Block Copolymerization

Block copolymers consist of two or more strands (blocks) of different polymers chemically attached to each other. The properties of these polymers depend on the copolymer sequence distribution, the chemical nature of the blocks, the average molecular weight and the molecular weight distribution of the blocks and the copolymer.

Block copolymers, particularly those of type A-B-A can exhibit properties that are very different from those of random copolymers.

In the case of copolymerization of vinyl polymers, block copolymers are often prepared by controlled sequential monomer addition. In this case, one of the two monomers is polymerized first. When polymerization of the first component is complete, a second (suitable) monomer is added to the "living" radical bearing polymers which then copolymerizes with the first component.

Another method involves monomers that have a strong tendency to homopolymerize but still copolymerize. This is the case, if the reactivity ratios, defined by

r₁ = k₁₁ / k₁₂

r₂ = k₂₂ / k₂₁

are (much) larger than unity:

r₁ ≈ r₂ > 1,
or

k₁₁ > k₁₂ ,  k₂₂ > k₂₁

where k_xy are the reaction constants of the four different types of polymerization reactions:

M₁^* + M₁ → M₁M₁^*      R₁₁ = k₁₁ [M₁^*] [M₁]

M₁^* + M₂ → M₂M₁^*      R₁₂ = k₁₂ [M₁^*] [M₂]

M₂^* + M₂ → M₂M₂^*      R₂₂ = k₂₂ [M₂^*] [M₂]

 M₂^* + M₁ → M₁M₂^*      R₂₁ = k₂₁ [M₂^*·] [M₁]

M₁^* and M₂^* represent reactive chain ends of type 1 and 2.
For r₁ > 1 and r₂ > 1, each radical has a strong preference to add monomers of its kind to the growing polymer chain. The addition of the same type of monomer will continue until there is a chance of addition of the other kind of monomer, and then polymerization of the other monomer will continue. In the case of a very low probability of crosspolymerization (r >>1), large M₁ and M₂ blocks will form and in the extreme case of both reactivity ratios very large, r₁ ≈ r₂ → ∞, the two monomers only react with themselves, leading to a mixture of two homopolymers. However, complete free radical homopolymerization in the presence of another monomer has not been observed yet.