Substitutional blocking

Synthesis of Saccharide-Substituted Block Copolymers Mimics of Glycoprotein Mucins... 

For the synthesis of carbohydrate-substituted block copolymers, it might be expected that the addition of acid to the polymerization reactions would result in a rate increase. Indeed, the ROMP of saccharide-modified monomers, when conducted in the presence of para-toluene sulfonic acid under emulsion conditions, successfully yielded block copolymers [52]. A key to the success of these reactions was the isolation of the initiated species, which resulted in its separation from the dissociated phosphine. The initiated ruthenium complex was isolated by starting the polymerization in acidic organic solution, from which the reactive species precipitated. The solvent was removed, and the reactive species was washed with additional degassed solvent. The polymerization was completed under emulsion conditions (in water and DTAB), and additional blocks were generated by the sequential addition of the different monomers. This method of polymerization was successful for both the mannose/galactose polymer and for the mannose polymer with the intervening diol sequence (Fig. 16A,B). 

There is evidence for process c in some systems where axial substitution blocks process d Os3(CO)10(MeCN)2 (334), Os3(CO)10(l,2-diazine) (304), and the Ru analog which has the tri-/i-CO form as the ground-state structure (339). Process c can also be identified for isomer 55 in the Ru3 series (336) and for the compounds l,l-Os3(CO)10(diene) (327,328). The fastest process in Os3(CO)10(C7H8)(C7H8 = norbornadiene) involves pairwise exchange of all... 

Branching at the a- or p-carbon is well known to retard the Sfj2 reaction in solution. " In fact, the diminution of reaction rate with increasing a-substitution is one of the hallmarks of the S 2 reaction, distinguishing it from the S 1 mechanism. The argument is basically one of the steric hindrance a-substitution blocks access to the backside of the C-LG bond, while the p-substitution precludes a linear attack of the nucleophile along the C-LG bond. 

The refractive index modulation of the block copolymer lb carrying cyano-azobenzene side-groups is more than twice that of the methoxy-substituted block copolymer 14. In addition, lb exhibits a much higher sensitivity than 14. This is just a consequence of (7), in which the sensitivity increases with the refractive index modulation. Furthermore, 14 exhibits a continuous decay after the writing laser is turned off, similar to that shown in Fig. 27. Block copolymer lb, in contrast, even shows an increase by 15% within a few hours, and the refractive index modulation is stable for years. In this case, the so-called post-development is not due to a different morphology as discussed above. Rather, the liquid crystalline character of the... 

The hydroboration studies on If (R = f-Bu) and 7 conclude that both bulky alkyl group and 1-trimethylsilyl substitution block the placement of second boron on that carbon. Consequently, the clean monohydroboration is achieved [14] with 9-BBN, under stoichiometric conditions when C C terminals possess these substituents. The reaction is highly regioselective (Eq. 5.27) and places the boron on the carbon bearing silyl substituent to afford the product in quantitative yields. 

Ohnishi, S., Orimoto, Y, Gu, F.L., Aoki, Y Nonlinear optical properties of polydiacetylene with donor-acceptor substitution block. J. Chem. Rhys. 127, 084702 (2007)... 

Henikoff S and J G Henikoff 1992. Amino Acid Substitution Matrices from Protein Blocks. Proceedir Hie National Academy of Sciences LISA 89 10915-10919. 

This aspect of electrophOic reactivity has been studied with several alkylthiazoles, and it is noteworthy that reduction of 4-(5-) nitrothia-zoles yields the amino derivatives that are good starting compounds for synthesis. Reaction takes place at the 5-position or at the 4-position if the 5-position is blocked, on the 4- and 5-positions at the same time but with poorer yields, if the 2-position is substituted (228, 231. 235-239, 244). 

Halogenation Bromination and chlorination of phenols occur readily even in the absence of a cata lyst Substitution occurs primarily at the position para to the hydroxyl group When the para position IS blocked ortho substitution is observed... 

A further development in the coumarin series is the use of derivatives of 3-phenyl-7-aminocoumarin ((13) where R, R = Cl or substituted amines) as building blocks for a series of light-stable brighteners for various plastics and synthetic fibers, and, as the quatemi2ed compounds, for brightening polyacrylonitrile (62). 

Other auxin-like herbicides (2,48) include the chlorobenzoic acids, eg, dicamba and chloramben, and miscellaneous compounds such as picloram, a substituted picolinic acid, and naptalam (see Table 1). Naptalam is not halogenated and is reported to function as an antiauxin, competitively blocking lAA action (199). TIBA is an antiauxin used in receptor site and other plant growth studies at the molecular level (201). Diclofop-methyl and diclofop are also potent, rapid inhibitors of auxin-stimulated response in monocots (93,94). Diclofop is reported to act as a proton ionophore, dissipating cell membrane potential and perturbing membrane functions. 

Polymerization ofiVIasked Disilenes. A novel approach, namely, the anionic polymerization of masked disilenes, has been used to synthesize a number of poly(dialkylsilanes) as well as the first dialkylamino substituted polysilanes (eq. 13) (111,112). The route is capable of providing monodisperse polymers with relatively high molecular weight M = lO" — 10 ), and holds promise of being a good method for the synthesis of alternating and block copolymers. 

Properties have been determined for a series of block copolymers based on poly[3,3-bis(ethoxymethyl)oxetane] and poly [3,3-bis(methoxymethyl)oxetane]- (9-tetrahydrofuran. The block copolymers had properties suggestive of a thermoplastic elastomer (308). POX was a good main chain for a weU-developed smectic Hquid crystalline state when cyano- or fluorine-substituted biphenyls were used as mesogenic groups attached through a four-methylene spacer (309,310). Other side-chain Hquid crystalline polyoxetanes were observed with a spacer-separated azo moiety (311) and with laterally attached mesogenic groups (312). 

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