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Tailored polymers by cationic ring-opening polymerization.

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Die Angewandte Makromolekulare Chemie 223 (1994) 1-11 (4003)
University of Ghent, Institute of Organic Chemistry, Polymer Division,
Krijgslaan 281 (S4-bis), B-9000 Ghent, Belgium
TAILORED POLYMERS BY CATIONIC RING-OPENING
POLYMERIZATION
Eric J. Goethals, Peter Van Caeter, Jan M. Geeraert, Filip E. Du Prez
SUMMARY
After a general classification of the cationic ring-opening polymerizations (CROP'S) according to
their polymerization mechanism, a number of examples of tailored polymers based on CROP are
presented. The monomers used for the synthesis of these tailored structures are tetrahydrofuran
(THF), N-tert-butyl aziridine (TBA), 2-methyl- 1,3-oxazoline (MeOX) and 1,3-dioxolane (DXL).
The polymer structures include different block and graft copolymers, macromonomers, star-shaped
polymers, polymer networks and interpenetrating polymer networks (IPNs).
Paper presented at the meeting of the GDCh-Fachgruppe "Makromolekulare Chemie" on "Progress
in Polymer Synthesis" in Bad Nauheim (Germany), March 21-22, 1994.
0 1994 Hiithig & Wepf Verlag, Zug
CCC 0003-3146/95/% 07.00
1
INTRODUCTION
A large variety of heterocyclic monomers can be polymerized by cationic ring-opening
polymerization (CROP) mechanism. However, only a limited number of these polymerizations
provide the possibility to prepare well-defined polymers, i.e. with predictable molecular weights
and dispersities, known micro- or macro structures, and provided with known (functional) end
groups. Whether a polymerization leads to such well-defined polymers or not, depends on the
reaction mechanism of that polymerization and more particularly on the occurrence of chain
transfer and / or termination reactions.
In this paper, a classification of the CROPS according to the mechanism of polymerization will be
presented and those monomers that have been found to give controllable polymerizations will be
discussed in more detail. Then, the use of these systems for the construction of some more
sophisticated polymer structures will be described
GENERAL MECHANISM AND CLASSIFICATION OF CROP's
It is now generally accepted that in the great majority of CROP's the active species are cyclic
onium ions and that the propagations take place by an SN2-type of reaction wherein the monomer
hetero-atom acts as the nucleophile and the active species as the electrophile. The initiation
reaction is, therefore, any reaction that produces the cyclic onium ion. The driving force of the
propagation is the ring-strain of the active species. Once polymer is formed, the hetero-atoms in the
polymer chain can compete with those of the monomer to react with the active species. We call this
reaction a "spontaneous termination" (in contrast to "deliberate" termination), since the result is an
onium ion that is non-strained and therefore less reactive than the active species. Such a species is
called "dormant" when the termination is reversible, i e. the species can be re-activated to an active
species. If re-activation is not possible, the species is a "dead" species and the reaction a real
termination. In contrast with the spontaneous termination, deliberate terminations are those which
are occurring with a terminating agent that is added to the polymerization mixture in order to endcap the polymer chains with well-defined end groups. The whole mechanism is presented in
Scheme 1.
The classification of CROP's is based on the nature of the spontaneous termination. Assuming that
fast and quantitative initiation systems are used, CROP's can be classified into three categories
which differ from each other by the "degree of controllability" of the structure of the end-product.
A first class of polymerizations are those that do not show any spontaneous termination reaction.
In that case the concentration of active species is entirely determined by the eficiency of the
initiation reaction and each polymer chain keeps an active chain end. Such polymerizations are the
typical "living" systems which allow the optimal control of polymer structure, including the
incorporation of functional end groups by end-capping.
