The coalescence rate in immiscible polymer blends predicted by all existing models is two or three orders of magnitude lower than that observed experimentally. The model developed in this study considers for the first time that coarsening is driven by van der Waals forces and there is slippage at the interfacial boundaries between the moving particles and the matrix. The growth rate of the average radius of the particles, R, can be approximately expressed as dR/dr = A exp (-sigma(2))/9pietabetaR(2) where beta is a function of the interfacial slippage length b, R, and the average surface distance between the particles h. The parameter b depends on the incompatibility and the degree of entanglement at the interface between the two polymers of the blend. The fact that the coarsening rate is decreased by the presence of a copolymer can be explained by a decrease in the value of b.