Monte Carlo Simulations of Atom Transfer Radical (Homo)polymerization of Divinyl Monomers: Applicability of Flory–Stockmayer Theory

It is well known that free radical (co)­polymerization of multivinyl monomers (MVMs) leads to insoluble gels even at a low monomer conversion, and the gelation point can be predicted by Flory–Stockmayer theory (F–S theory) based on two assumptions: (1) equal reactivity of all vinyl groups and (2) the absence of intramolecular cyclization. This theory has been experimentally studied and verified with conventional free radical (co)­polymerization (FRP) of several MVMs (e.g., divinylbenzene, DVB). However, it is still debatable whether this theory is applicable for the polymerization of MVMs using reversible deactivation radical polymerization (RDRP) approaches, such as atom transfer radical polymerization (ATRP). Herein, Monte Carlo simulations using two statistical modelswith cyclization (<b>w.c.</b>) and without cyclization (<b>wo.c.</b>, corresponding to F–S theory)and dynamic lattice liquid (DLL) models were conducted to study ATRP of divinyl monomers. The simulated gel points using <b>w.c.</b> and <b>wo.c.</b> models were compared with those obtained from ATRP experiments, from calculation using F–S theory, and from simulations using DLL models. The molecular weights, dispersity, and extent of intermolecular/intramolecular cross-linking were calculated as a function of double bond and cross-linker conversion. The results demonstrated that the gel points obtained from both <b>w.c.</b> and <b>wo.c.</b> models were lower than the values from DLL models and experiments. This indicates that F–S theory cannot be used to accurately predict the polymerization of divinyl monomers via ATRP. Our study shows that the limitation of F–S theory in predicting ATRP reaction of divinyl monomers is not only due to neglecting intramolecular cyclization but also due to spatial restrictions which can cause the reactivity and accessibility of vinyl groups becoming nonequivalent in ATRP of divinyl monomers

Solvent Effects on the Activation Rate Constant in Atom Transfer Radical Polymerization

Rate constants of activation (<i>k</i>_act) for the reactions of tertiary alkyl halides with the ATRP catalyst Cu^IBr/1,1,4,7,10,10-hexamethyltriethylenetetramine (HMTETA) have been determined in 14 different solvents. The measurements have been performed at 25 °C by spectrophotometrically following the time-dependent absorbances of the Cu^II species. A large excess of 2,2,6,6-tetramethylpiperidinyl-1-oxyl (TEMPO), which quantitatively trapped the alkyl radicals, ensured the irreversible generation of Cu^II. The rate constant for the least active solvent butanone is 30 times smaller than that of the most active solvent DMSO. In addition, the effect of increasing amounts of monomer in a solvent on the activation rate has been analyzed. A linear correlation of activation rate constants with previously determined equilibrium constants (<i>K</i>_ATRP) provides a Leffler–Hammond coefficient of 0.45. However, the activation rate constants do not correlate with dielectric constants and Dimroth’s and Reichardt’s <i>E</i>_T(30) values. Application of the linear solvation energy relationship of Kamlet and Taft revealed that the dipolarity/polarizability π* of the solvent, i.e., nonspecific solvent–solute interactions, mainly accounts for solvent effects on <i>k</i>_act, while the ability to donate a free electron pair is important for some solvents. Quantum chemical calculations showed that more polar solvents stabilize the Cu^II product complex to a higher degree than the Cu^I starting complex

Expanding the ATRP Toolbox: Methacrylate Polymerization with an Elemental Silver Reducing Agent

The atom transfer radical polymerization (ATRP) of methacrylates using Ag⁰ as a reducing agent was carried out. Optimized reaction conditions enabled the controlled preparation of polymethacrylates using elemental silver and a CuBr₂/PMDETA catalyst, with polymer dispersity values down to <i><i>Đ</i></i> = 1.06. In these reactions, the formation of AgBr was observed as a dark coating on the surface of the silver wire; however, the AgBr coating had minimal effect on polymerization, and the same silver wire could be used in several consecutive reactions without elaborate cleaning. Different polymethacrylates were prepared with good control, and a poly­(methyl methacrylate)-<i>b</i>-poly­(benzyl methacrylate)-<i>b</i>-poly­(ethyl methacrylate)-Br triblock copolymer was prepared with a molecular weight of 32 200 and a dispersity of <i>Đ</i> = 1.07. Additionally, it was shown that silver can act as a supplemental activator in the generation of radicals from ethyl α-bromophenylacetate, with a rate constant of surface activation of <i>k</i>_a0^app = 9.1 × 10^–6 cm s^–1

