In: Chemistry
Describe why termination in radical polymerization by disproportionation and combination result in different polydispersities. Which results in a higher PDI?
In chemistry the dispersity is a measure of heterogeneity.Polydispersity index is ratio of weight average molar mass and number average molar mass.It can also calculated according to degree of polymerization i.e ratio of weight average degree of polymerization and number average degree of polymerization.if a free radical polymerization ended by combination the reaction results in an increase of the chain length,doubling the molecular weight .Also combination will result in a polymer that is C2 symmetric about the point of combination.If the free radical process end by disproportionation process chain length remains constant during termination reaction,in this case a hydrogen atom from one chain end is abstructed to another,producing a polymer with a terminal unsaturated group and a polymer with terminal saturated group,also in this case chains with terminal double bond are produced ,which can therefore lead to branching.So its clear both the cases homogeneity,uniformity will be different and dispersity is measure of homogeneity and uniformity,so polydispersities will be different in both the cases.As ending with disproportionation leads to a non uniform chain so its PDI will be more.