Ene Addition Research Articles

Multi-functionalization of helical block copoly(α-peptide)s by orthogonal chemistry

Diblock copoly(α-peptide)s bearing functional side-chains (i.e., azido or allyl) that are readily derivatized by “click” chemistry have been synthesized by primary amine-initiated sequential polymerization of the corresponding N-carboxyanhydrides (NCAs). Facile derivation of the side-chains has been demonstrated, in which mannose moieties are quantitatively attached via the alkyne–azide [2 + 3] Huisgen cycloaddition to form amphiphilic block copoly(α-peptide)s that retain their helical conformations in solution. Allyl side-chains residing in the hydrophobic core can be further functionalized with high efficiency by radical thiol–ene addition reactions despite the tendency of the amphiphilic precursors to aggregate.

Photochromic materials with tunable color and mechanical flexibility

Florescence switches based on photochromic compounds have been fabricated previously and identified as potential candidates for information technology. Recently, optically responsive materials with tunable color have been prepared by dissolving photochromic compounds, such as spiropyran (SP), into solutions of various pH or embedding them into sol–gel matrices with adjusted chemical compositions. Here we report on fabricating flexible rubbers with tunable color by embedding SP molecules inside silicone elastomer networks (SENs) based on poly(vinylmethylsiloxane) (PVMS). SP-containing PVMS networks have been further modified either physically by exposing them to ultraviolet/ozone treatment or chemically by attaching functional thiols to the vinyl bonds in PVMS via UV-activated thiol–ene addition. The color hue of the SP–PVMS SENs after exposing to UV light depends on either the UVO dose or the chemical end-functionality of the thiol modifier, respectively. We also present simple methodologies enabling patterning regions in SP-doped SENs with various shapes and colors.

Amphiphilic Poly(ethylene oxide)-block-poly(butadiene-graft-liquid crystal) Copolymers: Synthesis and Self-Assembly in Water

What could be the self-assembly behavior in aqueous solution for an amphiphilic block copolymer with a very flexible hydrophobic backbone possessing however a liquid crystalline (LC) order? To answer this question, five amphiphilic block copolymers were prepared by grafting, via a thiol−ene addition reaction, four different thiol-functionalized LC mesogens or LC-like side groups on two commercially available poly(ethylene oxide)-block-poly(butadiene) (PEO-b-PB) polymers. These copolymers form monodisperse nanospheres in dilute deionized water with the aid of 1,4-dioxane as cosolvent (dioxane−H2O system), while in the THF−H2O system, some give heterogeneous polydisperse micelles and others form large hollow concentric spherical vesicles (“Russian dolls” arrangement). Smectic stripes inside the vesicles and the nanospheres are clearly visualized by cryo-TEM technique.

Synthesis of Novel Star Polymers with Vinyl-Functionalized Hyperbranched Core via “Arm-First” Strategy

Star polymers bearing vinyl-functionalized hyperbranched cores were successfully prepared at moderate-to-high monomer conversions (ca. 80%) via a modified “arm-first” strategy. This was achieved by reversible addition−fragmentation chain transfer polymerization of a methacrylate type asymmetrical divinyl monomer with thiobenzoate-capped polystyrenes as macro-chain-transfer agents (Macro-CTAs). Neither intermolecular nor intramolecular cross-linking occurred up to high levels of monomer conversion. The molecular weights (MWs) of Macro-CTAs significantly influence the structures of the resultant star polymers. A lower-MW Macro-CTA favors the formation of a star copolymer bearing big core and multiple short arms, whereas a higher-MW one is confined to the architecture bearing small core with only a few long arms. Moreover, the ca. 70% pendant vinyl groups remained in the hyperbranched core can be directly modified using reactions such as epoxidation and UV initiated thiol−ene addition reactions, resulting in star polymers with various functional groups in the core.

Functionalization of Polymer Nanoparticles by Thiol−Ene Addition

Syndiotactic 1,2-polybutadiene nanoparticles (volume average diameter 13 nm) were functionalized in aqueous dispersion by free radical mercaptan addition. By appropriate choice of both water-soluble radical initiator and mercaptan concentration, nanoparticles with a degree of functionalization of up to 85% were prepared using 3-mercaptopropionic acid methyl ester and 3-mercaptopropionic acid. Only a minor portion of double bonds formed cycles instead of the desired thiol−ene addition products. The composition and structure of the nanoparticles were elucidated by combination of elemental analysis, NMR, IR, DLS, and TEM. Highly hydrophilic mercaptans (3-mercaptopropanesulfonic acid sodium salts), in contrast, only reacted with surface accessible double bonds to afford stable and redispersible nanoparticles solely stabilized with covalently bound moieties on their surface. Analogous grafting of the tripeptide glutathione was demonstrated.