Temporal Control in Atom Transfer Radical Polymerization Using Zerovalent Metals

Polymer chain growth can be controlled in a spatiotemporal manner by external stimuli including chemical, redox, light, electrical current, and mechanical procedures. In this paper, atom transfer radical polymerization (ATRP) was investigated in the presence of zerovalent metals, such as copper or silver wire, as chemical stimuli to assert temporal control over ATRP reactions. The ATRP activator, L/Cu^I complex, was (re)­generated, or catalyst was switched “on”, in the presence of the metal wires to start the reaction whereas removing the wire from the solution stopped activator (re)­generation. In the absence of the metal zero wires, the residual activator in the solution was consumed by irreversible radical termination processes converting activators to deactivator speciescatalyst switched “off”and hence polymerization stopped. However, the nature of the ligand played a crucial role in defining the concentrations of deactivator and activator species, [L/Cu^II]/[L/Cu^I], present in the polymerization medium. More active catalysts shifted the ATRP equilibrium toward a higher concentration of L/Cu^II and hence lower concentration of activator L/Cu^I. In this case the reaction quickly stopped in the absence of metal wires. On the other hand, lower activity catalysts provided a higher concentration of the L/Cu^I species that took a longer time to be consumed by radical termination processes so that the reactions continued for longer times in the absence of the wires

Kinetics of Fe-Mediated ATRP with Triarylphosphines

Phosphines have been successfully used as additives for Fe-based ATRP. To understand their role in detail, the kinetics of Fe-based ATRP in the presence of tris­(2,4,6-trimethoxy­phenyl)­phosphine (TTMPP) and of triphenyl­phosphine (TPP) was studied via online vis/near-IR and ³¹P NMR spectroscopy. No significant effect on the ATRP equilibrium constant, <i>K</i>_ATRP, was observed upon the addition of up to 1.5 equiv of phosphine relative to Fe. The strongly Lewis basic TTMPP may coordinate to Fe^II species, but primarily it acts as a reducing agent for [Fe^IIIBr₄]⁻ in ATRP above 100 °C, thus transforming TTMPP to TTMPP-Br⁺. TPP is a weaker Lewis base and consequently a less effective reducing agent

Protection of Opening Lids: Very High Catalytic Activity of Lipase Immobilized on Core–Shell Nanoparticles

Various hydrophobic supports have been used for lipase immobilization since the active site of lipase can be opened in a hydrophobic environment. Nevertheless, the increase of lipase activity is still limited. This study demonstrates a hyperactivation-protection strategy of lipase after immobilization on poly­(<i>n</i>-butyl acrylate)–polyaldehyde dextran (PBA–PAD) core–shell nanoparticles. The inner hydrophobic PBA domain helps to rearrange lipase conformation to a more active form after immobilization into the PAD shell. More importantly, the outer PAD shell with dense polysaccharide chains prevents the immobilized lipase from contacting with outside aqueous medium and reverting its conformation back to an inactive form. As a result, under optimal conditions the activity of lipase immobilized in PBA–PAD nanoparticles was enhanced 40 times over the free one, much higher than in any previous report. Furthermore, the immobilized lipase retained more than 80% of its activity after 10 reaction cycles

Photoinduced Iron-Catalyzed Atom Transfer Radical Polymerization with ppm Levels of Iron Catalyst under Blue Light Irradiation

Photoinduced atom transfer radical polymerization (ATRP) has been mainly explored using copper-based catalytic systems. Recently developed iron-catalyzed photochemical ATRP employed high amounts of iron catalysts under high-energy UV light irradiation. Herein, a successful photoinduced iron-catalyzed ATRP mediated under blue light irradiation with ppm amounts (100–400 ppm) of iron­(III) bromide/tetrabutyl­ammonium bromide as the catalyst is reported. Several methacrylate monomers were polymerized with excellent control providing molecular weight in good agreement with theoretical values and low dispersity (<i>Đ</i> < 1.20). Near-quantitative monomer conversions (∼95%) enabled <i>in situ</i> chain extension and block copolymerization, indicating high retention of chain-end functionality. Notably, this system can tolerate oxygen, enabling synthesis of well-defined polymers with good chain-end functionality in the presence of air

