It is shown, that both the mobility of polymers as well their transition temperatures (glass transition and crystallization) depend on the "flexibility" of simple bonds, i.e., on their ability to promote by energetically stimulated rotations conformational changes to release stresses. The polymer class specific interdependence between melting temperature, T-m and glass temperature, T-g, suggests that the "flexibility" of polymers depends additionally on the probability of interactions between sequences of polymer chains. Interaction between polymer chain sequences controls at the same time the ordering necessary for crystallization. Char- acteristic of polymers is thus the dependence of both transition temperatures on the "mass /'flexible bond" of the monomeric (repeating) unit, mu / rho. This experimentally observed polymer class specific behavior is reflecting the similar probabilities of interaction within a given class of polymers. (C) 2003 Wiley Periodicals.