Synthesis of graft copolymer derivatives of brominated poly(isobutylene-co-isoprene)

Strategies for preparing graft copolymers from brominated poly(isobutylene-co-isoprene) (BIIR) are demonstrated. Selective dehydrohalogenation of the allylic bromide functionality within BIIR to give an exo-conjugated diene is described, along with subsequent cycloadditions of maleic anhydride (MAn) and its mono-ester and di-ester derivatives. Alcoholysis of the bicyclic anhydride product of BIIR dehydrobromination/MAn cycloaddition is used to produce an IIR-g-PE copolymer in low yield. An alternate approach involving bromide displacement from BIIR by the salts of maleate half-esters is shown to be an efficient means of generating isobutylene-rich copolymers containing polyethylene, polyethylene oxide and polycaprolactone grafts.     

Practical and Efficient Acylation and Tosylation of Sterically Hindered Alcohols Catalyzed with 1-Methylimidazole

Acylation of sterically hindered alcohols is frequently encountered in synthetic chemistry. An efficient and mild procedure for tosylation and esterification of sterically hindered hydroxyl groups with p-TsCl, Ac2O and Bz2O in the presence of 1-methylimidazole is described. It was observed that auxiliary base, triethylamine, accelerates the acylation reaction.     

Thermal stability of brominated poly(isobutylene‐co‐isoprene)

The thermal stability of brominated isobutylene–isoprene rubber (BIIR) was investigated through studies of the elastomer and a model compound that accurately represented the reactive functionality within the polymer. An analysis of commercial BIIR and reaction products of brominated 2,2,4,8,8‐pentamethyl‐4‐nonene (BPMN) by NMR demonstrated that bromination of isobutylene–isoprene rubber by 1,3‐dibromo‐5,5‐dimethylhydantoin yielded a kinetically favored exomethylene substitution product, 3‐bromo‐6,6‐dimethyl‐2‐(2,2‐dimethylpropyl)‐1‐heptene ( 2 ), as opposed to the more stable endo‐isomer, (E,Z)‐4‐(bromomethyl)‐2,2,8,8‐tetramethyl‐4‐nonene ( 3 ). The exposure of BIIR and the brominated model compound BPMN to vulcanization temperatures led to the isomerization of 2 to 3 at a rate strongly dependent on HBr concentration. The elimination of HBr from these allylic bromides to produce exo‐ and endo‐conjugated dienes proceeded concurrently with isomerization, and the kinetics of these processes could be rationalized on the basis of a polar reaction mechanism. The product distributions obtained from both the model system and BIIR were consistent, thereby justifying an extension of the model compound approach to an analysis of BIIR vulcanization chemistry. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 2019–2026, 2001     

Sulfuration and reversion reactions of brominated poly(isobutylene‐co‐isoprene)

The sulfuration and reversion products of brominated poly(isobutylene‐co‐isoprene) (BIIR) were characterized through the use of a model compound, brominated 2,2,4,8,8‐pentamethyl‐4‐nonene (BPMN). The reaction of BPMN with S₈ produced bisallylic polysulfides of various ranks, yielding sulfur bromide intermediates that likely contributed to the rapid oxidation of allylic sulfides into thiophenes. Reductive cure reversion to pentamethylnonene was also observed in the latter stages of vulcanization. The reaction of 2,2′‐dithiobisbenzothiazole with BIIR and BPMN produced a stable adduct that reduced the concentration of allylic bromide available for vulcanization. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 1915–1926, 2003     

Nucleophilic difluoromethylenation of aldehydes and ketones using diethyl difluoro(trimethylsilyl)methylphosphonate

A new nucleophilic difluoromethylenation of aldehydes and ketones using diethyl difluoro(trimethylsilyl)methylphosphonate (1) has been achieved. The reactive carbanion species were generated under mild reaction conditions with a catalytic amount of initiator. This methodology was compatible with non-enolizable and in some cases with enolizable aldehydes and ketones and provided a straightforward access to phosphates of syn and anti 2,2-difluoro-1,3-diols.     

