Monolith‐ and Silica‐Supported Carboxylate‐Based Grubbs–Herrmann‐Type Metathesis Catalysts

The synthesis of silica‐ and monolith‐supported Grubbs–Herrmann‐type catalysts is described. Two polymerizable, carboxylate‐containing ligands, exo, exo‐7‐oxanorborn‐2‐ene‐5,6‐dicarboxylic anhydride and 7‐oxanorborn‐2‐ene‐5‐carboxylic acid were surface‐immobilized onto silica‐ and ring‐opening metathesis (ROMP‐) derived monolithic supports using “grafting‐from” techniques. The “1^st generation Grubbs catalyst”, RuCl₂(CHPh)(PCy₃)₂, was used for these purposes. In addition, a poly(norborn‐2‐ene‐b‐exo, exo‐norborn‐2‐ene‐5,6‐dicarboxylic anhydride)‐coated silica 60 was prepared. The polymer supported anhydride and carboxylate groups were converted into the corresponding mono‐ and disilver salts, respectively, and reacted with the Grubbs–Herrmann catalyst RuCl₂(CHPh)(IMesH₂)(PCy₃) [IMesH₂=1,3‐bis(2,4,6‐trimethylphenyl)‐4,5‐dihydroimidazol‐2‐ylidene]. Heterogenization was accomplished by exchange of one chlorine ligand with the polymeric, immobilized silver carboxylates to yield monolith‐supported catalysts 4, 5 , and 6 as well as silica‐supported systems 7, 8 and 9 . The actual composition of these heterogenized catalysts was proven by the synthesis of a homogeneous analogue, RuCl[7‐oxanorbornan‐2‐(COOAg)‐3‐COO](CHPh)(IMesH₂)(PCy₃) ( 3 ). All homogeneous and heterogeneous catalysts were used in ring‐closing metathesis (RCM) of diethyl diallylmalonate, 1,7‐octadiene, diallyldiphenylsilane, methyl trans‐3‐pentenoate, diallyl ether, N,N‐diallyltrifluoracetamide and t‐butyl N,N‐diallylcarbamate allowing turnover numbers (TON's) close to 1000. In a flow‐through set‐up, an auxiliary effect of pendant silver carboxylates was observed with catalyst 5 , where the silver moiety functions as a (reversible) phosphine scavenger that both accelerates initiation and stabilizes the catalyst by preventing phosphine elution. Detailed catalytic studies were carried out with the monolith‐supported systems 4 and 6 in order to investigate the effects of temperature and chain‐transfer agents (CTA's) such as cis‐1,4‐diacetoxybut‐2‐ene. In all RCM experiments Ru‐leaching was low, resulting in a Ru‐content of the RCM products ≤3.5 μg/g (3.5 ppm).     

The relative reactivity of 2,3‐dicarbomethoxy‐5‐norbornenes in metathesis polymerization using the original n‐chelating ruthenium carbene complex

¹H NMR spectroscopy is used to study the kinetics of metathesis copolymerization of three isomeric 2,3‐dicarbomethoxy‐5‐norbornenes using the original N‐chelating ruthenium carbene complex. Based on the experimental data the copolymerization constants of isomeric 2,3‐dicarbomethoxy‐5‐norbornenes are calculated. It is shown that the relative reactivity of endic acid dimethyl ester—(1R,2S,3R,4S)‐dimethyl bicyclo[2.2.1]hept‐5‐ene‐2,3‐dicarboxylate is almost two times lower than the corresponding values for (1R,2R,3S,4S)‐dimethyl bicyclo[2.2.1]hept‐5‐ene‐2,3‐dicarboxylate, which confirms earlier findings of steric hindrance in the orientation of the monomer due to the carbene catalyst. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40130.     

