A new method to obtain enzymatic rate coefficients for synthesis of biological homopolymers is presented. The new method involves fitting molecular weight distributions (MWDs) obtained by taking samples at successive times from an in vitro enzyme-catalyzed system. Enzymatic rate coefficients can then be obtained by fitting the observed MWDs to an assumed model. These types of results are complementary to data from traditional methods. The new method is applied to the formation of linear 'starch' molecules (alpha-1,4-linked glucose units) formed by phosphorylase-catalyzed propagation of monomer. The entire set of MWDs over a range of times are successfully fitted by an extended Michaelis-Menten model, supporting the validity of the mechanistic assumptions; the resulting set of rate parameters provide a more detailed description of the reactions than a simple Michaelis-Menten treatment. (C) 2011 Elsevier Ltd. All rights reserved.