PEGylated Functional Nanoparticles from a Reactive Homopolymer Scaffold Modified by Thiol Addition Chemistry

Well-defined reactive polymer scaffolds are useful building blocks for a variety of biomedicine and nanotechnology applications. In this study, we have converted a RAFT-synthesized thiol-functional homopolymer scaffold (poly(pyridyl disulfide ethyl methacrylate), PPDSM) to poly(ethylene glycol) conjugated (PEGylated) nanoparticles via a straightforward approach. Poly(ethylene glycol) (PEG) was grafted to the reduced PPDSM via radical-mediated thiol−ene or Michael additions. The yield of PEG grafting via radical-mediated thiol−ene reaction and Michael addition was 68 ± 2 and 73 ± 1 mol %, respectively, of the total functional groups on the scaffold, as determined by 1H NMR spectroscopy. The grafting yield via Michael addition reactions was non-linearly proportional to the reducing agent concentration used (thus the number of free thiols created on the polymer chain). It was observed by UV−vis spectroscopy that PEG grafting via Michael addition to the PPDSM takes place simultaneously with inter- and intrach...

Highly Branched and Hyperbranched Glycopolymers via Reversible Addition−Fragmentation Chain Transfer Polymerization and Click Chemistry

We report a new strategy to synthesize densely functionalized highly and hyperbranched glycopolymers. Our methodology combines living radical polymerization and click chemistry. Hyperbranched “clickable” scaffolds synthesized via RAFT polymerization are functionalized with carbohydrate groups using an array of “click” chemistry reactions, namely Cu(I)-catalyzed Huisgen 1,3-cycloaddition of azides and alkynes (CuAAC), thiol−ene addition, and thiol−yne addition. This simple and flexible method yields glycopolymers of unique structures, which are potential candidates for biomedical applications.

Reaction of a triazolinedione with simple alkenes. Isolation and characterization of hydration products

The reaction of N-phenyltriazolinedione with three simple alkyl-substituted alkenes in water/alcohol or water/acetone solution was found to give a mixture of the corresponding ene and water addition products. The new hydration products were characterized by spectroscopy, and in one case, also by X-ray diffraction analysis. Thermodynamic parameters were determined for the reactions involving 2-methylbut-2-ene, TriME, and 2,3-dimethylbut-2-ene, TetraMe, in accordance with an ‘S N2-like’, nucleophilic attack on a closed aziridinium imide (AI) intermediate by water.

Preparation and reversible photo-crosslinking/photo-cleavage behavior of 4-methylcoumarin functionalized hyperbranched polyester

A novel hyperbranched polymer endcapped with 4-methylcoumarin group (MCTH40) was prepared via thiol–ene addition reaction of thiol-modified hyperbranched polyester (fully thioglycolic acetate of Boltorn™ H40, TAH40) with a vinyl monomer (7-(4-vinyl-benzyloxyl)-4-methylcoumarin, VBMC), and characterized with 1H NMR and FT-IR spectroscopies. Its reversible photo-crosslinking/photo-cleavage behavior was evaluated based on the UV–vis spectroscopic analysis, and compared with the linear polymer, poly(7-(4-vinyl-benzyloxyl)-4-methylcoumarin (PVBMC)). The absorbance at 319nm in the UV–vis spectrum gradually decreased under UVA irradiation (λmax=365nm), and then rapidly recovered under UVC irradiation (λmax=254nm). The fluorescence intensity of MCTH40 (λmax=469nm) recovered to 85.2% of original level after photo-cleavage under UVC irradiation, higher than 83.5% of PVBMC (λmax=472nm). The UV–vis analysis results indicated that MCTH40 performs more rapid photo-response than linear PVBMC under the same conditions. Furthermore, the average doses of UVA irradiation for the maximum degree of photo-crosslinking were 22.08Jcm−2 for MCTH40 and 28.29Jcm−2 for PVBMC. The average UVC doses of complete photo-cleavage were 9.44Jcm−2 for MCTH40 and 9.58Jcm−2 for PVBMC. The GPC analysis indicated that the average molecular weight and its PDI of MCTH40 showed a slight increase after three reversible cycles.

Immobilised porphyrins in monoterpene photooxidations

Porphyrins were covalently linked to modified Merrifield polymers by chlorosulphonation activation of the porphyrin nucleus. These supported porphyrins were used as photosensitizers to promote singlet oxygen oxidation of monoterpenes with an efficiency that depends on porphyrin structure and the spacer used to link it to the polymer structure. The performance of these photosensitizers was studied. Citronellol and α-terpinene gave the expected singlet oxygen ene addition products. α-Pinene and β-pinene also gave products from non-ene reactions, which is explained by the existence of an alternative radical pathway.