2
Scheme 1
Polymerizations that do show a termination but with a rate that is much smaller than the rate of the
propagation, may also be classified in this group because the polymerizations provide the same possibilities as the living ones on the condition that the factor rime is taken into consideration. If, for
example, after one hour, the polymerization has reached a yield of 98% and if at that moment still
99% of the polymer molecules contain an active species, t h e polymerization, including the endcapping, will lead, within the experimental error, to the same results as would have been obtained
with a real living system.
Polymerizations which do show a termination reaction with a rate comparable to the rate of
propagation can be subdivided into two classes depending on whether the termination is reversible
or not. If the termination is reversible, the concentration of (potential) active species remains
constant and the molecular weight can still be controlled by the ratio of monomer to initiator
concentration. Also, end-capping by the addition of a terminating agent is possible. However, due
to the continuous termination followed by re-initiation, the molecular weight distribution will
increase and eventually reach a theoretical value of 2 This phenomenon has been called
"scrambling". Another consequence of this scrambling is that polymers that originally contain one
active chain end per molecule are transformed into a mixture of polymers containing two, one and
no active species, which are continuously inter converted. This is of importance if the polymers are
end-capped with hnctional end groups with the purpose to build segmented polymers such as
block- or graft copolymers. Finally, polymerizations which show the scrambling, will always lead to
a fraction of cyclic oligomers the concentration of which decreases with the size according to the
Jacobson - Stockmayer theoryl. These cyclic oligomers are formed when the spontaneous
termhation reaction is occurring intramolecularly, i.e. the active species is attacked by a hetero
atom of its own chain.
3
If the termination reaction is irreversible, control of molecular weight and end-capping are not
possible. Polymerizations belonging to this category are not very usefbl for the construction of
well-defined polymers.
Tab.l gives an overview of the main characteristics of CROPS according to the category to which
they belong, and includes examples of monomers belonging to each category.
Tab. 1. Classification of CROP'S.
No termination
(or slow compared with propag.)
living polymerization
(or "slowly dying")
M W control
-a
narrow
end capping
cyclic oligomers
no
Termination
Reversible
Irreversible
Yes
no
broad ( = 2)
broad
Yes
no
Yes
variable
examples
WELL-DEFINED POLYMERS BY CROP.
Monomers b-longing to the first column of
are best suited for the synthesis of well-defined
polymers, provided that an appropriate initiating system is available. The typical example of such a
monomer is tetrahydrofuran (THF).
The mechanism of the polymerization of THF has been thoroughly studied in the seventies. It was
found that the (reversible) termination reaction is negligible when the conversion is kept low, for
ex. < 25%. The availability of a number of "clean" initiation systems allows the preparation of
polyTHFs with controlled molecular weights and provided with a variety of functional end
groups2. These end-groups can be introduced either by end-capping or by the use of functional
initiators. Monofbnctional as well as bifunctional living polymerizations are possible. The polymerization mechanism with methyl trifluoromethanesulfonate (methyl triflate) and with triflic anhydride
are shown in Schemes 2 and 3.
4
Scheme 2
CF3S03CH3
Scheme 3
3 - "3
+0
CH3-0
CF3S$
Another monomer that shows a polymerization with a high living character is N-tert-butylaziridine
(TBA). The ratio of the rate constant of propagation t o that of the spontaneous termination was
found t o be 12,000 L/mol. The dimension of this ratio (L/mol) comes from the observation that the
termination reaction obeys first order kinetics which is explained by assuming that it is
predominantly intramolecular with the formation of (macro)cyclic ammonium end group$. A
typical polymerization mechanism, including an end-capping reaction is given in Scheme 4.
Scheme 4
+
+
CF3S03CH3
0-20°C
t
The last example of polymerizations with a high living character is that of the 1,3-oxazolines. This
type of polymerization differs from the previous ones by the fact that during the propagation step,
the iminoether function in the monomer isomerizes to an amide function4. Since the nucleophilicity
of the latter is much lower than that of the former, the spontaneous termination reaction does not
occur. The polymerization of oxazolines is initiated by most alkylating agents such as tosylates,
iodides, trialkyloxonium salts, etc. Scheme 5 describes the polymerization of 2-methyl-2-oxazoline
(MeOX).
