It is shown that assuming linearity of the polymer class specific correlation between the glass transition temperature and the ratio mass to effective number of 'flexible' bonds of monomeric (repeat) unit, mu/rho, it is possible to evaluate by least square fit the 'flexibility' factor, rho, characterizing the share of those simple bonds within the repeat unit responsible for conformational rearrangements of the polymer. The evaluated effective number of simple bonds accounts for the influence of polymer class specific interactions and steric hindrances on the 'free rotation' of the simple bonds comprised in the repeat unit. It is shown that the well-known increase of the glass transition temperature of polymers bearing longer n-alkylic side chains may be the result of prefer-red 'crankshaft' like motions of neighboring methylenes, hindering the 'free rotation' of the simple bonds between the methylenes involved in the 'crankshaft'. It is also possible to quantify steric hindrances of bulky substituents. (c) 2004 Elsevier Ltd. All rights reserved.