After the fundamental structure of semicrystalline polymers—platelike crystallites with thicknesses in the nanometer range, embedded in a liquid matrix—was discovered in the late 1950s, attention turned to the mechanism of formation. After intense controversial discussions, an approach put forward by Hoffman and Lauritzen prevailed and was broadly accepted. The picture envisaged by the treatment—platelike crystallites with atomically smooth side faces and a surface occupied by chain folds, growing sideways layer by layer with a secondary nucleation as the rate-determining step—was easy to grasp and yielded simple relationships. The main control parameter is the supercooling below the equilibrium melting point of a macroscopic crystal $T_f^{\infty }$, which determines both the thickness of the crystallites and their lateral growth rate. The impression that the mechanism of polymer crystallization was understood and the issue essentially settled, however, was wrong. Experiments carried out during the last decade on various polymer systems provided new insights which are now completely changing our understanding of such systems. They revealed a number of laws that control polymer crystallization and melting in bulk, showing in particular that the crystal thickness is inversely proportional to the distance to a temperature $T_c^{\infty }$ which is located above the equilibrium melting point, and that crystal growth stops at a temperature $T_{\mathrm{zg}}$ which is below $T_f^{\infty }$. Observations indicate that the pathway followed in the growth of polymer crystallites includes an intermediate metastable phase. In a proposed model a thin layer with mesomorphic inner structure forms between the lateral crystal face and the melt. The first step in the growth process is attachment of the coiled chain sequences of the melt onto the mesomorphic layer, which subsequently is transformed into the crystalline state. The transitions between melt, mesomorphic layers, and lamellar crystallites can be described with the aid of a temperature-thickness phase diagram. $T_c^{\infty }$ and $T_{\mathrm{zg}}$ are identified with the temperatures of the (hidden) transitions between the mesomorphic and the crystalline phase, and between the liquid and the mesomorphic phase, respectively. Comparing predictions of the model theory with experimental results from small-angle x-ray scattering, optical microscopy, and calorimetry yields in addition to the three equilibrium transition temperatures latent heats of transition and surface free energies.

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