The addition of HPAM seems to play multiple roles in the crystal elongation phenomenon, firstly as a competing modulator. Since the pKa value for HPAM (4.5) is similar to that of TMA (pKa1 = 3.51, pKa2 = 3.89, pKa3 = 4.70), there will be competition with the linker to form coordination bonds with available Al. As a result, nucleation rate slows down and permits slow growth of large MOF crystals. Secondly, the amine (-NH2) groups in both HPAM and PAM increase the basicity of the reacting solution, providing ideal conditions for the formation of MIL-110 ascribed by the existence of HR crystals. HPAM is a copolymer composed of PAM and Na-PAA. To identify the dominant functional groups responsible to the formation of HR, MIL-96 was synthesized using equivalent concentrations of PAM and Na-PAA separately. Before the reaction, the solutions were very acidic upon addition of PAM (pH = 2), Na-PAA (pH = 2.4) and HPAM (pH =2.2). Such acidic environment increases the likelihood of -NH2 groups protonating to form primary ammonium ions (-NH3 + ) in preference to the ionization of the -COOH groups on the TMA due to higher pKa values. It is therefore expected to see the -NH2 groups contained in HPAM and PAM to govern the crystal elongation mechanism, transforming the HB shape into the HR habit more so than using Na-PAA.