Theoretical yield isn't a particularly useful concept in polymer chemistry. There are other ways of describing the outcome of a polymerization reaction that are more descriptive. Some of these are:
Percent conversion - when you are working with homopolymers or with random copolymers, you generally only want a few initiation events to get large polymers. When the reaction is finished, you typically have long chain polymers (-AAAAAAAAAAA-) and some leftover monomer, as the initiating/replicating sites get quenched before they run out of reagent. Percent conversion tells you how much monomer is leftover.
Average molecular weight - if you have a large number of initiation events in your reaction, you will start a large number of polymer chains, and the resulting chains will be relatively small when they run out of monomer to react with. If you have a small number of initiation events, you will only start a few polymer chains, and they can grow much larger before you run out of monomer. Either way, there will be a statistical variation in the speed with which the reactions occur (not every chain will grow at the same rate) and at the end of the reaction, rather than a single product with a single molecular weight, you will have a statistical spread of products of different molecular weights centered around an average molecular weight.