Dyads

There are several abstract concepts to deal with. You should become familar with what is meant by 'meso' and 'racemic' in the context of dyads. You should also be aware that this business of dyads ties in to the idea of tacticity.

can react with the next monomer to form

, or

can react with the next monomer to form

.

In the first instance the functional group of the adding monomer goes to the side opposite the functional group of the monomer to which it adds. In the second, the functional group of the adding monomer goes to the same side as the functional group of the monomer to which it adds.

The addition of a monomer is given a name depending on whether the functional group adds on the same side as that of the previous monomer, or if it goes to the other.

is called meso, and

is called racemic.

It should make sense that in going between any two monomers, you can make the claim that it is 'meso' or 'racemic.'

In the previous section a second monomer added to a first. For the Bernoulli model of monomer addition, it is assumed that the stereochemistry of the addition of a monomer only depends on the stereochemistry of the last monomer on the chain. More on this to be constructed later.

If we assume the probability of how the monomer adds depends on the last two monomers, then we have the following:

may go to

We now consider probability. Let us reconsider a Bernoulli system:

We defined

going to

as a meso propagation.

We now label the probability of this occuring as P(m). Textbooks will write P with an m subscript.

It follows that

going to

is a racemic proagation, and the probability of this occuring will be represented by P(r).

Using the Bernoulli model, if racemic propagations account for 60% of the addition, then by default, meso propagations must account for 40% of the propagation. We say for this example that P(r) = 0.6 and P(m) = 0.4.

P(m) + P(r) = 1

It's fair to ask, "What about yellow adding to yellow? Isn't that also a meso addition? And what about blue adding to yellow? Isn't that also a racemic addition?"

Yes. Yes. Assuming yellow to yellow is as common as blue to blue, we would say that yellow to yellow has a probability of 0.2 and blue to blue has a probability of 0.2 as well, which adds up to the 0.4 probability.

In our model we can tell the difference between blue to blue and yellow to yellow, but to an instrument such as an NMR, they are indistinguishable (both will contribute to the same peak), so it makes life easier to make the math work such that all meso additions contribute to one probability term.

Last Update- November 2, 1995- wld