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.