating units