The Spacer Concept

The geometrical constraints of vinyl polymers are the consequence of the excessive density of surfactant side chains. The problem is a general one encountered in functionalized polymers, and can be overcome by reducing the density by the incorporation of spacer groups. In the case of main chain 

If well defined spacer segments are desired, the synthesis of polysoaps becomes more complicated. Homopolymerization of other than vinylic surfactant monomers, e.g. of epoxides [312], butadienes [168, 313] or vinylcyclo-propanes [168] is needed (for examples see Tables 2 and 4). Alternatively, strictly alternating copolymerization enables the preparation of defined repeat units longer than C2-fragments [54, 82, 168-170, 184-187, 189-192, 195-197, 

usually of C4 (Fig. 13). Polyaddition or polycondensation reactions provide defined spacer segments as well, but only few such systems (e.g. hydrophobized ionenes [119, 120] or phenol-formaldehyde resins [241-243, 

315] ) have been reported (see Figs. 2e, 3b). Presumably this lack is caused by the sensitivity of most of the polycondensations to humidity and protic solvents as encountered in many polysoap systems. Here the use of the thiol/ene addition polymerization [316] may offer a promising new route (Fig. 14). 

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Poly(epichlorohydrin) (PECH) and poly(2,6 - dimethyl-1,4-phenylene oxide) (PPO) containing pendant mesogenic units separated form the main chain through spacers of zero to ten methylene units were synthesized and characterized in order to test the "spacer concept." Both polymers were modified by phase transfer catalyzed esterifications of the chloromethyl groups (PECH) or the bromobenzyl groups (brominated PPO) with potassium co -(4-oxybiphenyl) alkanoates and potassium u-(4-methoxy-4-oxybiphenyl)-alk.an oates. While PPO required ten methylene units as a spacer and 4,4 -methoxybiphenyl as mesogen to present thermotropic liquid crystalline mesomorphism,... 

Also note that the spacer concept, as verified by several laboratories (e.g.. see Perrson and Jann-asch ), has been moved forward to the immobilization of other proton solvents such as phosphonic acid, with very promising results (see Section 5). 

All subchannel codes are used to solve mass, momentum and energy transport equations for turbulent fluid flow. Since the governing equations are very complicated, in each code simplifying assumptions are made to expedite solutions. Empirical input by either experimental data or analytical results is required for the codes to obtain reliable computer results. The correlations strongly depend on the geometry considered, the spacer concept (grids or wire-wraps), etc. 

The first studies of the orientational distribution function in liquid crystalline side chain polymers have employed NMR on selectively labeled polymers [31]. By revealing a gradient of decreasing order from mesogen via spacer to main chain, these studies have shown the basic validity of the spacer concept [98,99], namely, the decou-... 

There are different polymer structures that exhibit LC phases. The extension of the mesogen s rigid aromatic core leads to main chain polper systems. Other approaches are based on the spacer concept. Here rigid cores that are common in LMWLCs are decoupled by flexible alkyl chains. The mesogenic units can be placed either in the main or in the side chain. Also combined MCLCP/SCLCPs have been prepared (figure 3). 

Figure 3.7. The concept of chemistry in iutcrphascs visualisation of two different ways to obtain such a material, sol-gel procedure using different reagents (left) and silica grafted materials with sufficiently long spacers (right). Solvent occupies the space between the spacers...

Initially, chiral stationary phases for chiral liquid chromatography were designed for preparative purposes, mostly based on the concept of three-point recognition .47 Pirkle and other scientists48 developed a series of chiral stationary phases that usually contain an aryl-substituted chiral compound connected to silica gel through a spacer. Figure 1-14 depicts the general concept and an actual example of such a chiral stationary phase. 

Fig. 9.18 The polymer spacer concept for the construction of a biomimetic cell membrane on solids. Mesogenic units, coupling groups and the flexible polymer can be combined either in form of a statistical terpolymer (above). Variation of the ratio of the three monomers allows an easy tuning of the system. In an alternative system, an end-functio-nalized linear hydrophilic polymer chain bearing a coupling group at the proximal and the mesogen at the distal end was employed.

These experiments clearly confirm the theoretical concept of the helical twist of cholesteric phases developed by Goossens and Vertogen 79,86), which is based on the hindered rotation of chiral molecules around their long molecular axis and the introduction of the order parameter D. Furthermore they support the concept of the flexible spacer, described in Chap. 2.1. 

Figure 5.5 Schematic drawing illustrating the sheet-flow and a tortuous-path spacer concept.

De Gennes (Nobel prize in Physics, 1991) and Hervet 51] published a statistical treatment of starburst dendrimers. They concluded from a mathematical growth model that steric hindrance limiting continued tier addition was dictated by the length of the spacer units that connect the branching centers. This reinforced the dense packing concept. 

In most cases, the metal atoms are in fact end groups necessary to define a starting site and an arrival site, while the spacer is usually purely organic. The concept of a molecular wire requires something to be transferred between these two sites more efficiently than if the two sites were separated by empty space. The present chapter will deal essentially with electron (or hole) transfer, but the related topic of energy transfer (treated extensively in Part 1, Chapter 3 of this volume) will be also evoked. Representative examples of electron and energy transfer in polynuclear transition metal complexes can also be found in recent reviews by Juris, Balzani et al. [3a] and by De Cola and Belser [3b. ... 

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