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21.
In the reaction of triisobutylene with AlBr3/DBr system, backbone isomerization and formation of higher oligomers takes place, in addition to deuteration. Both reactions are explained by the cleavage of the backbone bonds of the intermediate carbonium ions. The fragments formed can react with other components of the reaction system. The possible role of these reactions in the mechanism of termination and chain transfer in the cationic polymerization of isobutylene is discussed briefly. 相似文献
22.
Jaroslav Klal Milan Houska Otakar Sey
ek Petr Admek 《Macromolecular chemistry and physics.》1973,164(1):249-263
The method of azoisobutyronitrile (AIBN)-initiated bulk copolymerization of styrene (S) and methacrolein (M) was employed to determine the monomer reactivity ratios rS = 0.15 and rM = 0.55 (QM = 1.50, eM = 0.78). Quantitative IR spectroscopy in the region of 4000–450 cm?1 was used to elucidate the structure of copolymers. In addition to methacrolein units containing aldehyde groups, the copolymers may also contain cyclic hemiacetal structures formed by the reaction of the carbonyls of the neighboring methacrolein units. The carbonyl absorption at 1722 cm?1 showed that, up to 30 mole-% methacrolein in the copolymer, almost all methacrolein units have free aldehyde groups. The number of cyclic structures increases with the concentration of methacrolein in the copolymer. The experimental data are in agreement with the statistical theory on the sequence length distribution. The shift of the styrene band at 540 cm?1 to higher wavenumber was investigated and related to the shortening of sequence length of the styrene units. The intensity of the styrene band at 760 cm?1 was related with the distribution of the MSM triads and the copolymer structure. 相似文献
23.
24.
Serum progesterone at the time of human chorionic gonadotrophin does not predict pregnancy in in-vitro fertilization and embryo transfer 总被引:2,自引:0,他引:2
Bustillo Maria; Stern J.Jaroslav; Coulam Carolyn B. 《Human reproduction (Oxford, England)》1995,10(11):2862-2867
Controversy exists as to whether the serum concentration ofprogesterone on the day of human chorionic gonadotrophin (HCG)administration following ovarian stimulation for in-vitro fertilization(IVF) and embryo transfer can be used to predict the likelihoodof success. This retrospective study was undertaken to answerthis question by analysing a large population of IVF and embryotransfer cycles (n = 756). In addition to the concentrationof progesterone on the day of HCG administration, all variablesknown to impact on IVF and embryo transfer success (such aspatient age), indication for IVF and embryo transfer, numberof oocytes retrieved and the number of embryos generated andtransferred were examined. There was a significant increasein the number of oocytes retrieved with increasing progesteroneconcentration at the time of HCG administration. However, therewas no correlation of progesterone concentration at HCG administrationwith pregnancy and implantation rates. It is concluded thatprevious reports associating a slight elevation of progesteronein gonadotrophin- releasing hormone agonist ovarian stimulationcycles for IVF and embryo transfer may be misleading becauseof a small sample size or the presence of confounding variablesthat affect IVF and embryo transfer success. 相似文献
25.
26.
Vitezslav Kriz Jaroslav Mares Parri Wentzel Nina S Funa Gabriela Calounova Xiao-Qun Zhang Karin Forsberg-Nilsson Maud Forsberg Michael Welsh 《Developmental dynamics》2007,236(9):2485-2492
SHB is an Src homology 2 domain-containing adapter protein that has been found to be involved in numerous cellular responses. We have generated an Shb knockout mouse. No Shb-/- pups or embryos were obtained on the C57Bl6 background, indicating an early defect as a consequence of Shb- gene inactivation on this genetic background. Breeding heterozygotes for Shb gene inactivation (Shb+/-) on a mixed genetic background (FVB/C57Bl6/129Sv) reveals a distorted transmission ratio of the null allele with reduced numbers of Shb+/+ and Shb-/- animals, but increased number of Shb+/- animals. The Shb- allele is associated with various forms of malformations, explaining the relative reduction in the number of Shb-/- offspring. Shb-/- animals that were born were viable, fertile, and showed no obvious defects. However, Shb+/- female mice ovulated preferentially Shb- oocytes explaining the reduced frequency of Shb+/+ mice. Our study suggests a role of SHB during reproduction and development. 相似文献
27.