11C‐ and 18F‐Labeled Radioligands for P‐Glycoprotein Imaging by Positron Emission Tomography

P‐Glycoprotein (P‐gp) is an efflux transporter widely expressed at the human blood–brain barrier. It is involved in xenobiotics efflux and in onset and progression of neurodegenerative disorders. For these reasons, there is great interest in the assessment of P‐gp expression and function by noninvasive techniques such as positron emission tomography (PET). Three radiolabeled aryloxazole derivatives: 2‐[2‐(2‐methyl‐(¹¹C)‐5‐methoxyphenyl)oxazol‐4‐ylmethyl]‐6,7‐dimethoxy‐1,2,3,4‐tetrahydroisoquinoline ([¹¹C]‐ 5 ); 2‐[2‐(2‐fluoromethyl‐(¹⁸F)‐5‐methoxyphenyl)oxazol‐4‐ylmethyl]‐6,7‐dimethoxy‐1,2,3,4‐tetra‐hydroisoquinoline ([¹⁸F]‐ 6 ); and 2‐[2‐(2‐fluoroethyl‐(¹⁸F)‐5‐methoxyphenyl)oxazol‐4‐ylmethyl]‐6,7‐dimethoxy‐1,2,3,4‐tetrahydroisoquinoline ([¹⁸F]‐ 7 ), were tested in several in vitro biological assays to assess the effect of the aryl substituent in terms of potency and mechanism of action toward P‐gp. Methyl derivative [¹¹C]‐ 5 is a potent P‐gp substrate, whereas the corresponding fluoroethyl derivative [¹⁸F]‐ 7 is a P‐gp inhibitor. Fluoromethyl compound [¹⁸F]‐ 6 is classified as a non‐transported P‐gp substrate, because its efflux increases after cyclosporine A modulation. These studies revealed a promising substrate and inhibitor, [¹¹C]‐ 5 and [¹⁸F]‐ 7 , respectively, for in vivo imaging of P‐gp by using PET.     

Polymer‐Supported,Carbon Dioxide‐Protected N‐Heterocyclic Carbenes: Synthesis and Application in Organo‐ and Organometallic Catalysis

The synthesis of a resin‐supported, carbon dioxide‐protected N‐heterocyclic carbene (NHC) and its use in organocatalysis and organometallic catalysis are described. The resin‐bound carbon dioxide‐protected NHC‐based catalyst was prepared via ring‐opening metathesis copolymerization of 1,4,4a,5,8,8a‐hexahydro‐1,4,5,8‐exo,endo‐dimethanonaphthalene ( DMNH6 ) with 3‐(bicyclo[2.2.1]hept‐5‐en‐2‐ylmethyl)‐1‐(2‐propyl)‐3,4,5,6‐tetrahydropyrimidin‐1‐ium‐2‐carboxylate ( M1 ), using the well‐defined Schrock catalyst Mo[N‐2,6‐(2‐Pr)₂‐C₆H₃](CHCMe₂Ph)(OCMe₃)₂ and was used for a series of organocatalytic reactions, i.e., for the trimerization reaction of isocyanates, as well as for the cyanosilylation of carbonyl compounds. In the latter reaction, turn‐over numbers (TON) up to 5000 were achieved. In addition, the polymer‐supported, carbon dioxide‐protected N‐heterocyclic carbene served as an excellent progenitor for various polymer‐supported metal complexes. It was loaded with a series of rhodium(I), iridium(I), and palladium(II) precursors and the resulting Rh‐, Ir‐, and Pd‐loaded resins were successfully used in the polymerization of phenylacetylene, in the hydrogen transfer reaction to benzaldehyde, as well as in Heck‐type coupling reactions. In the latter reaction, TONs up to 100,000 were achieved. M1 , as a non‐supported analogue of poly‐M1‐b‐DMNH6 , as well as the complexes PdCl₂[1,3‐bis(2‐Pr)tetrahydropyrimidin‐2‐ylidene]₂ ( Pd‐1 ) and IrBr[1‐(norborn‐5‐ene‐2‐ylmethyl)‐3‐(2‐Pr)‐3,4,5,6‐tetrahydropyrimidin‐2‐ylidine](COD) ( Ir‐1 ) were used as homogeneous analogues and their reactivity in the above‐mentioned reactions was compared with that of the supported catalytic systems. In all reactions investigated, the TONs achieved with the supported systems were very similar to the ones obtained with the unsupported, homogeneous ones, the turn‐over frequencies (TOFs), however, were lower by up to a factor of three.     

