Alk‐1‐ene Polymerization in the Presence of a Monocyclopentadienyl Zirconium(IV) Acetamidinate Catalyst: Microstructural and Mechanistic Insights

Suitably activated, (Cp*){N(^tBu)C(Me)N(Et)}ZrMe₂ is known to initiate the ‘living’ and isotactic‐selective polymerization of alk‐1‐enes, and it can be used to synthesize block copolymers and stereoblock polymers. We report a full molecular kinetic investigation of propene, but‐1‐ene, and hex‐1‐ene polymerization with a MAO‐activated catalyst system. By combining NMR microstructural polymer analysis with QM modeling of the active species, the complicated regio‐ and stereochemistry of the polyinsertion process, as well as the active chain‐transfer pathways, are investigated. The perspectives and limitations of this catalyst for application in (stereo)block polymerizations are discussed.

     

Chain Transfer and Efficiency of End‐Group Introduction in Free Radical Polymerization of Methyl Methacrylate in the Presence of Poly(methyl methacrylate) Macromonomer

Summary: Experimental and modeling studies of addition–fragmentation chain transfer (AFCT) during radical polymerization of methyl methacrylate in the presence of poly(methyl methacrylate) macromonomer with 2‐carbomethoxy‐2‐propenyl ω‐ends (PMMA‐CO₂Me) at 60 °C are reported. The results revealed that AFCT involving PMMA‐CO₂Me formed in situ during methyl methacrylate polymerization has a negligible effect on the molecular weight distribution.

     

Synthesis and Dispersion Characteristics of Multi‐Walled Carbon Nanotube Composites with Poly(methyl methacrylate) Prepared by In‐Situ Bulk Polymerization

Multi‐walled carbon nanotubes (MWNT) purified by acidic solution were processed with PMMA via an in‐situ polymerization. Experimental evidences indicate the role of radical initiator (AIBN) and MWNT, showing increases of polymerization rate and MWNT diameter. Induced radicals on the MWNT by AIBN were found to trigger the grafting of PMMA. Moreover, the solvent cast film showed a better nanoscopic dispersion of MWNT and possibilities of CNT composites in engineering applications.

Fractured surface of multi‐walled carbon nanotube composite with PMMA prepared by in‐situ bulk polymerization.     

One‐Component Bimolecular Photoinitiating Systems, 2

The compounds 2‐thioxanthone‐thioacetic acid and 2‐(carboxymethoxy)thioxanthone, bimolecular photoinitiators for free radical polymerization, are synthesized and characterized. Their capability to act as initiators for the polymerization of methyl methacrylate was examined. The postulated mechanism is based on the intermolecular electron‐transfer reaction of the excited photoinitiator with the sulfur or oxygen atom of the ground state of the respective photoinitiator followed by decarboxylation. The resulting alkyl radicals initiate the polymerization.

Structures of the photoinitiators.     

Synthesis of a Multi Alternating‐Arm‐Containing Dendritic Star Copolymer by RAFT and Cationic Ring‐Opening Polymerization

A new dendritic heteroarm star copolymer that contains multi‐alternating arms of poly(ethylene oxide‐tetrahydrofuran) (P(EO‐THF)) and poly(methyl methacrylate) (PMMA) on a dendritic polyester core has been synthesized by a ‘core‐first’ approach by combination of sequential cationic ring‐opening polymerization (CROP) and reversible addition–fragmentation transfer (RAFT) polymerization initiated by a dendritic macroinitiator ( 3 ) capped with multi‐alternating terminal carboxylic acid groups (used directly to initiate the ROP of THF in the presence of EO as a polymerization promoter to attain P(EO‐THF) arms) and dithiobenzoate groups (used to initiate RAFT polymerization of MMA to attain PMMA arms). The structures of the products were confirmed by NMR spectroscopy, GPC‐MALLS, and DSC measurements.

     

Reversible Photocontrolled Swelling‐Shrinking Behavior of Micron Vesicles Self‐Assembled from Azopyridine‐Containing Diblock Copolymer

Poly(N‐isopropylacrylamide)‐block‐poly{6‐[4‐(4‐pyridyazo)phenoxy] hexylmethacrylate} (PNIPAM‐b‐PAzPy) was synthesized by successive reversible addition‐fragmentation chain transfer (RAFT) polymerization. In a water/tetrahydrofuran (H₂O/THF) mixture, amphiphilic PNIPAM‐b‐PAzPy self‐assembles into giant micro‐vesicles. Upon alternate ultraviolet (UV) and visible light irradiation, obvious reversible swelling‐shrinking of the vesicles was observed directly under an optical microscope. The maximum percentage increase in volume, caused by the UV light, reached 17%. Moreover, the swelling could be adjusted using the UV light power density. The derivation of this effect is due to photoinduced reversible isomerization of azopyridine units in the vesicles.

