Methylene spacer

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). 

The presence of three oxyethylene units in the spacer of PTEB slows down the crystallization from the meso-phase, which is a very rapid process in the analogous polybibenzoate with an all-methylene spacer, P8MB [13]. Other effects of the presence of ether groups in the spacer are the change from a monotropic behavior in P8MB to an enantiotropic one in PTEB, as well as the reduction in the glass transition temperature. This rather interesting behavior led us to perform a detailed study of the dynamic mechanical properties of copolymers of these two poly bibenzoates [41]. 

Monomers 24 and 25 behave differently when exposed to catalyst 14, shown in Fig. 8.15. Divinyltetramethyldisiloxane 24 is found to be metathesis inactive due to similar steric inhibitions experienced with divinyldimethylsilane. Monomer 25 is synthesized with one additional methylene spacer unit between the silicon atom and the olefin moiety, which then is reacted with Schrock s [Mo] catalyst. Here, metathesis occurs quite readily, exclusively forming a seven-membered cychc molecule (26) instead of polymer. The formation of the cyclic product can be explained by tire Tliorpe-Ingold effect.15... 

Other groups such as esters, silylethers, and imides are also successfully incorporated through ADMET depolymerization with 14 (Fig. 8.21).49 For an ester functionality, at least two methylene spacer units must be present between die olefin site and die functional group in order to achieve depolymerization. This is due to die negative neighboring group effect, a deactivation of the catalyst by coordination of the functionality heteroatoms to die catalyst.50 By physically... 

Imide-terminated telechelics are also synthesized by metathesis depolymerization, and it is found that phthalimide-substituted olefins allow for productive depolymerization when only one methylene spacer separates the nitrogen atom and the olefin (Fig. 8.21). This combination of steric hindrance around the nitrogen lone pair and decreased electron donation from resonance prevents the negative neighboring group effect. However, secondary acyclic amines are unable to produce telechelics through metathesis depolymerization because of unfavorable catalyst-amine interactions. 

The mesogenic units with methylenic spacers were prepared by reacting the sodium salt of either 4-methoxy-4 -hydroxybiphenyl or 4-phenylphenol with a bromoester in DMF at 82° C for at least 4 hours in the presence of tetrabutylammonium hydrogen sulfate (TBAH) as phase transfer catalyst. In this way, ethyl 4-(4-oxybi-phenyl)butyrate, ethyl 4-(4-methoxy-4 -oxybiphenyl)butyrate, ethyl 4-(4-oxybiphenyl)valerate, ethyl 4-(4-methoxy-4 -oxybiphenyl)-valerate, n-propyl 4-(4-oxybiphenyl)undecanoate and n-propyl 4-(4-methoxy-4 -oxybiphenyl)undecanoate were obtained. These esters were hydrolyzed with base and acidified to obtain the carboxylic acids. The corresponding potassium carboxylates were obtained by reaction with approximately stoichiometric amounts of potassium hydroxide. Experimental details of these syntheses were described elsewhere (27). 

A second strategy is to attach a linker (also referred to as a handle or anchor) to the resin followed by assembly of the molecule. A linker is bifunctional spacer that serves to link the initial synthetic unit to the support in two discrete steps (Fig. 3). To attach a linker to a chloromethyl-PS resin, a phenol functionality such as handle 4 is used to form an ether bond (Fig. 4). To attach the same handle to an amino-functionalized support, acetoxy function 5 or a longer methylene spacer of the corresponding phenol is applied to form an amide bond. Both of these resins perform similarly and only differ in their initial starting resin [4], An alternative approach is to prepare a preformed handle in which the first building block is prederivatized to the linker and this moiety is attached to the resin. For peptide synthesis, this practice is common for the preparation of C-terminal peptide acids in order to reduce the amount of racemization of the a-carbon at the anchoring position [5],... 

Figure 5.12 Dependence of peak frequencies in infrared spectrum of glucosamide bolaam-phiphiles NC( )GN-GLC (14) on methylene spacer length n. (a) The CH2 antisymmetric vas and symmetric vs stretching vibrations reveal gauche-included conformation for short chains and an all-trans conformation for longer chains, (b) Amide I and II frequencies show an even-odd effect for n > 10. Reprinted from Ref. 53 with permission of Wiley-VCH.

Fig. 6 Typical PET probes (a) and representative fluorescence light-up responses toward selected metal ions in tabulated (b) and graphical form (c trace 1 = 14, trace 2 = 14-(Zn2+)2, trace DMA = 9,10-dimethylanthracene in MeCN). Color code coordinating atoms in blue, atoms which take part in the complexation and show (main, in 14) PET activity in orange, fluorophore in green. Lincoln and co-workers have demonstrated that the attachment of two dimethylamino groups through propylene spacers to the 9,10-positions of anthracene has a more than 100-fold weaker PET activity than the attachment through methylene spacers [62]. The blue N atoms in 14 are thus predominantly responsible for coordination. For symbols, see Fig. 3. Quantum yield of 14 in MeCN estimated from intensity readings published in [61] and quantum yield data of the parent compound without active PET, DMA, published in [63]. (Reprinted in part with permission from [61]. Copyright 1988 American Chemical Society)...

Most fluorescent PET molecular sensors, including pH indicators of this type, consist of a fluorophore linked to an amine moiety via a methylene spacer. Photo-induced electron transfer (see Chapter 4, Section 4.3), which takes place from amino groups to aromatic hydrocarbons, causes fluorescence quenching of the latter. When the amino group is protonated (or strongly interacts with a cation), electron transfer is hindered and a very large enhancement of fluorescence is observed. 

However, separation of the carborane cage from The phos-phazene ring or chain by a methylene spacer group allows metals to be Inserted into the open face of the carborane. These syntheses were accomplished by the reaction routes shown in Schemes 3 and A. High polymeric analogues of these transformations have also been accomplished following polymerization of XXX. The rhodium-bound cyclophosphazenes and polyphosphazenes are catalysts for the hydrogenation of 1-hexene. In this, they show a similar behavior to metallocarboranes linked to polystyrene... 

Fig. 2. Comparison of HNMR-sig-nals between the concave pyridine bis-lactame 3c (Table 1) and the concave pyridine bissulfonamide 13a (s. Scheme 3). While for 13a only one set of signals exists for the pyridine H-atoms ( > 7 ppm) and the H-atoms of the methylene spacer ( < 5 ppm), the spectrum of 3e shows three sets of signals (ZZ, EZ and EE) [12a]...

The variations on the spacer module within 29 are available from Dabes-tani s laboratory [89]. The methylene spacer has been systematically extended to the tetramethylene case. As may be anticipated from previous studies on structurally similar aminoalkylaromatic PET systems [90-92], the fluorescence quantum yield (4>f) of the ion-free sensor rises gradually with increasing spacer length as the PET rate drops. Potassium ions cause moderate to small fluorescence enhancements, but the same trend can be discerned in the values, though muted. As usual, protons cause the largest enhancements of all and the ([>f values remain constant across the series since PET has been completely killed off. 

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