The microporous β polymorph of the title compound has been prepared as a crystalline product by slow removal of the guest from the inclusion compound with methylene chloride, [CuL 2 ]*2/3(CH 2 Cl 2 ) (L is 1,1,1trifluoro-5,5-dimethyl-5-methoxyacetylacetonate, {CF 3 COCHCOC(CH 3 ) 2 OCH 3 } -). Collapse of the crystals of the β polymorph into the thermodynamically stable R form has been studied by visual observation and DSC calorimetry, the product exhibiting much higher kinetic stability than the polycrystalline samples prepared so far. The crystal structure of the β polymorph was determined by single-crystal XRD methods both at 293 and 173 K. The β polymorph retains the same 3D polymeric coordination framework as found previously in inclusion compounds of the complex. The structures of the β and R polymorphs are compared and the variation of the β matrix in a series of compounds was analyzed. The β framework described in this work seems to be the most flexible of all coordination polymers known so far, preserving its microporous structure upon guest exchange or removal; the complete range of change in molar volume observed for the matrix is 8.6%.