Control Of Polymer Molecular Weight Research Articles

Polymer Molecular Weight Determination via Fluorescence Lifetime.

Control of polymer molecular weight is critical for tailoring structure-function properties; however, traditional molecular weight characterization techniques have limited ability to determine the molecular weight of polymers in real time without sample removal from the reaction mixture, with spatial resolution, and of insoluble polymers. In this work, a fluorescence lifetime imaging microscopy (FLIM) method was developed that overcomes these limitations. The method is demonstrated with polynorbornene and polydicyclopentadiene, polymers derived from ruthenium-catalyzed ring-opening metathesis polymerization (ROMP). The polymer Mw, ranging from 35 to 570 kg/mol as determined by gel-permeation chromatography, was quantitatively correlated with the fluorescence lifetime. The revealed correlation then enabled time-resolved measurement of Mw during an ongoing ROMP reaction, requiring only 1 s per measurement (of a 45 μm × 45 μm polymer sample area), and provided spatial resolution, resulting in simultaneous characterization of polymer morphology. To provide the fluorescence signal, the initial reaction solutions contained a very low doping of a reactive norbornene monomer labeled with fluorescent boron dipyrromethene (BODIPY), such that 1 in every 107 monomers contained a fluorophore. The resulting FLIM visualization method enables the rapid determination of the molecular weights of growing polymers without removal from the reaction mixture and regardless of polymer solubility.

Dehydropolymerization of H3B·NMeH2 Using a [Rh(DPEphos)]+ Catalyst: The Promoting Effect of NMeH2.

[Rh(κ2-PP-DPEphos){η2η2-H2B(NMe3)(CH2)2tBu}][BArF4] acts as an effective precatalyst for the dehydropolymerization of H3B·NMeH2 to form N-methylpolyaminoborane (H2BNMeH)n. Control of polymer molecular weight is achieved by variation of precatalyst loading (0.1–1 mol %, an inverse relationship) and use of the chain-modifying agent H2: with Mn ranging between 5 500 and 34 900 g/mol and Đ between 1.5 and 1.8. H2 evolution studies (1,2-F2C6H4 solvent) reveal an induction period that gets longer with higher precatalyst loading and complex kinetics with a noninteger order in [Rh]TOTAL. Speciation studies at 10 mol % indicate the initial formation of the amino–borane bridged dimer, [Rh2(κ2-PP-DPEphos)2(μ-H)(μ-H2BN=HMe)][BArF4], followed by the crystallographically characterized amidodiboryl complex [Rh2(cis-κ2-PP-DPEphos)2(σ,μ-(H2B)2NHMe)][BArF4]. Adding ∼2 equiv of NMeH2 in tetrahydrofuran (THF) solution to the precatalyst removes this induction period, pseudo-first-order kinetics are observed, a half-order relationship to [Rh]TOTAL is revealed with regard to dehydrogenation, and polymer molecular weights are increased (e.g., Mn = 40 000 g/mol). Speciation studies suggest that NMeH2 acts to form the precatalysts [Rh(κ2-DPEphos)(NMeH2)2][BArF4] and [Rh(κ2-DPEphos)(H)2(NMeH2)2][BArF4], which were independently synthesized and shown to follow very similar dehydrogenation kinetics, and produce polymers of molecular weight comparable with [Rh(κ2-PP-DPEphos){η2-H2B(NMe3)(CH2)2tBu}][BArF4], which has been doped with amine. This promoting effect of added amine in situ is shown to be general in other cationic Rh-based systems, and possible mechanistic scenarios are discussed.

