Heterogeneous catalyst used for olefin polymerization are suspended in a fluid phase containing the monomer, and get encapsulated by the polymer formed. At the microparticle level, this deposited polymer offers diffusional resistance to the monomer transport to the catalyst fragment surface. We consider an additional effect that further resistance may be offered by the slow deformation of the deposited polymer mass that must move away from the catalyst surface where it is formed. We find that this may limit the overall reaction rates more severely than what is predicted if the viscoelastic resistance of the deforming polymer is neglected.