The growth rate of polymer crystals decreases exponentially with rising temperature, thus indicating a growing activation barrier. The barrier is proportional to the inverse of the supercooling below the "zero growth temperature" T-zg, which is always located below the equilibrium melting point. In order to see the effect of counits on T-zg, we studied with time-dependent light attenuation measurements the growth kinetics of s-polypropylene and a derived statistical copolymer with 15% of octene counits. Using a home-built device, we achieved a sensitivity superior to DSC or X-ray experiments. An accurate extrapolation procedure yielded for both samples the zero growth temperature. The counits lead to a downward shift of T-zg by 40 K, which is larger than the downward shift of the melting point (30 K). We interpret the results in the framework of our multistage model which assumes participation of a transient mesophase in the growth process of polymer crystals. T-zg is identified with the equilibrium melting point of the mesophase. Knowledge of T-zg allows to establish for both samples a size-temperature nanophase diagram. Data evaluation yields the beat of melting and the surface free energy of mesomorphic layers as well as the activation barrier per monomer.