A New Class of 3′‐Sulfonyl BINAPHOS Ligands: Modulation of Activity and Selectivity in Asymmetric Palladium‐Catalysed Hydrophosphorylation of Styrene

Palladium‐catalysed monophosphorylation of (R)‐2,2′‐bisperfluoroalkanesulfonates of BINOL (R^F=CF₃ or C₄F₉) by a diaryl phosphinate [Ar₂P(O)H] followed by phosphine oxide reduction (Cl₃SiH) then lithium diisopropylamide‐mediated anionic thia‐Fries rearrangement furnishes enantiomerically‐pure (R)‐2′‐diarylphosphino‐2′‐hydroxy‐3′‐perfluoralkanesulfonyl‐1,1′‐binaphthalenes [(R)‐ 8ab and (R)‐ 8g–j ], which can be further diversified by Grignard reagent (RMgX)‐mediated CF₃‐displacement [→(R)‐ 8c–f ]. Coupling of (R)‐ 8a–j with (S)‐1,1′‐binaphthalene‐2,2′‐dioxychlorophosphine (S)‐ 9 generates 3′‐sulfonyl BINAPHOS ligands (R,S)‐ 10a–j in good yields (43–82%). These new ligands are of utlility in the asymmetric hydrophosphonylation of styrene ( 1 ) by 4,4,5,5‐tetramethyl‐1,3,2‐dioxaphospholane 2‐oxide ( 2 ), for which a combination of the chiral ligands with either [Pd(Cp)(allyl)] or [Pd(allyl)(MeCN)₂]⁺/NaCH(CO₂Me)₂ proves to be a convenient and active pre‐catalyst system. A combination of an electron‐rich phosphine moiety and an electron‐deficient 3′‐sulfone moiety provides the best enantioselectivity to date for this process, affording the branched 2‐phenethenephosphonate, (−)‐iso‐ 3 , in up to 74% ee with ligand (R,S)‐ 10i , where Ar=p‐anisyl and the 3′‐SO₂R group is triflone.     

Evidence of isomerization of 5-methylenebicyclo[2.2.1] hept-2-ene cation by13C NMR spectroscopy

Summary Cations obtained by reaction of various protonic acids with 5-Methylenebicyclo [2.2.1] hept-2-ene (5-methylene-2-norbornene) have been studied by C NMR spectroscopy. The isomerization of initial carbocation has been pointed out. A correlation of these results with the structure of the corresponding polymers has been establishedThe cationic polymerization of 5-Methylenebicyclo [2.2.1] hept2-ene (5-methylene-2-norbornene) has been investigated several times and KENNEDY (1974) published a review on this topic. A structure of the polymer obtained by classical cationic initiation has been suggested; this results from infrared and X.R. diffraction studies.In a recent work IVIN et al. (1980) reported the¹³C NMR spectrum of a poly[5-methylenebicyclo [2.2.1] hept-2-ene] resulting from a Ziegler-Natta initiation. We obtained the same¹³C NMR spectrum as IVIN et al. when the initiator were TiCl₄, CF₃ COOH, CCl₃COOH and concentrated H₂SO₄ (see fig.1). Chemical shifts and coupling constants fit with the generally accepted structure of the polymers: KENNEDY and MAKOWSKY (1967) suggested an isomerization of the active species:     

Organosoluble and light‐colored fluorinated polyimides based on 1,1‐bis[4‐(4‐amino‐2‐trifluoromethylphenoxy)phenyl]‐1‐phenylethane and various aromatic dianhydrides

To investigate the CF₃ group affecting the coloration and solubility of polyimides (PI), a novel fluorinated diamine 1,1‐bis[4‐(4‐amino‐2‐ trifluoromethylphenoxy)phenyl]‐1‐phenylethane (2) was prepared from 1,1‐ bis(4‐hydrophenyl)‐1‐phenylethan and 2‐chloro‐5‐nitrobenzotrifluoride. A series of light‐colored and soluble PI 5 were synthesized from 2 and various aromatic dianhydrides 3a–f using a standard two‐stage process with thermal 5a– f(H) and chemical 5a–f(C) imidization of poly(amic acid). The 5 series had inherent viscosities ranging from 0.55 to 0.98 dL/g. Most of 5a–f(H) were soluble in amide‐type solvents, such as N‐methyl‐2‐pyrrolidone (NMP), N,N‐ dimethylacetamide (DMAc), and N,N‐dimethylformamide (DMF), and even soluble in less polar solvents, such as m‐Cresol, Py, Dioxane, THF, and CH₂Cl₂, and the 5(C) series was soluble in all solvents. The GPC data of the 5a–f(C) indicated that the Mn and Mw values were in the range of 5.5–8.7 × 10⁴ and 8.5–10.6 × 10⁴, respectively, and the polydispersity index (PDI) Mw /Mn values were 1.2–1.5. The PI 5 series had excellent mechanical properties. The glass transition temperatures of the 5 series were in the range of 232–276°C, and the 10% weight loss temperatures were at 505–548 °C in nitrogen and 508–532 °C in air, respectively. They left more than 56% char yield at 800°C in nitrogen. These films had cutoff wavelengths between 356.5–411.5 nm, the b* values ranged from 5.0–71.1, the dielectric constants, were 3.11–3.43 (1MHz) and the moisture absorptions were in the range of 011–0.40%. Comparing 5 containing the analogous PI 6 series based on 1,1‐bis[4‐(4‐aminophenoxy)phenyl]‐1‐ phenylethane (BAPPE), the 5 series with the CF₃ group showed lower color intensity, dielectric constants, and better solubility. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 2399–2412, 2005     