Replacing sulfonate by carboxylate: application of pyridyliminocarboxylato copper(II) complexes in rac-lactide polymerization and Chan–Evans–Lam coupling

Copper(II) complexes carrying pyridylmethyleneaminobenzoate or –propanoate ligands, LCuX, were prepared in one-pot reactions from pyridinecarboxaldehyde, aminobenzoic acid or β-alanine, and CuX2 (X = Cl, NO3, OAc, or OTf). All complexes were characterized by single-crystal X-ray diffraction studies and formed either dimers, tetramers, or coordination polymers. Attempted preparation of the respective alkoxide complexes, LCu(OR), was unsuccessful, but use of LCuX/NaOMe mixtures in rac-lactide polymerization indicated under some conditions coordination–insertion polymerization via a copper alkoxide as the mechanism. The complexes performed poorly in rac-lactide polymerization, showing low activities (12 h to completion at 140 °C), low to moderate heterotacticity (Pr = 0.6–0.8), and poor polymer molecular weight control (intramolecular transesterification). They were competent catalysts for Chan–Evans–Lam couplings with phenylboronic acid, without any indication of side reactions such as deboration or aryl homocoupling. The complexes were active in undried methanol, without addition of base, ligand, or molecular sieves. Aniline, n-octylamine, and cyclohexylamine were coupled quantitatively under identical reaction conditions. There is only little influence of the anion on activities (less than a factor of 2) but a strong influence on induction periods. The complexes were not active in CEL coupling with alcohols, phenols, or alkylboronic acids.

Biobased poly(vinyl ether)s derived from soybean oil, linseed oil, and camelina oil: Synthesis, characterization, and properties of crosslinked networks and surface coatings

A series of novel plant oil (PO)-based poly(vinyl ether)s were produced that varied with respect to PO composition and molecular weight (MW). The POs investigated were soybean oil, linseed oil, and camelina oil. All of the polymers were liquids at room temperature and were used to produce crosslinked networks, both as free-standing films and as surface coatings on steel substrates. Crosslinking was achieved at ambient conditions through the process of autoxidation. Viscosity of the neat polymers as well as the viscoelastic and mechanical properties of crosslinked networks were highly dependent on parent PO composition. At a given polymer MW, viscosity decreased with increasing PO unsaturation, while glass transition temperature, Young’s modulus, and tensile strength of crosslinked networks increased with increasing PO unsaturation. For polymers derived from the most highly unsaturated PO, i.e. linseed oil, impact resistance of coatings was significantly compromised, due to the relatively high crosslink density of these coatings. Overall, these results demonstrated that viscosity and the properties of crosslinked films based on these novel PO-based poly(vinyl ether)s could be tailored through selection of the parent PO and control of polymer MW. This class of highly bio-based polymers appears to have particular utility for the production of one-component, ambient-cured coatings. One component, ambient-cured thermoset coatings are highly desired because of their ease of use, lower waste production, and energy cost savings compared to other thermoset coating systems.

Biodegradable PHB from rac-β-Butyrolactone: Highly Controlled ROP Mediated by a Pentacoordinated Aluminum Complex

The diphenoxyimine five-coordinated aluminum complex [AlMeL2] (1), (L = N-(2,6-diisopropylphenyl)phenoxyimine), has been synthesized and fully characterized. The mononuclear structure observed in the solid state by X-ray diffraction analysis is maintained in solution, as shown by diffusion-ordered NMR spectroscopy (DOSY). Complex 1 is an active ROP catalyst able to provide well-controlled living ring-opening polymerization of rac-BBL at 100 °C, with PDI = 1.03, in the presence of BnOH to give the biodegrable polyester poly(hydroxybutyrate) (PHB). The suggested active species of this catalytic process, the aluminum complex [Al(OBn)L2] (2), has been synthesized and fully characterized. DFT calculations have been carried out to study the influence of the catalytic active species pocket over the polymer molecular weight control. Moreover, 1 is also active in the copolymerization of rac-β-butyrolactone and l-lactide to provide random copolymers.

Ultrafast Observation of a Photoredox Reaction Mechanism: Photoinitiation in Organocatalyzed Atom-Transfer Radical Polymerization.

