Divinylsiloxane‐bisbenzocyclobutene‐based polymer modified with polystyrene–polybutadiene–polystyrene triblock copolymers

Divinylsiloxane‐bisbenzocyclobutene (DVS‐bisBCB) polymer has very low dielectric constant and dissipation factor, good thermal stability, and high chemical resistance. The fracture toughness of the thermoset polymer is moderate due to its high crosslink density. A thermoplastic elastomer, polystyrene–polybutadiene–polystyrene triblock copolymer, was incorporated into the matrix to enhance its toughness. The cured thermoset matrix showed different morphology when the elastomer was added to the B‐staged prepolymer or when the elastomer was B‐staged with the DVS‐bisBCB monomer. Small and uniformly distributed elastomer domains were detected by transmission electron micrographs (TEM) in the former case, but TEM did not detect a separate domain in the latter case. A high percentage of the polystyrene–polybutadiene–polystyrene triblock copolymer could be incorporated into the DVS‐bisBCB thermoset matrix by B‐staging the triblock copolymer with the BCB monomer. The elastomer increased the fracture toughness of DVS‐bisBCB polymer as indicated by enhanced elongation at break and increased K1c values obtained by the modified edge‐lift‐off test. Elastomer modified DVS‐bisBCB maintained excellent electrical properties, high T_g and good thermal stability, but showed higher coefficient of linear thermal expansion values. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 1591–1599, 2006     

[2+2]-Photocycloaddition von Cyclohexen an 2-Acetyl-5,5-dimethyl-1,3-cyclohexandion und 3-Acetyl-1,5,5-trimethyl-2,4-pyrrolidindion

Summary Irradiation of solutions of excess cyclohexene and 2-acetyl-5,5-dimethyl-1,3-cyclohexanedione (1), and 3-acetyl-1,5,5-trimethyl-2,4-pyrrolidinedione (4) results mainly in the formation of 1,5-diones2 and5. These originate from intermediate cycloadducts of cyclohexene and theexo-enols of the cyclic 1,3-diketones. The yields decrease with increasing polarity of the solvent. In solution2 and5 are in equilibrium with the cyclic hemiacetales3 and6.

     

1.2-Bis(pentamethylcyclopentadienyl)tetrachlorodisilane and its reduction to decamethylsilicocene

Reaction of pentamethylcyclopentadienyl(pentachloro)disilane (2), prepared from hexachlorodisilane and potassium pentamethylcyclopentadienide (Cp^*K), with a further equivalent of Cp^*K leads selectively to the title compound Cp^* ₂ Si ₂ Cl ₄ (3) which was characterized by NMR and X-ray structural data. Dehalogenation of 3 with four equivalents of sodium naphthalenide offers an alternative route for the synthesis of decamethylsilicocene (1). Dedicated to Professor Mitsuo Kira on the occasion of being honoured with the Wacker Silicon Award 2005.     

Two types of intramolecular addition of an Al-N multiple-bonded monomer LAlNAr' arising from the reaction of LAl with N3Ar' (L = HC[(CMe)(NAr)]2, Ar' = 2,6-Ar2C6H3, Ar = 2,6-iPr2C6H3)

The reaction of beta-diketiminated aluminum(I) monomer LAl with a large bulky azide N3Ar' (L = HC(CMeNAr)2, Ar' = 2,6-Ar2C6H3, Ar = 2,6-iPr2C6H3) in the temperature range from -78 degrees C to room temperature affords two different isomers 2 and 3, which have been characterized by spectroscopic and X-ray structural analyses, as well as elemental analysis. The variable-temperature 1H NMR kinetic studies of this reaction indicate the existence of the monomer LAlNAr' (1) at low temperature and the thermal stability of the compounds increases in the order of 1 < 2 < 3.     

Form fluctuations of carbosilane dendrimers in dilute solution: a study using neutron spin echo spectroscopy

A series of carbosilane dendrimers with perfluorinated end groups has been prepared. The structure of these molecules in dilute solution is studied using small angle neutron scattering. For generations g<3 we find a non-spherical shape of the particles and a tendency for aggregation. This result is supported by the analysis of the diffusion coefficient obtained from photon correlation spectroscopy. The overall shape of the molecules is that of a core-shell particle. The generation 4 molecule is obtained as a compact sphere. Neutron spin echo spectroscopy reveals a relaxation time which is attributed to the form fluctuations of this particle.     

High regio-, chemo-, and stereoselectivity via low-temperature 4 + 3 cycloadditions. Convergent synthesis of multifunctionalized vinylmetals (M = Si, Sn) and S-vinyl benzenecarbothioates

A series of enantiomerically pure 8-oxabicyclo[3.2.1]oct-6-en-3-ones functionalized in the unsaturated two-carbon bridge has been prepared by the title reaction. Carbocation reactivity has been fined-tuned at -95 degrees C and adjusted to diene nucleophilicity. Conventional electrophilic substitution of 3-silylated and 3-stannylated furan is suppressed in favor of the rapid 4 + 3 cycloaddition mode. In the case of cycloadduct 13A, stereoselectivity (17:1) is perfectly matched to regioselectivity (17:1). High stereoselection as well as unprecedented regioselection and chemoselection is attributed to the low-temperature cycloaddition protocol and the design of chiral auxiliary and tether.