Limits to expanding the PN‐F series of polyphosphazene elastomers

Mixed‐substituent fluoroalkoxyphosphazene polymers bearing ～15% 1H,1H,2H,2H‐perfluorooctan‐1‐oxy or 1H,1H,2H,2H‐perfluorodecan‐1‐oxy side groups together with trifluoroethoxy cosubstituent groups were synthesized. The low reactivity of the long‐chain fluoroalkoxides and their limited solubility in organic solvents prevented higher levels of substitution. Moreover, the sodium alkoxides with two methylene residues adjacent to the oxygen proved to be unstable in solution due to elimination of NaF and precipitation of side products, and this limited the time available for chlorine replacement reactions. The resulting cosubstituent polymers were characterized by proton nuclear magnetic resonance (¹H‐NMR), ³¹P‐NMR, ¹⁹F‐NMR, gel‐permeation chromatography, and differential scanning calorimetry. Unlike homo‐ or mixed‐substituent fluoroalkoxyphosphazene polymers, such as [NP(OCH₂CF₃)₂]_n (a microcrystalline thermoplastic, T_g ～ ?63°C, T_m ～ 242°C) or [NP(OCH₂CF₃)(OCH₂(CF₂)_xCF₂H)]_n (PN‐F, a rubbery elastomer, T_g ～ ?60°C, but no detectable T_m), the new polymers are gums (T_g ～ ?50°C, but no detectable T_m) with molecular weights in the 10⁵ g/mol rather than the 10⁶ g/mol range. POLYM. ENG. SCI., 54:1827–1832, 2014. © 2013 Society of Plastics Engineers     

Kinetics of azo‐initiated 1‐vinyl‐2‐pyrrolidone polymerizations at low conversions in aqueous media

The free‐radical polymerization behavior of 1‐vinyl,2‐pyrrolidone (NVP) was studied at low conversions, using capillary dilatometry. The aqueous media were kept at neutral pH and the studies were conducted isothermally, at 40 or 45°C. The azo‐type initiators used were 4,4′‐azobis‐4‐cyanopentanoic acid (ACPA), 2,2′‐azobisisobutyronitrile (AZBN), and 2,2′‐azobis[2‐(2‐imidazolin‐2‐yl)propane dihydrochloride] (ABDH). The monomer concentration and initiator concentration ranges were 1.17–2.34 mol L⁻¹ and 1–8 mmol L⁻¹, respectively. The rates of polymerization (R_p) and orders of reaction with respect to NVP and the initiator were evaluated and the kinetic equations were found to be R_p ∝ [NVP] [ACPA]^1.2; R_p ∝ [NVP] [AZBN]^1.1; and R_p ∝ [NVP]^2.2 [ABDH]^1.1. The polymers obtained were characterized by their viscosity numbers and correlation of the viscosity average molecular weights made with the type and amount of the azo initiator. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 75: 239–246, 2000     

Optically Active 4‐Substituted (S)‐2‐Phenyl‐2‐oxazolines

(R)‐4‐Hydroxymethyl‐2‐phenyl‐2‐oxazoline (R)‐ 1 ) was prepared from (L)‐serine. The respective tosylate ((S)‐ 2 ) was converted into sulfides (S)‐ 4 and (S)‐ 5 , and sulfone (S)‐ 6 , useful starting materials for the elaboration of additional chiral centers. A previously reported [ α]_D ²⁵ value for (R)‐ 4 is corrected.     

