I don't think fractional precipitation will separate polymers of different densities. However, density gradient columns can do this....such columns exist for common polymers such as polyester,probably for other polymers as well.

One way to distinguish between water soluble polymers of different densities is to place them on a test tube that contains a discontinuous sucrose density gradient which is prepared by layering successive decreasing sucrose density solutions upon one another. The polymer sample is then placed at the top of the tube and centrifuged. Each polymer will go to the sucrose layer that best matches its density.

In principle a similar method could be applied to polymers that are non water soluble by using a gradient of organic solvents

Here a mixture of polymers is placed on a gel and an electric current is applied. Low molecular weight move faster through the gel than high molecular weight polymers.