Model interactions have been studied by phosphorescence quenching to obtain a better understanding of the chain length dependence of interpolymeric chain end-chain end reactions such as those involved in the termination step of free radical polymerization. For small molecule-polymer interactions in dilute cyclohexane solution, quenching rate constant (k(q)) data agree with the Smoluchowski equation prediction that k,scales as polymer molecular weight (MW) to the -1/2 power, confirming self-diffusion control. For polymer-polymer interactions in dilute solution, the chain length dependence is weaker than that predicted by translational diffusion control, as described by the Smoluchowski equation, but is stronger than that predicted by renormalization group theory. For interactions between 70000 MW benzil-end-labeled polystyrene and varying MWs of anthracene-end-labeled polystyrene at 300 g/L polymer, k(q) decreases by a factor of 10 in going from MWs of 100 to 1000 g/mol; beyond 1000 g/mol, k(q) is nearly independent of chain length. Such effects indicate that the importance of oligomeric radical self-diffusion and polymer radical chain-end segmental mobility must be carefully considered in understanding the termination process in free radical polymerization.