The Non-Free Draining Effect For Small Cyclics In Solution

It is commonly believed that cyclic polymers have smaller hydrodynamic radii (R(h)s) than their linear counterparts due to the constraint topology. This consensus is based on the results from polymers with high molecular weights (MWs), where the chains are considered as coils. Herein, we investigate the R(h)s of low MW cyclic polystyrenes (3000-11,000), cyclic alkanes and their linear counterparts by diffusion ordered NMR spectroscopy. For the first time, a non-free draining effect for small cyclics is revealed, due to the depletion of solvent molecules to pass through the rings, which leads to the increment of hydrodynamic size. With the decrement of polymer MW, this effect becomes more significant and R(h)s of cyclic polystyrenes approach that of their linear counterparts. Below a crossover point at MW around 660, R(h)s of cyclic polystyrenes are larger than their linear counterparts from the established scaling laws. This is supported by the observed phenomena that cycloalkanes have larger R(h)s than their n-alkane counterparts. The results are helpful in understanding the thermodynamic behaviors of small cyclics in solution, especially those bioactive small ring molecules in life science.