     

Gold Nanoparticles Functionalized by a Dextran‐Based pH‐ and Temperature‐Sensitive Polymer

A dextran‐based dual‐sensitive polymer is employed to endow gold nanoparticles with stability and pH‐ and temperature‐sensitivity. The dual‐sensitive polymer is prepared by RAFT polymerization of N‐isopropylacrylamide from trithiocarbonate groups linked to dextran and succinoylation of dextran after polymerization. The functionalized nanoparticles show excellent stability under various conditions and can be stored in powder‐form. UV and DLS measurements confirm that the temperature‐induced optical changes and aggregation behaviors of the particles are strongly dependent on pH.

     

SET‐RAFT Polymerization of Progargyl Methacrylate and a One‐Pot/One‐Step Preparation of Side‐chain Functionalized Polymers via Combination of SET‐RAFT and Click Chemistry

A clickable alkyne monomer, PgMA, was successfully polymerized in a well‐controlled manner via single electron transfer initiation and propagation through the radical addition fragmentation chain transfer (SET‐RAFT) method. The living nature of the polymerization was confirmed by the first‐order kinetic plots, the linear relationships between molecular weights and the monomer conversions while keeping relatively narrow (≤1.55), and the successful chain‐extension with MMA. The better controllability of SET‐RAFT than other CRP methods is attributed to the less competitive termination in view of the presence of the CTA as well as the Cu(II) that is generated in situ. Moreover, a one‐pot/one‐step technique combining SET‐RAFT and “click chemistry” methods has been successfully employed to prepare the side‐chain functionalized polymers.

     

Simulation of RAFT Dispersion Polymerization in Supercritical Carbon Dioxide

The RAFT radical polymerization of vinyl monomers in supercritical carbon dioxide was modeled using the Predici® simulation package. The sensitivity of polymerization responses on formulation and process variables was analyzed. The simulations were carried out using kinetic and physical parameters corresponding to the polymerization of methyl methacrylate in supercritical carbon dioxide, using AIBN as initiator, at 65 °C and 200 bar, and using values of the addition and fragmentation kinetic rate constants of a “typical” RAFT agent, as reference conditions. This is the first report in the literature addressing the modeling or simulation of RAFT polymerization in supercritical carbon dioxide.

     

Easy One‐Pot Synthesis of a Reactive Hydrogel from Commercially Available 2‐Isocyanatoethyl Methacrylate and K2S2O5

Addition of commercially available 2‐isocyanatoethyl methacrylate to a water solution of potassium bisulfite unexpectedly yields an anionically charged hydrogel. Structural analysis shows the presence of sulfonato‐blocked isocyanates (with no detectable residual isocyanates), a full polymerization of the vinyl groups, and the appearance of urea functional groups. A mechanism is proposed to explain the gel formation, based on a combination of bisulfite‐initiated free‐radical polymerization, crosslinking by urea bridge formation, and addition of bisulfite ions to isocyanate groups. Some basic physical properties (TGA, swelling) of the gel are presented.

     

Chitinase‐Catalyzed Synthesis of a Chitin‐Xylan Hybrid Polymer: A Novel Water‐Soluble β(1 → 4) Polysaccharide Having an N‐Acetylglucosamine‐Xylose Repeating Unit

Summary: A chitin‐xylan hybrid polysaccharide having β(1 → 4)‐linked alternating structure of N‐acetyl‐D ‐glucosamine and D ‐xylose was synthesized via chitinase‐catalyzed polymerization. An oxazoline derivative of D ‐xylosyl‐β(1 → 4)‐N‐acetyl‐D ‐glucosamine ( 1 ) was effectively polymerized by the catalysis of chitinase from Bacillus sp., giving rise to a water‐soluble chitin‐xylan hybrid polysaccharide ( 2 ) in good yields. Molecular weights ( ) of 2 reached 1 500, which corresponds to 8–10 saccharide units.

A chitin‐xylan hybrid polysaccharide ( 2 ) synthesized via chitinase‐catalyzed polymerization.     

Cu(S2CNEt2)Cl‐Catalyzed Reverse Atom‐Transfer Radical Polymerization of Vinyl Monomers

A new copper catalyst containing chlorine and a photo‐labile diethylthiocarbamoylthiyl group was successfully employed in the reverse ATRP of methyl methacrylate (MMA). The polymeric chains were end‐capped with S₂CNEt₂, due to pseudo‐halogen atom‐transfer reaction between active and dormant species. Photopolymerization of this PMMA in the presence of fresh MMA and styrene monomers at ambient temperature yielded chain‐extended PMMA and MMA/styrene block copolymers, respectively.

GPC traces of (A) PMMA end‐capped with a photo‐labile group (pre‐PMMA), (B) chain‐extended PMMA (post‐PMMA), and (C) PMMA/styrene block copolymer (PMMA‐b‐PSt).     