Nucleophilic, radical, and electrophilic (phenylsulfonyl)difluoromethylations

Selective incorporation of a fluoroalkyl moiety to modulate the properties of an organic molecule has become a frequently used strategy in life science- and materials science-related applications. In this context, selective introduction of a (phenylsulfonyl)difluoromethyl group (PhSO₂CF₂) into organic molecules has attracted much attention, since the PhSO₂CF₂ group can be regarded as a “chemical chameleon” that can be readily transformed into difluoromethyl (CF₂H), difluoromethylene (–CF₂–), and difluoromethylidene (CF₂) functionalities. This article overviews the recent development of (phenylsulfonyl)difluoromethylation reactions from 2003, including the nucleophilic (phenylsulfonyl)difluoromethylations with PhSO₂CF₂H, PhSO₂CF₂SiMe₃ and PhSO₂CF₂Br reagents, free radical (phenylsulfonyl)difluoromethylations with PhSO₂CF₂I reagent, and electrophilic (phenylsulfonyl)difluoromethylations with a hypervalent iodine(III)-CF₂SO₂Ph reagent.     

Nucleophilic (phenylsulfinyl)difluoromethylation of carbonyl compounds with difluoromethyl phenyl sulfoxide

We have successfully achieved nucleophilic (phenylsulfinyl)difluoromethylation of both enolizable and non-enolizable aldehydes and ketones by using difluoromethyl phenyl sulfone (1) as the fluoroalkylating agent. Although the chemical yields of the reactions are good to excellent, the observed diastereoselectivity is poor (dr = 1:1.04-2.03). The present synthetic methodology provides a convenient alternative for the preparation of α-(phenylsulfinyl)difluoromethylated carbinols that were previously synthesized via a two-step procedure.     

Enantioselective direct Mannich reactions of 3-substituted oxindoles catalyzed by chiral phosphine via dual-reagent catalysis

A combination of an amino acid-derived chiral phosphine catalyst and methyl acrylate to catalyze the direct Mannich reaction of 3-substituted oxindoles and imines has been reported to afford 3-tetrasubstituted oxindole derivatives which are key structures for biological activities. The products are formed with a quaternary carbon and featured with two adjacent chiral centers. Various N-EDG(electron-donating group) and N-EWG(electron-withdrawing group) protected oxindoles, including 3-aryl and 3-alkyl substituted ones, have been evaluated with aromatic and aliphatic imines under this catalytic system, smoothly giving desired products in good yields as well as excellent diastereo- and enantioselectivities.     

Synthesis and characterization of ether derivatives of brominated poly(isobutylene‐co‐isoprene)

Variations of the Williamson ether synthesis were employed to prepare a range of new derivatives of brominated poly(isobutylene‐co‐isoprene) (BIIR). Unambiguous characterization of the polymeric products was accomplished by spectroscopic comparisons to low‐molecular‐weight analogues derived from brominated 2,2,4,8,8‐pentamethyl‐4‐nonene, which served as a model for the reactive functionality found within BIIR. The substitution of bromide from BIIR occurred at moderate temperatures with stoichiometric amounts of quaternary ammonium phenoxide to yield O‐alkylation products in high yields. Simple mixtures of BIIR, KOH, and aliphatic alcohols generated the desired allylic ethers when heated above 110 °C in the absence of quaternary ammonium salts. Knowledge gained from these small‐molecule alkylations was used to prepare graft copolymers from BIIR and poly(ethylene oxide) through the exploitation of the apparent ability of polyethers to activate potassium alkoxides in nucleophilic substitutions. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 983–992, 2006     

p-(tricyanovinylidene)phenylhydrazones. Nucleophilic attack on carbon-carbon double bonds

The dispacement of a —CN group inp-(tricyanovinylidene)-phenylhydrazones, induced by amines, is investigated. The rate of the reaction is dependent on the polarity of the solvent and the observed second-order rate coefficientsk _obs ² for the —CN displacement increase with the amine concentration. A X-ray analysis of one of the substituted compounds is also reported.This work was supported by Ministero Pubblica Istruzione (Rome, 60%).     

Nucleophilic substitution of hydrogen in naphthalene by chloride(Cl~-)in ionic liquids

Nucleophilic aromatic substitution of hydrogen in non-activated aromatic ring,a very rare phenomenon in organic chemistry,is found in ionic liquids containing Cl^- as anion under mild reaction conditions.The reaction may be carried out by the addition of the halogen-bonding adduct（Br₂Cl^-） as nucleophile to aromatic ring carbon atom,leading to the formation of the nucleophilic substitution product.     

Nucleophilic Addition Reactions of Tricarbonyl [η5-1-(Phenylthio)Cyclohexadienyl]Iron(I) Complex

Hydride abstraction of tricarbonyl[η⁴phenylthio)-l,3-cyclohexadiene]iron(0) complex 2 with Ph₃C⁺PF₆^? regiospecifically provided the title compound 3 in excellent yield. Cationic complex 3 could react with a variety of nucleophiles in good yield. Hard nucleophiles prefer to attack at the C-l position, whereas soft and hindered nucleophiles favor attack at the C-5 position. Some synthetic applications were also studied.     