Initiators for Continuous Activator Regeneration Atom Transfer Radical Polymerization of Methyl Methacrylate and Styrene with <i>N</i>‑Heterocyclic Carbene as Ligands for Fe-Based Catalysts

Iron-based <i>N</i>-heterocyclic carbene (FeX₃(NHC)) complexes were used in initiators for continuous activator regeneration atom transfer radical polymerization (ICAR ATRP). A series of ICAR ATRPs of methyl methacrylate (MMA) and styrene (St) were carried out using FeX₃(IDipp) where IDipp = 1,3-bis­(2,6-diisopropylphenyl)­imidazol-2-ylidene with X = Cl (<b>I–Cl</b>) or Br (<b>I–Br</b>) and FeX₃(HIDipp) (HIDipp =1,3-bis­(2,6-diisopropylphenyl)­imidazolidin-2-ylidene) with X = Cl (<b>H–Cl</b>) or Br (<b>H–Br</b>). The polymerizations showed good activity, resulting in polymers with controlled molecular weights and narrow molecular weight distribution (MWD) with low loading of catalysts (50 ppm). In particular, <b>I–Br</b> and <b>H–Br</b> generated polymers with narrower MWD. For example, ICAR ATRP of MMA with 50 ppm catalysts (<b>H–Br</b>) after 24 h at <i>T</i> = 60 °C in 50% v/v anisole resulted in PMMA with <i>M</i>_w/<i>M</i>_n = 1.20 at 65% monomer conversion. ICAR ATRP of St after 72 h resulted in PSt with <i>M</i>_w/<i>M</i>_n = 1.15 at 53% monomer conversion

Iron-Based ICAR ATRP of Styrene with ppm Amounts of Fe^IIIBr₃ and 1,1′-Azobis(cyclohexanecarbonitrile)

A successful ICAR (initiators for continuous activator regeneration) ATRP (atom transfer radical polymerization) of styrene was conducted with iron­(III) bromide and 1,1′-azobis­(cyclohexanecarbonitrile) (ACHN) as the thermal initiator. A polymerization, started with 50 ppm of FeBr₃ and 50 mol equivalents of ACHN in 33% (v/v) anisole at 90 °C, reached 70% conversion in 24 h and was well controlled, giving a polymer with a narrow molecular weight distribution (<i>M</i>_w/<i>M</i>_n = 1.15). The number average molecular weight (<i>M</i>_n) corresponded well to theoretical values, as conversion increased. The rate of polymerization was dependent on the amount of ACHN initially added to the reaction. A polymer with a relatively narrow molecular weight distribution, <i>M</i>_w/<i>M</i>_n = 1.29 at 65% of conversion, was obtained with 5 ppm of FeBr₃ and the appropriate amount of ACHN. This procedure therefore provides an efficient controlled polymerization in addition to creating a robust, cheap, and environmentally friendly catalytic system. Control of polymerization with ACHN was better than with <i>tert</i>-butyl peracetate as a thermal initiator or tin­(II) 2-ethylhexanoate, Fe⁰, or Zn⁰ wire as reducing agents

Photoinduced Miniemulsion Atom Transfer Radical Polymerization

Photomediated atom transfer radical polymerization (photoATRP) of (meth)­acrylic monomers was conducted in miniemulsion media. The polymerization procedures took advantage of an ion-pair catalyst formed by interaction of Cu/TPMA² (TPMA = tris­(2-pyridylmethyl)­amine) and an anionic surfactant, sodium dodecyl sulfate (SDS). The ion-pair catalyst was efficient in controlling ATRP reactions with catalyst loadings as low as 100 ppm. The effect of different polymerization parameters, such as the size of the reaction vial, amount of surfactant, and solids content influencing the photoATRP in miniemulsion, was studied. The polymerization was conducted with solids content ranging from 5 to 50 vol % under a moderate surfactant loading (<5 wt % relative to monomer). Excellent temporal control was achieved upon switching the UV light on and off multiple times, and the polymer was successfully chain extended, indicating high retention of chain-end fidelity