Does polychloroethylene have only van der Waals/London forces between its molecules? Surely, if all the chlorine atoms are on one side, due to the fact that chlorine is more electronegative than carbon, shouldn’t it have permanent dipole-dipole forces?
No, but for polymers of any reasonable size, the dispersion (and excluded volume) forces completely overwhelm any dipole-dipole forces, since they grow with the size of the molecule and dipole-dipole forces do not. Also, bear in mind that the chlorines aren't all on one side of the molecule first because the way it's manufactured tends to put Cls randomly on each side of the chain, and second because the molecule has considerable conformational flexibility. That's true even in the solid state, which for polymers is usually substantially glassy with interspersed domains of crystal.