Facile, Efficient Routes to Diverse Protected Thiols and to Their Deprotection and Addition to Create Functional Polymers by Thiol−Ene Coupling

Carbazole, dinitrobenzoate, phenol, pyridine, and 4-cyano-4‘-alkoxybiphenyl side groups were grafted onto pendant vinyl groups of polybutadiene (PB) by thiol−ene addition to yield functional polymer of polydispersity equal to that of the precursor material. Synthesis protocols that are clean (>90% conversion to desired product) and scalable were developed to incorporate a protected thiol group into functional precursors. The resulting acetyl or benzoyl thioesters are suitable for long-term storage, and are conveniently deprotected and added to polymer in a matter of hours in a straightforward procedure that does not require isolation of the thiol intermediate. Alternatively, PB functionalization could be achieved with equal success by reaction with β-mercaptoethanol (BME), and subsequent esterification of the incorporated hydroxyl groups with a suitable acyl halide. The chemistry described can potentially be applied to add any side group onto any polymer, copolymer, or block copolymer displaying pendant vinyl groups.

Indium Triflate Catalysed Ene and Friedel Crafts Additions to α-Imino Esters: A One-Pot Synthesis of α-Amino Acid Derivatives

The indium catalysed additions of electron-rich alkenes and heteroaromatics to an activated α-imino ester is described. The moisture-sensitive substrate is prepared in situ facilitating a convenient synthesis of unnatural α-amino acid derivatives in one-pot from commercially available reagents. Keywords: Indium, Lewis acid, Catalysis, Friedel-Crafts, Imino-ene

Radical‐mediated modification of polypropylene: Selective grafting via polyallyl coagents

AbstractSelective graft modifications of polypropylene (PP) are demonstrated in which desirable functionality is introduced without the degradation that accompanies conventional radical‐mediated processes. A range of modification strategies is presented, each exploiting triallyl trimellitate (TATM) or its derivatives to counteract the effects of macroradical fragmentation on the molecular weight. Model compound studies, as well as examinations of atactic PP reaction products, show that allylic ester activation occurs predominately by a radical‐addition/hydrogen‐transfer sequence, with a limited propensity for telomerization. The cografting of TATM and maleic anhydride leads to maleated PP of a high melt viscosity, whereas the apparent incompatibility of TATM with vinyltrimethoxysilane requires the use of TATM‐assisted thiol–ene addition and/or diallyl silane grafting to produce moisture‐curable PP derivatives. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 4882–4893, 2005

Terminal Functionalization of Polypropylene by Radical-Mediated Thiol−Ene Addition

The radical-mediated addition of alkanethiols to the terminal unsaturation of polypropylene (PP) is used to prepare functional polyolefin derivatives without altering the molecular weight of the parent material. The sulfide product of thiol−ene addition, as well as the internal olefin resulting from a remarkable thiyl radical-mediated olefin migration, is characterized using a model hydrocarbon, 2,4-dimethylhept-1-ene. The yield of 3-mercaptopropyltrimethoxysilane (MPTMS) addition to atactic-PP is examined in detail, and the functionalization of high molecular weight isotactic-PP is used to produce moisture-curing resins that bind to siliceous fillers. The limited unsaturation content of PP, along with the high temperatures needed to modify the resin in its melt state, is shown to impact negatively on the reaction conversion due to the reversibility of thiyl radical attack on olefin.

Fireproofing of polyurethane elastomers by reactive organophosphonates

AbstractPolyurethane elastomers were prepared with hydroxytelechelic polybutadiene (HTPB) as polyol, modified 4,4′‐diphenylmethane diisocyanate (modified MDI) as liquid polyisocyanate, and phosphonate diols as chain extenders and flame retardant compounds. These phosphonate diols were synthesized by radical thiol–ene addition of allyl or vinyl dialkyl phosphonate to 3‐mercapto‐1,2‐propanediol. For various percentages of phosphorus (0 to 3%, w/w), polyurethane elastomers remain stable up to 250 °C. The percentage of residual char at 600 °C increases with increasing phosphorus content. For the soft segments, no variation in the glass transition temperature (Tg) is observed as the percentage of P increases, whereas the Tg of hard segments increases. Above 0.5% phosphorus content, the limiting oxygen index (LOI) becomes higher than the percentage of oxygen in the air.© 2003 Society of Chemical Industry

Synthesis and properties of ultraviolet‐curable resins via a thio–ene (thiol and allyl) addition reaction