Controlling the Regioselectivity of Baeyer–Villiger Monooxygenases by Mutation of Active‐Site Residues

Baeyer–Villiger monooxygenase (BVMO)‐mediated regiodivergent conversions of asymmetric ketones can lead to the formation of “normal” or “abnormal” lactones. In a previous study, we were able to change the regioselectivity of a BVMO by mutation of the active‐site residues to smaller amino acids, which thus created more space. In this study, we demonstrate that this method can also be used for other BVMO/substrate combinations. We investigated the regioselectivity of 2‐oxo‐Δ³‐4,5,5‐trimethylcyclopentenylacetyl‐CoA monooxygenase from Pseudomonas putida (OTEMO) for cis‐bicyclo[3.2.0]hept‐2‐en‐6‐one ( 1 ) and trans‐dihydrocarvone ( 2 ), and we were able to switch the regioselectivity of this enzyme for one of the substrate enantiomers. The OTEMO wild‐type enzyme converted (?)‐ 1 into an equal (50:50) mixture of the normal and abnormal products. The F255A/F443V variant produced 90 % of the normal product, whereas the W501V variant formed up to 98 % of the abnormal product. OTEMO F255A exclusively produced the normal lactone from (+)‐ 2 , whereas the wild‐type enzyme was selective for the production of the abnormal product. The positions of these amino acids were equivalent to those mutated in the cyclohexanone monooxygenases from Arthrobacter sp. and Acinetobacter sp. (CHMO_Arthro and CHMO_Acineto) to switch their regioselectivity towards (+)‐ 2 , which suggests that there are hot spots in the active site of BVMOs that can be targeted with the aim to change the regioselectivity.     

Facile,Efficient Copolymerization of Ethylene with Bicyclic,Non‐Conjugated Dienes by Titanium Complexes Bearing Bis(β‐Enaminoketonato) Ligands

Copolymerizations of ethylene with 5‐vinyl‐2‐norbornene or 5‐ethylidene‐2‐norbornene under the action of various titanium complexes bearing bis(β‐enaminoketonato) chelate ligands of the type, [R¹NC(R²)CHC(R³)O]₂TiCl₂ ( 1 , R¹=Ph, R²=CF₃, R³=Ph; 2 , R¹=C₆H₄F‐p, R²=CF₃, R³=Ph; 3 , R¹=Ph, R²=CF₃, R³=t‐Bu; 4 , R¹=C₆H₄F‐p, R²=CF₃, R³=t‐Bu; 5 , R¹=Ph, R²=CH₃, R³=CF₃; 6 , R¹=C₆H₄F‐p, R²=CH₃, R³=CF₃), have been shown to occur with the regioselective insertion of the endocyclic double bond of the monomer into the copolymer chain, leaving the exocyclic vinyl double bond as a pendant unsaturation. The ligand modification strongly affects the copolymerization behaviour. High catalytic activities and efficient co‐monomer incorporation can be easily obtained by optimizing the catalyst structures and polymerization conditions.     

Synthesis and characterization of homo‐ and copolymers of 3‐(2‐cyano ethoxy)methyl‐ and 3‐[methoxy(triethylenoxy)]methyl‐3′‐methyl‐oxetane

Two oxetane‐derived monomers 3‐(2‐cyanoethoxy)methyl‐ and 3‐(methoxy(triethylenoxy)) methyl‐3′‐methyloxetane were prepared from the reaction of 3‐methyl‐3′‐hydroxymethyloxetane with acrylonitrile and triethylene glycol monomethyl ether, respectively. Their homo‐ and copolyethers were synthesized with BF₃· Et₂O/1,4‐butanediol and trifluoromethane sulfonic acid as initiator through cationic ring‐opening polymerization. The structure of the polymers was characterized by FTIR and¹H NMR. The ratio of two repeating units incorporated into the copolymers is well consistent with the feed ratio. Regarding glass transition temperature (T_g), the DSC data imply that the resulting copolymers have a lower T_g than pure poly(ethylene oxide). Moreover, the TGA measurements reveal that they possess in general a high heat decomposition temperature. The ion conductivity of a sample (P‐AN 20) is 1.07 × 10^?5 S cm^?1 at room temperature and 2.79 × 10^?4 S cm^?1 at 80 °C, thus presenting the potential to meet the practical requirement of lithium ion batteries for polymer electrolytes. Copyright © 2005 Society of Chemical Industry     

