A typical polymer sample contains a variety of chain sizes. The average molecular weight of these chains, as well as the breadth and shape of their distribution, can influence bulk polymer properties. Adjusting how quickly a polymerization initiator is added can help tune that distribution, report Dillon T. Gentekos, Lauren N. Dupuis, and Brett P. Fors of Cornell University (J. Am. Chem. Soc. 2016, DOI: 10.1021/jacs.5b13565). Working with polymerization of styrene initiated by an alkyl nitroxide, the team added the initiator in various ways. Compared with adding the initiator all at once, adding it steadily over as long as two-and-a-half hours broadened the polystyrene molecular weight distribution: The width increased for longer addition times, but the average chain length stayed the same. Adding the initiator at an increasing rate shifted the shape of the distribution curve, indicating a greater proportion of chains with lower molecular weight. And adding the initiator in two distinct doses created a bimodal distribution. “This approach is applicable to any controlled polymerization that uses a discrete initiator, and it allows the use of molecular weight distribution composition as a parameter to tune material properties,” the researchers write.