Styrene and methyl methacrylate have been polymerized
at 90 °C with azobis(isobutyronitrile) (AIBN) as a thermal initiator. The molecular weight
distributions were analyzed using matrix-assisted laser desorption ionization (MALDI) mass spectrometry and size
exclusion chromatography (SEC).
By synthesizing polymer of low molecular weight
(M
p = 1000), we were able to achieve excellent
agreement
between molecular weight data from MALDI and SEC for the same polymer.
Evaluation of the termination
mode in free-radical polymerization was performed by integrating the
peaks on the mass spectra which
corresponded with either one or two initiator fragments per polymer
chain. Evidence for some primary
radical termination was noted at high initiator concentrations. An
additional peak in the spectra for
polystyrene was ascribed to the copolymerization of styrene with a
single methacrylonitrile unit originating
from the initiation process with AIBN. Two other anomalous peaks
were noted in the polystyrene mass
spectrum; the first relates to chain scission under MALDI conditions
and the second to a Diels−Alder
rearrangement thermal initiation product. The ratios of the
termination modes (disproportionation to
recombination, k
td/k
tc)
were evaluated for methyl methacrylate and styrene as 4.37 ± 1.1 and
0.057 ±
0.032, respectively, at 90 °C. These values are in excellent
agreement with data reported previously in
the literature. Potential errors arising from the use of MALDI for
quantitative chain end analyses are
discussed.
The technique of pulsed-laser polymerization (PLP) was used to
measure homopropagation
rate coefficients for styrene (STY), methyl methacrylate (MMA), ethyl
methacrylate (EMA), n-butyl
methacrylate (BMA), and benzyl methacrylate (BzMA). Special
attention was given to the molecular
weight analysis of PLP distributions by size exclusion chromatography
(SEC) with online low-angle laser-light scattering (LALLS), differential viscometry (DV), and differential
refractive index (DRI) detectors
connected in series. The PLP distributions were analyzed using two
different approaches: first, by
transformations to the universal calibration curve via
Mark−Houwink−Sakurada (MHS) constants and,
second, by direct analysis of the molecular weight distributions using
absolute detection methods. The
accuracy and precision of the two approaches is discussed with
reference to the measured Arrhenius
parameters. The variation in the propagation rate coefficients
with ester side chain size is attributed
primarily to the preexponential factor. This is consistent with
theoretical predictions but in opposition
to a recent PLP study of this series. An explanation for this
apparent contradiction is suggested.
A comprehensive set of propagation rate coefficients for
styrene−methyl methacrylate bulk
copolymerization has been obtained from pulsed laser polymerization
experiments performed over a range
of temperatures (18−57 °C). The copolymer molecular weight
distributions were analyzed using size
exclusion chromatography with on-line low-angle laser-light scattering
(LALLS) and differential viscometry
(DV) detectors. The molecular weight data obtained using these
absolute detection methods were
compared with data obtained by assuming the copolymer hydrodynamic
volumes (HDVs) can be described
by a weighted average of the homopolymer HDVs. The analyses via
these three different approaches
yielded virtually identical molecular weight data, validating previous
work on this copolymerization
system. The precision of the data obtained from LALLS and DV was
worse than that obtained from the
HDV-weighted approach. However, the errors in the molecular weight
data from DV and LALLS analyses
are believed to be a more correct representation of the true errors in
the molecular weight measurement.
A strategy for reducing the effect of the molecular weight errors
in the k̄
p measurements, via the
selection
of slow laser flashing rates, is presented.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.