Doktorarbeit
Synthesis of tailored polymer structures by the combination of cationic and anionic polymerizations
Jesper Feldthusen (01/1998-01/1998)
Johannes-Gutenberg-Universität Mainz
Summary
A new method
for the synthesis of tailored polyisobutylene-based
(PIB-based) block copolymers by the combination of controlled/living
cationic
and controlled/living anionic polymerizations has been developed. In
addition,
parallel to this subject new synthetic routes for preparation of
telechelic
PIBs and amphiphilic networks have been investigated.
The PIB
precursors used for subsequent controlled/living anionic
polymerization of various methacrylates and ethylene oxide were
prepared by
controlled/living cationic polymerization of isobutylene followed by
quenching
with 1,1-diphenylethylene (DPE) under selected conditions. Depending on
the
quenching procedure two different endgroups, either a diphenylmethoxy-
or a
diphenylvinyl endgroup were obtained.
Since the
diphenylmethoxy endgroup can be obtained in quantitative yield
by quenching with methanol under carefully selected conditions,
quenching
experiments involving the diphenyl-cation with different alcohols were
performed, e.g. with 2-hydroxyethyl methacrylate (HEMA) leading to in situ formation of a macromonomer.
However, since the competing reaction between the Lewis acid (TiCl4) and the alcohol is more pronounced in
case of these alcohols only partial functionality was reached (with
HEMA » 65 %
was detected). The side product is the
diphenylvinyl endgroup formed as consequence of the reduced Lewis
acidity (TiCl4 + xROH Ti(OR)xCl4-x). Further experiments
with these
alcohols using different Lewis acids might lead to improvements.
Preliminary
model studies involving the transformation from cationic to
anionic polymerization with low molecular weight compounds having the
same
active sites as the corresponding PIB precursors were performed. The
compounds
used in these experiments were; diphenylmethane (DPM), triphenylmethane
(TPM),
3,3,5,5-tetramethyl-1,1-diphenylhex-1-ene (TMP-DPV), and
1-methoxy-3,3,5,5-tetramethyl-1,1-diphenyl (TMP-DPOMe).
With the
first two compounds (DPM and TPM) < 85 % conversion was
reached under different conditions. Lithiation of TMP-DPV was performed
with n/s-BuLi. However, in contrast to DPE,
the addition of BuLi is not quantitative due to sterical hindrance.
Direct
metalation of TMP-DPV with K/Na alloy or Cs led to quantitative
conversion
within 15 min verified by 1H NMR and
UV spectroscopy. Interestingly, coupling was not detected which is the
case in
a similar reaction with DPE. Under the same conditions metalation of
TMP-DPOMe
was completed also in 15-20 min. In both experiments the same product
was
formed after quenching with methanol, 3,3,5,5-tetramethylhexane. By
addition of
LiCl, the corresponding lithium initiator was achieved. That means that
by this
procedure (macro)initiators with various counterions; K+,
Cs+, and Li+ are available
in quantitative yield in
less than 20 min (metalation with Na-mirror is rather slow). Anionic
polymerizations of methacrylates with the synthesized initiator,
3,3,5,5-tetra-methylhexyllithium from TMP-DPV and/or TMP-DPOMe indicate
quantitative initiating efficiency.
In order
to analyze effects of potential Clt-terminated
PIB (due to incomplete DPE-capping) metalation experiments were carried
out
with a sample containing 85 % DPV and 15 % Clt
endgroups and one with 100 % Clt-terminated
PIB. With the former sample coupling was observed due to nucleophilic
substitution of the Clt endgroup with
PIB-DPE-. Experiments with the 100 %
Clt-terminated PIB led to interesting
observations. Metalation with K/Na alloy in n-hexane
contain-ing N,N,N,N-tetramethylethyldiamine resulted in quantitative
formation
of isopropenyl capped PIB faster and more selectively than existing
methods
(instead of the expected generation of a PIB mac-rocarbanion which is
partially
formed in pure n-hexane together with
the isopropenyl endgroup).
Metalation
of the resulting DPE-capped PIB precursor with K/Na alloy or Cs led
also to
quantitative formation of a PIB macrocarbanion within less than 60 min
independent of the molecular weight. The independence of the metalation
and
anionic polymerization on the ratio of the diphenylvinyl and
diphenylmethoxy
endgroups is a important finding. This means that it is not necessary
to
consider the chain end composition of the DPE-capped PIB in the course
of its
preparation by controlled/living cationic polymerization.
Well-defined
OH-terminated PIBs with quantitative functionality were synthesized by
adding
ethylene oxide to the lithium salt of the PIB macroinitiator. Using the
corresponding potassium or cesium salt of the PIB macroinitiator
tailored
amphiphilic i.e. water soluble PIB-b-PEO
block copolymers were prepared.
Different
tailored PIB-based AB, ABA, and (AB)3 block
copolymers of various methacrylates were synthesized in
order to obtain materials which can subsequently be characterized. Near
to
quantitative blocking efficiencies were normally reached proved by SEC
analysis
(and 1H NMR).
Thermoplastic
elastomers (TPEs) based on linear poly(methyl
methacrylate)-b-poly-isobutylene-b-poly(methyl
methacrylate) PMMA-b-PIB-b-PMMA and
star-shaped (PIB-b-PMMA)3 block
copolymers with tensile strength
exceeding 20 MPa have been prepared. The synthesized samples have
similar
properties compared to corresponding polystyrene-based block copolymers
(PSt-b-PIB-b-PSt). In addition, the morphology was examined by small
angle
X-ray scattering. Lamellar and cylindrical morphologies were observed.
Amphiphilic
polyisobutylene-b-poly(methacrylic acid) (PIB-b-PMAA) block copolymers have been
synthesized and characterized. The characterization of these block
copolymers
in aqueous media with dynamic and static light scattering, fluorescence
correlation spectroscopy, ultracentrifu-gation, and transmission
electron
microscopy verified the formation of well-defined micelles with an
aggregation
number and a hydrodynamic radius which depend primarily on the chain
length of
the PIB block segment. Compared to other polymer systems, an extremely
low CMC
was detected for all the PIB-b-PMAA
samples, about 2-3 orders of magnitude lower than comparable PMMA-b-PMAA block copolymers.
Preliminary
studies regarding synthesis of amphiphilic networks (APNs) based on
block
copolymer units have been carried out using different methods: A series
of APNs
were prepared by adding ethylene glycol dimethacrylate to living block
copolymer units. A second curing process involving crosslinking
reactions
between alcohol groups at the polymer chain ends and isocyanates or
acid
chlorides was examined, too. Optimization of the curing procedures and
systematic characterization of the resulting APNs have to be performed
in a
future work.