= Solution
The end groups associate reversibly through a complementary hydrogen-bond array. At low <concentration>, bringing the two ends of the same <molecule> together avoids the concentration-dependent cost of finding another <molecule>, so an \b[intramolecular ring] is favored.
For A, increasing <concentration> permits end-to-end intermolecular association to compete successfully with ring closure. It produces long, reversible supramolecular chains. Increasing chain length and entanglement account for the highly viscous solution without requiring covalent <polymerization>.
B has more favorably positioned attachment points on its rigid aromatic core. Its arms can close a less strained ring, so intramolecular association remains strongly competitive over the stated range. Predominantly discrete rings do not form the same extended chain network, explaining the retained fluidity. Both compounds can make the same local <hydrogen bonds>; their different balance of ring <strain>, preorganization and intermolecular association explains the macroscopic difference.
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