AbstractBenzophenone diallyl ester (I) and benzophenone tetraallyl ester (II) based on 3,3′,4,4′‐benzophenone tetracarboxylic dianhydride (BTDA) with allyl alcohol (AAL) were synthesized. Glycidyl methacrylate (GMA) was added to I and formed diallyl diglycidyl methacrylate (III). These BTDA‐based allyl‐containing compounds (II and III) reacted with 1,4‐butanedithiol and 4,4′‐thiol‐bisbenzene‐thiol to produce ultraviolet (UV)‐curable resins via a thio–ene addition reaction. The ester (III) was cured easily when exposed to UV or sunlight radiation without any photoinitiator and only required a lower thermal curing temperature. The diallyl ester (I) and tetraallyl ester (II) required the addition of benzophenone to increase the photosensitivity, which reduced the exposition time. These resins used AAL as a monomer to successfully reduce the oxygen effect of the photocuring. The resin BTDA–2Allyl–2GMA had a glass‐transition temperature of 166°C and a hardness of 6H. The resultant UV‐curable coatings had excellent hardness, chemical resistance, adhesion, and tensile properties. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 1878–1885, 2002

Ene synthesis of bicyclic arylmethylenedihydropyrazoles using 4-phenyl-4,5-dihydro-3H-1,2,4-triazole-3,5-dione

8-Acetyl-7-aryl-2-arylmethylene-8,9-diaza- and 4,8,9-triazabicyclo[4.3.0]non-9-enes react with4-phenyl-4,5-dihydro-3H-1,2,4-triazole-3,5-dione, following the ene addition pattern. Under similar conditons 7-aryl-2-arylmethylene-8-methyl-8,9-diazabicyclo[4.3.0]non-9-enes give rise to both mono- and polyadditionproducts. The product structures were studied by 1H and 13C NMR, IR, and UV spectroscopy and singlecrystal X-ray diffraction.

The formyl C–H⋯O hydrogen bond as a critical factor in enantioselective Lewis-acid catalyzed reactions of aldehydes

X-Ray crystallographic studies have provided experimental evidence for the existence of intramolecular formyl C–H hydrogen bonds to oxygen or fluorine ligands in complexes of aldehydes and boron Lewis acids. This type of hydrogen bond can be regarded as ‘induced’ or ‘cooperative’ in the sense that its strength can be expected to increase as the bonding between the formyl oxygen and the Lewis acid becomes stronger. Coplanarity of the formyl group and the metal–X subunit to which it is bound in a five-membered ring effectively restricts rotation about the donor–acceptor bond between the formyl oxygen and the metal center of the Lewis acid, thus creating an additional organizing element in these complexes. This organizing element provides a simple and logical basis for understanding the mechanistic basis for enantioselectivity in many reactions of achiral aldehydes which are catalyzed by chiral Lewis acids. These reactions include aldol, allylation and ene addition to the formyl CO group and Diels–Alder reactions of α,β-unsaturated aldehydes with 1,3-dienes. The idea of the induced formyl C–H hydrogen bond can serve as a guide in the design of new enantioselective catalysts as well as a mechanistic principle for understanding preferred transition state assemblies.

Photoinitiated cross-linking of a thiol–methacrylate system

The photoinitiated thiol–ene cross-linking of a dimethacrylate polyether of Bisphenol A and pentaerythritol tetrakis 2-mercaptopropionate was studied in the presence of 2,2-dimethyl-2-hydroxy acetophenone (Darocur 1173). Two complementary techniques were used: photocalorimetry and real-time infrared spectroscopy. In the first part, the kinetic reaction was characterized by a stoichiometric mixture in reactive functions. The influence of temperature, photoinitiator concentration and ultraviolet (UV) light intensity was investigated. The results mainly show that the methacrylate homopolymerization is faster than the thiol–ene addition so that the reaction is usually stopped because of the complete consumption of the methacrylate double bonds. A theoretical approach has allowed us to determine a transfer constant of 0.26. Moreover, an increase in the autoacceleration rate was observed in the presence of the thiol monomer. In the second part, the molar fraction of methacrylate double bonds and thiol functions was changed to determine the effect on the kinetic reaction and the glass transition temperature of the final material.

ChemInform Abstract: Tandem Ene Addition/Diels-Alder Cycloaddition Reactions of Methyl Linoleate with 4-Phenyl-1,2,4-triazoline-3,5-dione and Diethyl Azodicarboxylate.

Treatment of methyl linoleate with 4-phenyl-1,2,4-triazoline-3,5-dione affords 1:1 adducts resulting from ene addition, and 1:2 adducts formed by ene addition followed by Diels Alder cycloaddition....