Convenient Synthesis of (E)‐5‐Aryl(halo)methylenebicyclo‐ [2.2.2]oct‐2‐enes and ‐[2.2.1]hept‐2‐enes via Lewis Acid‐ Promoted Carbohalogenation of Cyclic 2,6‐Enynols

An efficient synthesis of (E)‐5‐aryl(halo)methylenebicyclo[2.2.2]oct‐2‐enes is reported. Lewis acid‐promoted carbohalogenation of 4‐(3‐arylprop‐2‐ynyl)‐cyclohex‐2‐enols in dichloromethane proceeds rapidly to afford the exo‐methylene‐bridged bicycles in good yields. This method also provides an easy access to (E)‐5‐aryl(halo)methylenebicyclo[2.2.1]hept‐2‐enes from the five‐membered ring 2,6‐enynols. The reactions are procedurally simple and high yielding, producing the aryl(halo)methylene‐bridged bicycles in minutes under air and mild conditions.

     

Synthesis of Pyrrolizidine,Indolizidine, and Quinolizidine Derivatives Using Ruthenium‐Catalyzed Ring‐Opening Metathesis and Ring‐Closing Metathesis of Cycloalkene‐ynes

Ring‐opening metathesis and ring‐closing metathesis (ROM‐RCM) of a cyclopentene‐yne having an ester moiety was demonstrated using first‐ and second‐generation Grubbs’ catalysts. When the reaction of cycloalkene‐yne was carried out in the presence of 5 mol % of a ruthenium carbene complex under an ethylene atmosphere at room temperature, ROM‐RCM proceeded smoothly to give a pyrrolidine derivative in good yield, which could be converted to a pyrrolizidine derivative. Furthermore, ROM‐RCM of azabicyclo[2.2.1]heptene‐ynes using the second‐generation Grubbs’ catalyst was investigated. When an azabicycloheptene derivative was exposed to a catalytic amount of a ruthenium carbene complex, pyrrolizidine and indolizidine derivatives were obtained in good yields. The distribution of these products depends on the substituents on the alkyne. When azabicyclo[2.2.1]heptene‐ynes bearing large substituents on the alkyne were treated with ruthenium catalyst 1b , a pyrrolizidine derivative was obtained as the major product. ROM‐RCM of azabicyclo[2.2.2]octene‐ynes with 1b afforded quinolizidine derivative 20 , although the yield was moderate.     

[3H]UR‐DE257: Development of a Tritium‐Labeled Squaramide‐Type Selective Histamine H2 Receptor Antagonist

A series of new piperidinomethylphenoxypropylamine‐type histamine H₂ receptor (H₂R) antagonists with different substituted “urea equivalents” was synthesized and characterized in functional in vitro assays. Based on these data as selection criteria, radiosynthesis of N‐[6‐(3,4‐dioxo‐2‐{3‐[3‐(piperidin‐1‐ylmethyl)phenoxy]propylamino}cyclobut‐1‐enylamino)hexyl]‐(2,3‐³H₂)propionic amide ([³H]UR‐DE257) was performed. The radioligand (specific activity: 63 Ci mmol^?1) had high affinity for human, rat, and guinea pig H₂R (hH₂R, Sf9 cells: K_d, saturation binding: 31 nM , kinetic studies: 20 nM ). UR‐DE257 revealed high H₂R selectivity on membranes of Sf9 cells, expressing the respective hH_xR subtype (K_i values: hH₁R: >10 000 nM , hH₂R: 28 nM , hH₃R: 3800 nM , hH₄R: >10 000 nM ). In spite of insurmountable antagonism, probably due to rebinding of [³H]UR‐DE257 to the H₂R (extended residence time), the title compound proved to be a valuable pharmacological tool for the determination of H₂R affinities in competition binding assays.     

