An experimental study of the finite strain response of annealed a and crystalline isotactic polypropylene (iPP) was conducted over a range of temperatures (25, 75, 110 and 135 degreesC) using uniaxial compression tests. Uniaxial compression results indicate nearly identical macroscopic stress vs. strain behavior for alpha-iPP and for beta-iPP to true strains in excess of -1.1 at room temperature despite the different initial morphologies. At larger compressive strains (> 1.2), beta-iPP shows more rapid strain hardening. The orientation of crystalline planes during straining differs at room temperature from that at high temperature, indicating a change of slip mechanisms as temperature increases. In addition, strain-induced crystallization occurred at the highest temperature examined in alpha-iPP. A continuous transformation of beta crystals to a crystals with inelastic deformation at room temperature was observed and it was facilitated at higher deformation temperatures. Scanning electron microscopy (SEM) observations of deformed beta-iPP provide strong evidence that the transformation is achieved via a solid-to-solid mechanism despite the different helical hands in alpha and beta crystal structures. Molecular simulations were used to investigate a conformational defect in the 31 helical chains of beta-iPP, characterized by a 120degrees helical jump. The propagation of this conformational defect along molecular chains provides the reversal of helical hand required by the solid-to-solid transformation. The beta-->alpha phase transformation in iPP is proposed to be accomplished via a solid transformation that includes slip along beta(110) and beta(120) planes during shear of the crystal lattice. (C) 2004 Elsevier Ltd. All rights reserved.