Selective nucleophilic substitution reactions on poly(epichlorohydrin) using aromatic and aliphatic thiol compounds

Poly(epichlorohydrin) has been modified chemically using aromatic and aliphatic thiol compounds. The reactivity and kinetics of these modifiers with respect to substitution and elimination was studied. Therefore, the chemical structure of the reaction products was analysed using ¹³C NMR, ¹H NMR and ¹³C-DEPT spectroscopies. It is shown that both, aromatic as well as aliphatic thiols, are highly selective with respect to nucleophilic substitution as reaction conditions can be found which allow one to achieve degrees of modification of up to 90% without any elimination side-reaction. As a consequence no degradative chain-scission takes place what has been confirmed by GPC analysis.A comparison between both types of thiol modifiers shows that aromatic ones react faster and that higher degrees of modification are reached than with their aliphatic homologues.     

Synthesis of thioether derivatives of brominated poly(isobutylene‐co‐isoprene): Direct coupling chemistry for silica reinforcement

Thioether derivatives of brominated poly(isobutylene‐co‐isoprene) (BIIR) were prepared through base‐mediated nucleophilic substitution reactions of the elastomer with thiolate ions formed in situ from thiols. The characterization of the reaction products was accomplished with studies of a model compound, brominated 2,2,4,8,8‐pentamethyl‐4‐nonene, the reactivity of which was shown to be consistent with that of the polymer. The reaction of aliphatic thiols with the allylic bromide functionality of BIIR required a strong base, and base‐catalyzed air oxidation of the thiol to disulfide operated concurrently to reduce the yield of the desired substitution products. An efficient and direct method of coupling BIIR to silica with (3‐mercaptopropyl)trimethoxysilane improved filler dispersion, thereby overcoming problematic agglomeration of reinforcing siliceous fillers in these compounds. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 2937–2944, 2002     

Nucleophilic attack by bromide ions as a first step of conversion of Se(VI) to Se(IV)

Numerous commonly used analytical methods allow only determination of a total amount of selenium in a given sample. Electroanalytical methods as well as those based on hydride generation or on formation of piazselenol allow only determination of Se(IV). To determine Se(VI) by these procedures, present alone or in mixtures with Se(IV), it is first necessary to convert Se(VI) to Se(IV). Such conversion is effective in the presence of excess of halides in acidic media or by photoreduction. In the often used conversion of Se(VI) in the presence of chlorides or less frequently of that of bromides, it has been assumed that the halide ion acts as a reducing agent. Kinetic studies of conversion of Se(VI) in acidic solutions containing an excess of bromide ions indicated that the rate determining first step of the reaction with Se(VI) is a nucleophilic substitution of the OH₂⁺ group in the protonated form of H₂SeO₄ by bromide ions. For the overall reaction with rate −d[Se(VI)]/dt = k₁[H⁺][Br⁻]^1.15[Se(IV)] the rate constant 1 × 10⁻³ L² mol⁻² s⁻¹ was found. The following formation of Se(IV) from the bromo derivative is a fast reaction probably resulting in elimination of HBrO.     

Blends of poly(carbonate) with poly(styrene) and poly(methylmethacrylate): phase morphology,mechanical, and thermal properties

This paper is concerned with the dependence of mechanical and thermal properties of heterogeneous blends of poly(carbonate) (PC) with poly(methyl-methacrylate) (PMMA) and with poly(styrene) (PS) on the concentration of the components. PS displays a very weak phase coupling in blends with PC, whereas PMMA is characterized by a strong coupling to PC. Experimental results as well as predictions based on composite theories are reported. The general finding is that mechanical properties, such as the tensile modulus and the dynamic shear modulus, as well as thermal properties, such as thermal expansion, are (i) only weakly affected by the occurrence of a phase inversion and of a continuous phase morphology, (ii) vary continuously with the concentration of the components, and (iii) are rather insensitive to the strength of the phase coupling. The theoretical predictions on the concentration—property relationship for these properties, based on a self-consistent approach, agree very well with those observed experimentally. The elongation at break as well as the yield stress, on the other hand, are strongly influenced by the nature of the phase coupling: a discontinuous variation of these properties with the composition is observed for PS/PC blends but not for PMMA/PC blends. The general conclusion is that a set of mechanical and thermal properties of heterogeneous blends can satisfactorily be predicted on the basis of rather simple composite theories.