Photoredox catalysis has driven a revolution in the field of organic chemistry, but direct mechanistic insights into reactions of genuine synthetic utility remain relatively scarce. Herein we report ultrafast time-resolved spectroscopic observation of a bimolecular organocatalyzed photoredox reaction, from catalyst photoexcitation through to photoinduced electron transfer (PET) and intermediate formation, using transient vibrational and electronic absorption spectroscopy with sub-picosecond time resolution. Specifically, the photochemical dynamics of initiation in organocatalyzed atom-transfer radical polymerization (O-ATRP) are elucidated for two complementary photoredox organocatalysts (N,N-diaryl-5,10-dihydrophenazines). Following photoexcitation, a dissociative bimolecular electron transfer is observed from the first excited singlet state of both photocatalysts to methyl 2-bromopropionate in dichloromethane, toluene, and dimethylformamide. The photocatalyst excited donor state, ground state, and radical cation are tracked in real time alongside the debrominated radical fragment. Our work challenges previously proposed mechanisms of initiation in O-ATRP and indicates that PET from short-lived excited singlet states can exert control of polymer molecular weight and dispersity by suppressing the steady-state concentration of the reactive debrominated radical. More broadly, we aim to demonstrate the potential of ultrafast absorption spectroscopy to observe directly transient, open-shell intermediates in mechanistic studies of photoredox catalysis.

High pressure ethylene polymerization with a post‐metallocene bis‐phenyl phenoxy catalyst

ABSTRACTThe use of post‐metallocene bis‐phenylphenoxy catalysts to polymerize ethylene under high ethylene pressures (>25,000 psi) results in some remarkable catalytic properties. The high ethylene pressure produces molar ethylene concentrations in the reactor as much as 40 times higher than in typical low pressure ethylene polymerizations. This high ethylene concentration results in high catalyst efficiency at high temperatures and low reactor residence time, between 180 °C and 240 °C the catalyst efficiency surprisingly increases with increasing temperature, allowing for use of these catalysts at temperatures much higher than can be utilized in the low pressure processes. It has further been demonstrated that under these conditions increasing hydrogen levels up to 0.5 mol% does not significantly affect the polymer molecular weight; however, polymer molecular weight control can be realized with varying reactor temperature. The polymer produced is shown to be high density polyethylene made from a single site catalyst and not free radical initiated low density polymer. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017, 55, 861–866

Automatic Control of Polymer Molecular Weight during Synthesis

The continuous record of monomer and polymer concentrations, Cm and Cp, and cumulative weight-average mass, Mw, furnished by automatic continuous online monitoring of polymerization reactions (ACOMP) has been harnessed to provide feedback to control reactor monomer flow in order to follow a target trajectory Mw,t(t) during linear chain growth free radical polymerization. This was achieved without a detailed kinetic model. Two proportionality parameters to pilot the controller, α and p, result from (i) reaction rate = αCm and (ii) Mw,inst = pCm, where Mw,inst is instantaneous Mw. Using Ansatz values for α and p, the controller periodically recomputes these, based on the ACOMP data stream, in order to follow Mw,t(t). A histogram of concentration vs Mw,inst estimates the molecular weight distribution width. Invoking an instantaneous distribution provides polydispersities. Results are compared to GPC analysis on end products. The concept of “isomorphic reaction pair” is introduced: two reactions that follow t...

Optimization of PDTS-DTffBT-Based Solar Cell Performance through Control of Polymer Molecular Weight

Herein, we investigated the effect of molecular weight (MW) on the efficiency of PDTS-DTffBT based polymer solar cells (PSCs). PDTS-DTffBTs with three different MWs were synthesized by controlling the polymerization conditions. The performance of PSCs improved significantly as the number-average molecular weight (Mn) increased from 9 to 36 kDa. Combined with UV–vis absorption and electrochemical cyclic voltammetry measurements, the absorption properties and frontier orbital energy levels of the polymers were estimated, indicating the red-shifted light absorption and up-shifted highest occupied molecular orbital (HOMO) energy level when MW increased. PDTS-DTffBT with high MW also provided increased charge mobility, smoother film surface, and reduced domain size in morphology of the PDTS-DTffBT:PC71BM active layer. The performance of PDTS-DTffBT based PSCs was improved owing to these MW related properties, and both short circuit current density (JSC) and power conversion efficiency (PCE) went up significantly with increasing MW. The best PCE of 6.40% was achieved by the devices based on the PDTS-DTffBT with a Mn value of 36 kDa.

