The fractions of triads in a regular vinyl polymer that are of the
mm,
rr and
mr =
rm types, respectively. In cases where triad analysis is not attainable, the diad isotacticity and diad syndiotacticity may be defined as the fractions of diads in a regular vinyl polymer that are of the
m and
r types, respectively. (
m =
meso;
r =
racemic).
Source:
Purple Book, 1st ed., p. 45 (http://old.iupac.org/publications/books/author/metanomski.html)