Free radical kinetics of N‐methyl N‐vinyl acetamide polymerization at low conversions in aqueous media

N‐methyl N‐vinyl acetamide (NMNVA) monomer was polymerized at low conversions and its free radical kinetics were detailed using capillary dilatometry. The polymerizations were conducted isothermally, at 40°C using 2,2′‐azobis[2‐(2‐imidazolin‐2‐yl) propane dihydrochloride] (ABDH) as initiator. Monomer concentration and initiator concentration ranges were 1.10–1.70 mol · L⁻¹ and 1–4 mmol · L⁻¹, respectively. The aqueous polymerization media were kept at neutral pH. The rates of polymerization (R_p) and orders of reaction with respect to NMNVA and ABDH concentrations were evaluated and the kinetic expression was found to be ideal, with R_p ∝ [NMNVA]^1.07 [ABDH]^0.61. The polymers obtained were characterized by their viscosity numbers and correlation of viscosity average molecular weights was made with the amount of ABDH initiator. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 79: 337–341, 2001     

Spectroscopic and Theoretical Study on Electronically Modified Chromophores in LOV Domains: 8‐Bromo‐ and 8‐Trifluoromethyl‐Substituted Flavins

Two chemically synthesized flavin derivatives, 8‐trifluoromethyl‐ and 8‐bromoriboflavin (8‐CF₃RF and 8‐BrRF), were photochemically characterized in H₂O and studied spectroscopically after incorporation into the LOV domain of the blue light photoreceptor YtvA from Bacillus subtilis. The spectroscopic studies were paralleled by high‐level quantum chemical calculations. In solution, 8‐BrRF showed a remarkably high triplet quantum yield (0.97, parent compound riboflavin, RF: 0.6) and a small fluorescence quantum yield (0.07, RF: 0.27). For 8‐CF₃RF, the triplet yield was 0.12, and the fluorescence quantum yield was 0.7. The high triplet yield of 8‐BrRF is due to the bromine heavy atom effect causing a stronger spin–orbit coupling. Theoretical calculations reveal that the decreased triplet yield of 8‐CF₃RF is due to a smaller charge transfer and a less favorable energetic position of T₂, required for intersystem crossing from S₁ to T₁, as an effect of the electron‐withdrawing CF₃ group. The reconstitution of the LOV domain with the new flavins resulted in the typical LOV photochemistry, consisting of triplet state formation and covalent binding of the chromophore, followed by a thermal recovery of the parent state, albeit with different kinetics and photophysical properties.     

Stereochemical and compositional assignments of trans‐4‐methacryloyloxyazobenzene–methyl methacrylate copolymers by one‐ and two‐dimensional NMR spectroscopy

The microstructure of trans‐4‐methacryloyloxyazobenzene–methyl methacrylate copolymers prepared by solution polymerization process using AIBN as initiator is analyzed by one‐and two‐dimensional spectroscopy. Sequence distribution was calculated from the ¹³C(¹H)‐NMR spectra of the copolymers. Comonomer reactivity ratios were determined using the Kelen–Tudos and the nonlinear error‐in‐variables methods are r_A = 1.14 ± 0.08 and r_M = 0.51 ± 0.03; r_A = 1.13 ± 0.1 and r_M = 0.50 ± 0.04, respectively. The sequence distribution of A‐ and M‐centered triads determined from ¹³C(¹H)‐NMR spectra of copolymer is in good agreement with triad concentration calculated from a statistical model. The 2‐D heteronuclear single‐quantum correlation and correlated spectroscopy (TOCSY) was used to analyze the complex ¹H‐NMR spectrum. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 3016–3025, 1999     

Synthesis of 1,3‐benzodioxole end‐functionalized polymers via reversible addition–fragmentation chain transfer polymerization

Reversible addition–fragmentation chain transfer (RAFT) polymerization of styrene was carried out in the presence of a novel RAFT reagent, bearing 1,3‐benzodioxole group, benzo [1,3]dioxole‐5‐carbodithioic acid benzo [1,3]dioxol‐5‐ylmethyl ester (BDCB), to prepare end‐functionalized polystyrene. The polymerization results showed that RAFT polymerization of styrene could be well controlled. Number–average molecular weight (M_n(GPC)) increased linearly with monomer conversion, and molecular weight distributions were narrow (M_w/M_n < 1.4). The successful reaction of chain extension and analysis of ¹H NMR spectra confirmed the existence of the functional 1,3‐benzodioxole group at the chain‐end of polystyrene. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 99: 3535–3539, 2006