Modeling the Influence of Diffusion-Controlled Reactions and Residual Termination and Deactivation on the Rate and Control of Bulk ATRP at High Conversions

In high-conversion atom transfer radical polymerization (ATRP), all the reactions, such as radical termination, radical deactivation, dormant chain activation, monomer propagation, etc. could become diffusion controlled sooner or later, depending on relative diffusivities of the involved reacting species. These diffusion-controlled reactions directly affect the rate of polymerization and the control of polymer molecular weight. A model is developed to investigate the influence of diffusion-controlled reactions on the high conversion ATRP kinetics. Model simulation reveals that diffusion-controlled termination slightly increases the rate, but it is the diffusion-controlled deactivation that causes auto-acceleration in the rate (“gel effect”) and loss of control. At high conversions, radical chains are “trapped” because of high molecular weight. However, radical centers can still migrate through (1) radical deactivation–activation cycles and (2) monomer propagation, which introduce “residual termination” reactions. It is found that the “residual termination” does not have much influence on the polymerization kinetics. The migration of radical centers through propagation can however facilitate catalytic deactivation of radicals, which improves the control of polymer molecular weight to some extent. Dormant chain activation and monomer propagation also become diffusion controlled and finally stop the polymerization when the system approaches its glass state.

Mechanistic studies of the dehydrocoupling and dehydropolymerization of amine-boranes using a [Rh(Xantphos)]⁺ catalyst.

A detailed catalytic, stoichiometric, and mechanistic study on the dehydrocoupling of H3B·NMe2H and dehydropolymerization of H3B·NMeH2 using the [Rh(Xantphos)](+) fragment is reported. At 0.2 mol % catalyst loadings, dehydrocoupling produces dimeric [H2B-NMe2]2 and poly(methylaminoborane) (M(n) = 22,700 g mol(-1), PDI = 2.1), respectively. The stoichiometric and catalytic kinetic data obtained suggest that similar mechanisms operate for both substrates, in which a key feature is an induction period that generates the active catalyst, proposed to be a Rh-amido-borane, that reversibly binds additional amine-borane so that saturation kinetics (Michaelis-Menten type steady-state approximation) operate during catalysis. B-N bond formation (with H3B·NMeH2) or elimination of amino-borane (with H3B·NMe2H) follows, in which N-H activation is proposed to be turnover limiting (KIE = 2.1 ± 0.2), with suggested mechanisms that only differ in that B-N bond formation (and the resulting propagation of a polymer chain) is favored for H3B·NMeH2 but not H3B·NMe2H. Importantly, for the dehydropolymerization of H3B·NMeH2, polymer formation follows a chain growth process from the metal (relatively high degrees of polymerization at low conversions, increased catalyst loadings lead to lower-molecular-weight polymer), which is not living, and control of polymer molecular weight can be also achieved by using H2 (M(n) = 2,800 g mol(-1), PDI = 1.8) or THF solvent (M(n) = 52,200 g mol(-1), PDI = 1.4). Hydrogen is suggested to act as a chain transfer agent in a similar way to the polymerization of ethene, leading to low-molecular-weight polymer, while THF acts to attenuate chain transfer and accordingly longer polymer chains are formed. In situ studies on the likely active species present data that support a Rh-amido-borane intermediate as the active catalyst. An alternative Rh(III) hydrido-boryl complex, which has been independently synthesized and structurally characterized, is discounted as an intermediate by kinetic studies. A mechanism for dehydropolymerization is suggested in which the putative amido-borane species dehydrogenates an additional H3B·NMeH2 to form the "real monomer" amino-borane H2B═NMeH that undergoes insertion into the Rh-amido bond to propagate the growing polymer chain from the metal. Such a process is directly analogous to the chain growth mechanism for single-site olefin polymerization.

