High-density polydicyclopentadiene

The invention provides a method of contacting a membrane having a highly cross-linked polydicyclopentadiene matrix with a feed solution having a) a first component with a molecular weight in the range of from about 100 g mol−1 to about 600 g mol−1 and a cross-sectional area of less than about 0.40 nm2 and b) a second component with a molecular weight in the range of from about 100 to about 600 grams g mol−1 and a cross-sectional area of greater than about 0.50 nm2 so that the feed solution is fractionated into a permeate comprising the first component and a retentate enriched in the second component.