Simulation of Styrene Polymerization by Monomolecular and Bimolecular Nitroxide‐Mediated Radical Processes over a Range of Reaction Conditions

Simulations of polymerization rate, molecular weight development and evolution of the concentrations of species participating in the reaction mechanism over a range of operating conditions, and a parameter sensitivity analysis showing the effects of temperature, activation/deactivation equilibrium constant and initial concentrations of controller and initiator (if present) on these variables are presented for the nitroxide‐mediated radical polymerization of styrene. The simulations were performed with a computer program based on a detailed reaction mechanism. The simulated profiles of conversion, number average molecular weight ( ), and polydispersity agree well with experimental data. Previously unknown activation energies for reactions involved in the mechanism are estimated. The temperature dependence of the kinetic rate constants obtained in this study will be useful for future modeling and optimization studies.

     

Microstructure,Kinetics, and Transport in Oil‐in‐Water Microemulsion Polymerizations

The kinetics of microemulsion polymerization depend on the structure of the initial microemulsion and the transport of species between the aqueous domain, the micelles, and the polymer particles. The water solubility of the monomer and the proximity of the initial microemulsion composition to a phase boundary are key considerations for studying microemulsion polymerization kinetics and producing the desired products. Complications frequently arise in the synthesis of copolymers or the incorporation of controlled polymerization mechanisms because of the compartmentalized nature of microemulsion polymerizations.

     

Automated Parallel Temperature Optimization and Determination of Activation Energy for the Living Cationic Polymerization of 2‐Ethyl‐2‐oxazoline

Sixteen parallel polymerization reactions of 2‐ethyl‐2‐oxazoline have been performed at different temperatures in an automated synthesizer that allowed individual heating of each reactor. During the reactions samples were taken automatically, which were characterized by means of both online GPC and offline GC, in order to optimize the reaction temperature and to determine the activation energy of the polymerization.

     

RAFT Miniemulsion Polymerization Kinetics, 1 – Polymerization Rate

The polymerization kinetics of a RAFT‐mediated radical polymerization inside submicron particles (30 < D_p < 300 nm) is considered. When the time fraction of active radical period, ϕ_A, is larger than ca. 1%, the polymerization rate increases with reducing particle size, as for the cases of conventional emulsion polymerization. The rate retardation by the addition of RAFT agent occurs with or without intermediate termination in zero‐one systems. For the particles with D_p < 100 nm, the statistical variation of monomer concentration among particles may not be neglected. It was found that this monomer‐concentration‐variation (MCV) effect may slow down the polymerization rate. An analytical expression describing the MCV effect is proposed, which is valid for both RAFT and conventional miniemulsion polymerizations.

     

Mechanism of Dithiobenzoate‐Mediated RAFT Polymerization: A Missing Reaction Step

Summary: The debate on the mechanism of dithiobenzoate‐mediated RAFT polymerization may be resolved by including the reaction between a propagating radical and the star‐shaped combination product from irreversible termination into the kinetic scheme. By this step, a highly reactive propagating radical and a not overly stable three‐arm star species are transformed into the resonance‐stabilized RAFT intermediate radical and a very stable polymer molecule. The time evolution of concentrations is discussed for the main‐equilibrium range of CDB‐mediated methyl acrylate polymerization.

Illustration of the novel understanding of the RAFT mechanism in dithiobenzoate‐mediated RAFT polymerization.     

High‐Molecular‐Weight Polycarbonates Synthesized by Enzymatic ROP of a Cyclic Carbonate as a Green Process

High‐molecular‐weight PTeMC and PHMC were prepared by the lipase‐catalyzed polymerization of butane‐1,4‐diol or hexane‐1,6‐diol and diphenyl carbonate via the formation of a cyclic dimer by a green process. Cyclic carbonate dimers were prepared by the lipase‐catalyzed condensation of diphenyl carbonate with butane‐1,4‐diol or hexane‐1,6‐diol in dilute toluene solution using an immobilized lipase from Candida antarctica, and was followed by the ring‐opening polymerization of the cyclic dimer in bulk with the same lipase to produce PTeMC with = 119 000 g · mol^?1 and PHMC with = 399 000 g · mol^?1, respectively. Additionally, enzymatic polymerization of cyclic carbonate dimer was analyzed with respect to the K_m and V_max for the lipase.

     

Diversified Applications of Chemically Modified 1,2‐Polybutadiene

Commercially available 1,2‐PB was transformed into a well‐defined reactive intermediate by quantitative bromination. The brominated polymer was used as a polyfunctional macroinitiator for the cationic ring‐opening polymerization of 2‐ethyl‐2‐oxazoline to yield a water‐soluble brush polymer. Nucleophilic substitution of bromide by 1‐methyl imidazole resulted in the formation of polyelectrolyte copolymers consisting of mixed units of imidazolium, bromo, and double bond. These copolymers, which were soluble in water without forming aggregates, were used as stabilizers in the heterophase polymerization of styrene and were also studied for their ionic conducting properties.