Rigid group principle

The synthesis of macrocycles with the aid of the dilution principle still remains a dominating cyclization method although there have been many attempts to avoid the standard procedure, i.e., the simultaneous addition of reactants to the reaction medium (see Sect. 4). The dilution principle can be avoided in some cases [90,92] by application of template-directed reactions and/or by use of the rigid group principle . 

The preparation of thiamacrocycles of the cyclophane type by substitution of thiolates on suitable substrates usually leads to the desired products in good yields [55], favoured by the rigid group principle [56] and by application of the dilution principle [1]. On the other hand yields of conformative flexible thia-cycloalkanes under similar conditions as a rule are not satisfactory and often low... 

This illustrates a general principle rigid groups - in the polymer backbone, or attached to it - make a polymer inflexible. 

Despite his numerous achievements, Mendeleyev is remembered mainly for the periodic table. Central to his concept was the conviction that the properties of the elements are a periodic function of their atomic masses. Today, chemists believe that the periodicity of the elements is more apparent when the elements are ordered by atomic number, not atomic mass. However, this change affected Mendeleyevs periodic table only slightly because atomic mass and atomic number are closely correlated. The periodic table does not produce a rigid rule like Paulis exclusion principle. The information one can extract from a periodic table is less precise. This is because its groupings contain elements with similar, but not identical, physical and chemical properties. 

---