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 kxy 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.

  • Summary

    Block Copolymers

    consist of two or more blocks of different polymers that are chemically linked to each other.

  • In the case of vinyl monomers, block copolymers are often prepared by controlled polymerization of one monomer, followed by chain extension with a second monomer.

  • The properties of block copolymers depend on the chemical nature of the monomers, the size, number and sequence of the blocks, and the average molecular weight and its distribution.

  • Block copolymers in which the components are incompatible, undergo phase separation. In many cases, one phase will be dispersed in a contiunuous matrix formed by the other component. These phase domains are usually microscopic small and lead to unique properties.

  • Block copolymers exhibit properties that are very different from those of random copolymers. Usually, the properties of block copolymers are superior to those of random copolymers.

PRODUCT OF REACTIVITY
RATIOS OF MONOMERS
       Table