This paper presents an investigation of the kinetics of the decomposition of cumene hydroperoxide (1-methyl-1-phenylethyl hydroperoxide) (CHP) in benzene/methanol (95:5, by vol.) in the presence of copper(II) acetate (A) and different aminoalcohols (L), such as 2-aminoethanol, 2-(2-hydroxyethylamino)ethanol, 2-[bis(2-hydroxyethyl)amino]ethanol, 1-amino-2-propanol, and 2-(diethylamino)ethanol. It has been found that the decomposition of CHP takes place via a complex formed from the components of the reaction system. The individual components of the catalytic system (i. e. A or L), when applied separately, do not contribute to an acceleration of the decomposition reaction. Under the given conditions the initial rate of the CHP decomposition vo is expressed formally by the equation where [A2]0, [L]0, and [CHP]0 are the initial concentrations of the above mentioned compounds; k is the monomolecular rate constant for the decomposition of the complex between metal compound and CHP, and KL, Kc and KA are the equilibrium constants for the formation of the adduct from aminoalcohol and copper(II) acetate, the formation of the complex consisting of this adduct and CHP and for the dimerisation of copper(II) acetate, resp. 相似文献
28.
The interaction between alternating styrene/maleic anhydride copolymer as polymeric acceptor and styrene as low molecular weight donor in acetone and/or tetrahydrofuran was investigated by UV spectroscopy and polymerization technique. The equilibrium constant for the complex formation between styrene and the maleic anhydride structural unit of the styrene/maleic anhydride copolymer at 20°C was found to be 0,02 ± 0,001 dm3·mol?1 in acetone and 0,06 ± 0,003 dm3·mol?1 in tetrahydrofuran. The results of the thermally initiated polymerization of styrene in acetone in the presence of the alternating styrene/maleic anhydride copolymer indicate that the copolymer or the products of the interaction between the monomeric units of maleic anhydride in the copolymer and styrene do not initiate a radical polymerization of styrene. 相似文献
29.
Two paralogous mitochondrial malate dehydrogenase 2 (Mdh2) genes of Xenopus laevis have been cloned and sequenced, revealing 95% identity. Fluorescence in-situ hybridization (FISH) combined with tyramide amplification discriminates both genes; Mdh2a was localized into chromosome q3
and Mdh2b into chromosome q8. One kb cDNA probes detect both genes with 85% accuracy. The remaining signals were on the paralogous
counterpart. Introns interrupt coding sequences at the same nucleotide as defined for mouse. Restriction polymorphism has
been detected in the first intron of Mdh2a, while the individual variability in intron 6 of Mdh2b gene is represented by an
insertion of incomplete retrotransposon L1Xl. Rates of nucleotide substitutions indicate that both genes are under similar
evolutionary constraints. X. laevis Mdh2 genes can be used as markers for physical mapping and linkage analysis. 相似文献
30.
The effect of 2,2′-azoisobutyronitrile (AIBN) on the kinetics and mechanism of emulsion polymerization of butyl methacrylate was studied in the presence of anionic emulsifier disodium dodecylphenoxybenzene disulfonate (Dowfax® 2A1) at 60°C. The ratio between the proportion of the polymerization in monomer droplets and that of the polymerization in the aqueous phase was determined for the overall initial rate of butyl methacrylate polymerization in the region of the increasing polymerization rate (interval I). Using the model of polymerization in discrete particles, the portion of the polymerization in monomer droplets with a diameter of 100 nm in the overall polymerization rate is 24,4%; the portion of the polymerization in the water phase is only 0,022% for a concentration of Dowfax® 2A1 of 5 · 10?2 mol · dm?3, and 60,4% and 0,054% for a Dowfax® 2A1 concentration of 1 · 10?2 mol · dm?3. The exponent of the emulsifier concentration in the equation for the polymerization rate is 0,56 for interval I and 0,36 for interval II; the exponent for the concentration of AIBN over the conversion range between 0 and 30% is 0,34. For the proposed reaction mechanism it is assumed that 2-cyanoisopropyl radicals, generated from AIBN in the water phase, are responsible for the initiation of polymerization in micelles swollen by monomer and in polymer/monomer particles. Polymer/monomer particles are formed also by co-precipitation of oligomer radicals, which in turn are formed by polymerization of monomer molecules present in the water phase. Polymerization within monomer droplets has no significant influence on the course of emulsion polymerization. 相似文献