Crystallization of isotactic poly(N,N-diethyl acrylamide) (i-PDEAA) prepared by ytterbium triflate/H O-catalyzed radical polymerization in the presence of an organotellurium chain transfer agent was studied. i-PDEAA single crystals were successfully grown from solution using a mixture of methanol and diethyl ether as the solvent. Selected-area electron diffraction patterns obtained by transmission electron microscopy provided clear information about the unit cell and suggested polymorphisms of the i-PDEAA crystals. The lattice constants of the Form-1 crystal were determined to be a = 0.530 nm, b = 0.342 nm, c = 1.009 nm, and α = β = γ = 90°. On the other hand, the lattice constants of the Form-2 crystal were only roughly estimated to be a = 0.73 nm, b = 0.49 nm, and γ = 90°. 2