This paper presents a mixing model in the laminar regime without any adjustable parameters. It employs ideas from the micromixing theory as developed by Ottino. Striation thinning behaviour is obtained from Computational Fluid Dynamics (CFD). The model accounting for striation thinning predicts the Molecular Weight Distribution (MWD) of polypropylene (PP) during a degradation reaction in a static mixer by coupling CFD to the chemical model. Validation is performed by comparing model results to MWDs of PP degradation experiments in an experimental static mixer.