Reactivity ratios indicate preference for propagation. Large  indicates a tendency for  to add , while small  corresponds to a tendency for  to add . Values of  describe the tendency of  to add  or . From the definition of reactivity ratios, several special cases can be derived:
*  If both reactivity ratios are very high, the two monomers only react with themselves and not with each other. This leads to a mixture of two homopolymers.
* . If both ratios are larger than 1, homopolymerization of each monomer is favored. However, in the event of crosspolymerization adding the other monomer, the chain-end will continue to add the new monomer and form a block copolymer.
* . If both ratios are near 1, a given monomer will add the two monomers with comparable speeds and a statistical or random copolymer is formed.
*  If both values are near 0, the monomers are unable to homopolymerize. Each can add only the other resulting in an alternating polymer. For example, the copolymerization of maleic anhydride and styrene has reactivity ratios  = 0.01 for maleic anhydride  and  = 0.02 for styrene. Maleic acid in fact does not homopolymerize in free radical polymerization, but will form an almost exclusively alternating copolymer with styrene.
*  In the initial stage of the copolymerization, monomer 1 is incorporated faster and the copolymer is rich in monomer 1. When this monomer gets depleted, more monomer 2 segments are added. This is called composition drift.  
* When both , the system has an azeotrope, where feed and copolymer composition are the same.