Melt-crystallization of syndiotactic polystyrene (sPS) is studied using simultaneous small/wide angle X-ray scattering (SAXS/WAXS). Emergence, intensification, and saturation of WAXS reflections within 120 s at isothermal crystallization temperature T-c = 250 degrees C after quenching from 300 degrees C illustrate crystallinity development through successive nucleation of instantaneously stabilized crystallites of lateral (hk0) coherence length Lambda approximate to 75 nm. The corresponding SAXS profiles exhibit increased density heterogeneity due to formation of nanograins; fitting of time-resolved SAXS data with a model of arrayed disks reveals slightly thickened (l(c) = 7.4-8.4 nm) disks of radius R approximate to 9 nm and long period L approximate to 19 nm with polydisperse stacking number slowly enhanced from an average of 1.2 to 2.4. The moderate ratio of Lambda/2R approximate to 4 indicates imperfect coalescence of nanograins in the lamellar assembly process. More importantly, with successively increased T-c from 250 to 260 degrees C the development of density heterogeneity of SAXS invariant Q(inv) is found to precede that of the WAXS-determined crystallinity X-c increasingly more, suggesting a precursor mesophase with non-crystalline nanograins. Avrami analysis for the developments of X-c and Q(inv) further reveals the same Avrami exponent n approximate to 3, consistent with heterogeneous nucleation of crystallites; the corresponding Avrami rate constant kappa(1/n)(T-c) extracted from X-c is correlated to the crystal growth rates G(T-c) described by the Hoffman-Lauritzen theory with a common surface-nucleation rate constant K-g. A lateral surface energy sigma approximate to 17 mJ m(-2) can be deduced on the basis of K-g using previously determined fold surface energy sigma(e) approximate to 27 mJ m(-2). Correspondingly, kappa(1/n)(T-c) extracted from Q(inv) is described by a mesophase model proposed by Strobl with a zero-growth temperature of the nanograin mesophase ca. 30 K below the beta-crystal melting point of sPS. Together, these results suggest that sPS melt-crystallization proceeds with nucleation of non-crystalline nanograins (SAXS-before-WAXS regime) at the crystal growth front, followed by crystallization of the nanograins with similar kinetics to sustain the growth front. Branching/twisting can occur at the lamellar growth front due to frustrated nanograin alignment, favoring the microscopically observed sheaf-like crystal morphology. (C) 2016 Elsevier Ltd. All rights reserved.