Controlled Radical Polymerization at High Conversion: Bulk ICAR ATRP of Methyl Methacrylate

Methyl methacrylate (MMA) was polymerized in bulk via atom transfer radical polymerization (ATRP) with azobis(isobutyronitrile) (AIBN) and tert-butyl peroxybenzoate (TBPB) as initiator for continuous activator regeneration (ICAR). The ATRP was first carried out up to high conversion at 70 °C. When the system reached its glass state at about 94%, the polymerization temperature was elevated to 120 °C to maximize the conversion. Livingness of the ICAR ATRP process and control of polymer molecular weight at high conversion were carefully examined to elucidate the mechanisms of diffusion-controlled reactions. The “gel effect” started at about 50% conversion, and it was accompanied by molecular weight distribution broadening. A dramatic increase in polydispersity was observed at above 90% conversion. The loss of control over molecular weight was caused by diffusion-controlled deactivation, which could be reduced to some extent by increasing deactivator concentration. However, despite the loss of control at high conversion, livingness of the system did not suffer, owing to diffusion-controlled termination. The polymer chains collected at very high conversion could be easily extended with high efficiency.

2‐(Vinyloxy)ethyl soyate as a versatile platform chemical for coatings: An overview

A plant oil‐based polymer technology was recently developed that enables high molecular weight polyvinylether polymers to be produced using cationic polymerization. The plant oil monomer synthesis involves simple base‐catalyzed transesterification of a plant oil triglyceride with a hydroxy‐functional vinyl, ether such as 4‐(vinyloxy)butanol or 2‐(vinyloxy)ethanol, to produce a mixture of vinyl ether monomers that possess fatty acid ester groups in their structure. A key aspect of the technology involves the polymerization process that allows for high molecular weight polymers to be produced by polymerization exclusively through the vinyl ether double bonds. This feature enables polymers to be produced that possess unsaturation in the fatty acid ester pendent groups derived from the plant oil starting material. As has been done for conventional plant oil triglycerides, the double bonds can be used to produce thermoset materials either directly through an oxidative process or by derivatization to produce other functional groups, such as epoxides, acrylates, and hydroxyls that can be subsequently used to produce crosslinked networks. The polymerization process also results in a “living” polymerization that allows for control of polymer molecular weight, narrow molecular weight distributions, and the production of unique polymer architectures such as block copolymers. Another very important feature of the technology involves the ability to copolymerize the plant oil‐based vinyl ether monomers with other monomers to tailor polymer properties such as glass transition temperature, mechanical properties, and solubility/compatibility with other materials. This document provides an overview of the present state of the technology using soybean oil as a representative plant oil source.

Emulsion polymerization of vinyl acetate using iodine-transfer and RAFT radical polymerizations

This study deals with control of the molecular weight and molecular weight distribution of poly(vinyl acetate) by iodine-transfer radical polymerization and reversible addition-fragmentation transfer (RAFT) emulsion polymerizations as the first example. Emulsion polymerization using ethyl iodoacetate as the chain transfer agent more closely approximated the theoretical molecular weights than did the free radical polymerization. Although 1H NMR spectra indicated that the peaks of α- and ω-terminal groups were observed, the molecular weight distributions show a relatively broad range (Mw/Mn = 2.2–4.0). On the other hand, RAFT polymerizations revealed that the dithiocarbamate 7 is an excellent candidate to control the polymer molecular weight (Mn = 9.1 × 103, Mw/Mn = 1.48), more so than xanthate 1 (Mn = 10.0 × 103, Mw/Mn = 1.89) under same condition, with accompanied stable emulsions produced. In the Mn versus conversion plot, Mn increased linearly as a function of conversion. We also performed seed-emulsion polymerization using poly(nonamethylene L-tartrate) as the chiral polyester seed to fabricate emulsions with core-shell structures. The control of polymer molecular weight and emulsion stability, as well as stereoregularity, is also discussed. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013

Emulsion copolymerization of styrene and butyl acrylate by reverse atom transfer radical polymerization

AbstractReverse atom transfer radical copolymerization of styrene (St) and butyl acrylate was carried out in emulsion under normal emulsion conditions, using CuBr2/bpy complex as catalyst. The effects of surfactant type, initiator type and concentration, and CuBr2 addition on the system livingness, polymer molecular weight control, and latex stability were examined in detail. It was found that the Polysorbate 80 (Tween 80) and azodiisobutyronitrile gave the best exhibition in this system, polymer samples were got with narrow molecular‐weight dispersity (Mw/Mn = 1.1–1.2) and linear relationships of molecular weight versus monomer conversion, as well as a relatively low polydispersity index (<0.1). Through the GPC and SEM analysis, the polymerization processes under these conditions showed good living/control characteristics relative to the processes under normal emulsion polymerization, although the latex stability was susceptible to the CuBr2 catalyst. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

A homogeneous, recyclable polymer support for Rh(I)-catalyzed C-C bond formation.

