Handbook of Size Exclusion Chromatography and Related ...

Mw, so that Mh=Mw is a valuable measure of polydispersity. A third method, which
applies when dead chains are produced on a time scale that is fast compared to
total conversion (for example, free radical polymerization, but not anionic or
controlled radical polymerization), involves finding the instantaneous weightaverage
mass Mw,inst, which is related to the measured, cumulative Mw via
Mw,inst( f ) ¼ Mw( f ) þ f dMw
df
(12)
Figure 5 shows Mw and Mw,inst for a PVP reaction in which an additional
amount of hydrogen peroxide initiator (“booster”) was added after about 20%
conversion. When a fixed amount of hydrogen peroxide was added at the outset of
the PVP reactions, Mw would remain constant throughout conversion, as found in
Ref. 24. Hence, a booster of hydrogen peroxide was expected abruptly to cause
subsequent conversion to proceed at a fixed, lower Mw. The curve of Mw in Fig. 5
Figure 5 Mw is the cumulative value of Mw measured directly by light scattering,
whereas Mw,inst represents the instantaneous value of Mw, obtained from the Mw data via
Eq. (12). An initiator “boost” at 20% conversion led to the production of smaller chains for
the remainder of the reaction. The on-line histograms are derived from the Mw,inst data at two
different conversion points (20 and 90%), and show how the initial, unimodal population
becomes bimodal after the initiator boost. (From Ref. 25.)
© 2004 by Marcel Dekker, Inc.