The Polydispersity Index (PDI) is a dimensionless number that quantifies the distribution of molecular weights in a given polymer sample, calculated using the formula $$pdi = \frac{m_w}{m_n}$$, where $m_w$ is the weight average molecular weight and $m_n$ is the number average molecular weight. This index provides insight into the heterogeneity of polymer chains, with lower values indicating a more uniform sample and higher values suggesting a wider range of chain lengths and weights. Understanding PDI is crucial for characterizing materials and predicting their behavior in various applications.