A robust and practical polymer-supported, homogeneous, recyclable biphephos rhodium(I) catalyst has been developed for C-C bond formation reactions. Control of polymer molecular weight allowed tuning of the polymer solubility such that the polymer-supported catalyst is soluble in nonpolar solvents and insoluble in polar solvents. Using the supported rhodium catalysts, addition of aryl and vinylboronic acids to the electrophiles such as enones, aldehydes, N-sulfonyl aldimines, and alkynes occurs smoothly to provide products in high yields. Additions of terminal alkynes to enones and industrially relevant hydroformylation reactions have also been successfully carried out. Studies show that the leaching of Rh from the polymer support is low and catalyst recycle can be achieved by simple precipitation and filtration.

Hybrid atom transfer radical polymerization system for balanced polymerization rate and polymer molecular weight control

A hybrid polymerization system that combines the fast reaction kinetics of conventional free radical polymerization and the control of molecular weight and distribution afforded by ATRP has been developed. High-free radical initiator concentrations in the range of 0.1–0.2 M were used in combination with a low concentration of ATRP catalyst. Conversions higher than 90% were achieved with ATRP catalyst concentrations of less than 20 ppm within 2 h for the hybrid ATRP system as compared with ATRPs where achieving such conversions would take up to 24 h. These reaction conditions lead to living polymerizations where polymer molecular weight increases linearly with monomer conversion. As in living polymerization and despite the fast rates and low ATRP catalyst concentrations, the polydispersity of the produced polymer remained below 1.30. Chain extension experiments from a synthesized macroinitiator were successful, which demonstrate the living characteristics of the hybrid ATRP process. Catalyst concentrations as low as 16 ppm were found to effectively mediate the growth of over 100 polymer chains per catalytic center, whereas at the same time negating the need for post polymerization purification given the low-catalyst concentration. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 2294–2301, 2010

RAFT GRAFTING POLYMERIZATION OF MMA/St FROM SURFACE OF SILICON WAFER

Polystyrene(PSt),polymethylmethacrylate(PMMA) and copolymers(poly(MMA-co-St)) were grafted from silicon wafer via RAFT agent that was anchored to the surface.X-ray photoelectron spectroscopy showed that while the polymers were successfully grafted,the control of polymer molecular weight(MW) behaved very differently.The Si-PMMA system yielded MW much higher than the theoretically targeted,broad molecular weight distribution and low grafting density(ca.0.03 chains/nm2).In contrast,the Si-PSt system gave MW close to the theoretical value with polydispersities around 1.3.The grafting density increased to 0.21 chains/nm2.In Si-poly(MMA-co-St) system,the polymer layer thickness increased linearly with monomer conversion and molecular weight of free polymer in solution,suggesting that the RAFT copolymerization grafting of MMA and St from silicon wafer was well controlled.The MW was close to the theoretical value.The grafting density reached 0.31 chains/nm2,which was higher than those obtained in their homo-polymerizations.

A homogeneous, recyclable rhodium(I) catalyst for the hydroarylation of Michael acceptors.

A robust and practical polymer-supported, recyclable biphephos rhodium(I) catalyst has been developed. Control of polymer molecular weight allowed the tuning of solubility such that the polymer-supported catalyst is soluble in nonpolar solvents and not soluble in polar solvents. Thus, catalytic addition of aryl- and vinylboronic acids to enones occurs under completely homogeneous conditions and catalyst recycle can be achieved by simple precipitation and filtration.

Smart Polymer‐Streptavidin Conjugates

AbstractFor Abstract see ChemInform Abstract in Full Text.