Silyl chains

The triplet-sensitized photoreaction of stilbenophanes that caused trans-ds photoisomerization of stilbenophanes trans,trans-2, Z = SiMe2CH2-l,3-C6H4CH2SiMe2 trans,trans-, Z = CH2SiMe2CH2-l,4-CH2 and cis,cis-3, Z = CH2) was reported [98], A series of macrocyclic and medium-size stilbenophanes tethered by silyl chains... 

Water soluble p-sulfonato calbc[w]arenes (w = 8, la-lb and n=6, 2a-2b) was employed as host to control the outcome of photodimerization and photoisomerization of 4-stilbazoles [17]. Novel macrocydic and medium-size stilbenophanes tethered by silyl chains were synthesized, and their photochemical and photophysical properties were examined (Figure 5.5) [18]. Direct irradiation of macrocydic stilbenophanes stereoselectively gave intramolecular photocydoadducts, and the efficiency increased with decreasing distance between the two stilbene units. The triplet-sensitized photoreaction of stilbenophanes caused cis-trans photoisomerization. Photoreactions of as-fixed stilbenophanes under an oxygen atmosphere selectively led to phenanthrenophanes. Fluorescence quantum yields increased with the introduction of silyl substituents, and hence those of silyl-tethered stilbenophanes were larger than that of unsubstituted fraws-stilbene. Intramolecular excimer emis-... 

The term —2Djj K J +1) has Ihe efifecl of separating Ihe (J+ 1) componenls of each J + ) J Iransilion wilh dififerenl values of K. This is illuslraled in Figure 5.7, which shows Ihe eighl componenls of Ihe J = 8 — 7 Iransilion of silyl isolhiocyanale (HsSi—N=C=S), which has a linear SiNCS chain and is a prolate symmelric rolor. 

Chemical Grafting. Polymer chains which are soluble in the suspending Hquid may be grafted to the particle surface to provide steric stabilization. The most common technique is the reaction of an organic silyl chloride or an organic titanate with surface hydroxyl groups in a nonaqueous solvent. For typical interparticle potentials and a particle diameter of 10 p.m, steric stabilization can be provided by a soluble polymer layer having a thickness of - 10 nm. This can be provided by a polymer tail with a molar mass of 10 kg/mol (25) (see Dispersants). 

Anionic polymerisation techniques aie one of many ways to synthesise a special class of block copolymers, lefeiied to as star block copolymers (eq. 25) (33). Specifically, a "living" SB block is coupled with a silyl haUde coupling agent. The term living polymerisation refers to a chain polymerisation that proceeds in the absence of termination or transfer reactions. 

Substitution at C(5) of the penicillanic ring system has been relatively little explored. The 5a-phenyl analog has been prepared by total synthesis as shown in Scheme 26 75JMC486). It was interesting that epimerization (step iv) occurs without side chain silylation (cf. Section 5.11.3.8.4). 

It is also possible to prepare them from amino acids by the self-condensation reaction (3.12). The PAs (AABB) can be prepared from diamines and diacids by hydrolytic polymerization [see (3.12)]. The polyamides can also be prepared from other starting materials, such as esters, acid chlorides, isocyanates, silylated amines, and nitrils. The reactive acid chlorides are employed in the synthesis of wholly aromatic polyamides, such as poly(p-phenyleneterephthalamide) in (3.4). The molecular weight distribution (Mw/Mn) of these polymers follows the classical theory of molecular weight distribution and is nearly always in the region of 2. In some cases, such as PA-6,6, chain branching can take place and then the Mw/Mn ratio is higher. 

Recent progress of basic and application studies in chitin chemistry was reviewed by Kurita (2001) with emphasis on the controlled modification reactions for the preparation of chitin derivatives. The reactions discussed include hydrolysis of main chain, deacetylation, acylation, M-phthaloylation, tosylation, alkylation, Schiff base formation, reductive alkylation, 0-carboxymethylation, N-carboxyalkylation, silylation, and graft copolymerization. For conducting modification reactions in a facile and controlled manner, some soluble chitin derivatives are convenient. Among soluble precursors, N-phthaloyl chitosan is particularly useful and made possible a series of regioselective and quantitative substitutions that was otherwise difficult. One of the important achievements based on this organosoluble precursor is the synthesis of nonnatural branched polysaccharides that have sugar branches at a specific site of the linear chitin or chitosan backbone [89]. 

The reactions are radical chain processes (Scheme 3) and, therefore, the initial silyl radicals are generated by some initiation. The most popular thermal initiator is azobisisobutyronitrile (AIBN), with a half-life of 1 h at 81 °C. Other azocompounds are used from time to time depending on the reaction conditions. EtsB in the presence of very small amounts of oxygen is an excellent initiator for lower temperature reactions (down to —78°C). The procedures and examples for reductive removal of functional groups by (TMSlsSiH are numerous and have recently been summarized in the book Organosilanes in Radical Chemistry. ... 

---