The resulting polymers are acylated polyethylenimines and therefore, polymerization of oxazolines,
followed by hydrolysis in basic or acidic medium, is the most used route for the synthesis of linear
polyethylenimine (LPEI)S.
6
Scheme 5
I
isomerization-polymerization
("living")
poly MeOX
IR'm2
Polymerizations which are characterized by a reversible termination reaction can be used to prepare
polymers with predictable molecular weights but having a polydispersity of 2 Functional endgroups can be introduced by the "end-blocker'' method This method is in fact based on a transfer
reaction that is similar t o the "transfer to polymer" but taking place with an added transfer reagent
The end-blocker consists of a hnctionalized compound that has the same structure as the recurring
unit of the polymer chain This chemistry has been used in the synthesis of hnctionalized
polysiloxanes6 and, recently, for the synthesis of hnctionalized polyacetals7 Thus, a,o-acrylateterminated poly( 1,3-dioxolane) has been prepared by carrying out the cationic ring-opening
polymerization of 1,3-dioxolane (DXL) in the presence of the formal of 2-hydroxyethyl acrylate
This reaction is shown in Scheme 6
Scheme 6
7
SEGMENTED COPOLYMERS BY CROP
The availability of several living polymerizations occurring by cyclic onium ions as the active
species gives the possibility to prepare block-copolymers by the sequential monomer addition
method, provided the active species of the first polymerization is a fast and quantitative initiator for
the polymerization of the second. Since oxonium ions are good initiators for the polymerization of
cyclic amines, living polyTHF is a good initiator for the polymerization of TBA and oxazolines. AB
and ABA types of block-copolymers in which A stands for the polyamine8 or polyoxazoline9,10
segments have been prepared. The low molecular weight poly(THF-b-MeOX) shows interesting
properties as a non-ionic surfactant which is due to the amphiphilic character of this blockcopolymer9. ABA block-copolymers in which the polyether segments have molecular weights of
appr. 20,000 and the two MeOX segments appr. 2000, show interesting mechanical properties
which were explained by assuming cluster formation of the polar polyMeOX segments in the apolar
polyTHF matrixlo.
STAR-SHAPED POLYMERS BY CROP
By means of living polymerizations star-shaped polymers can be obtained in two ways : termination
of the polymerization with a polyhnctional end-capper, or initiation with a polfinctional initiator.
~ , ~ ~ .
Both methods have been used for the synthesis of star-shaped ~ O I ~ T H FAs~ polyfhctional
end-cappers, ammonia or di- or poly-amines can be used. In order to control the reaction it is
necessary to add an amount of a proton trap that is at least equivalent to the number of protons
which have to be released from the amine. A suitable proton trap was 2,2,6,6-tetramethyl
piperidine (TMP). It was found that ammonia can accept four polyTHF chains whereas diethylene
triamine can accept as much as seven, on the condition that the molecular weight of the polyTHF
molecules is not too high The reaction is represented in Scheme 7.
Scheme 7
8
For polyfimctional initiation, combinations of a polyacid chloride and a silver salt having a nonnucleophilic counter ion have been used. Thus, tri- and tetra-functional initiators were obtained
from the acid chlorides of benzene-l,3,5-tricarboxylicacid and of pyromellitic acid12.
Star-shaped polyTBA has been prepared by end-capping of the living polymer with pyrornellitic
acid, leading to a four-armed, and by diethylene triamine, leading to a five-armed ~ t a r - p o l y m e r l ~ .
REGIOSPECIFIC GRAFTING REACTIONS
Polymers which contain amino functions at well defined parts of their chain can be grafted in the
same manner as described above for the polyfunctional end-capping reactions. In this way the
grafting reaction takes place at well defined spots of the polymer. As an example, amino-terminated
polyTHF can be grafted with living polyTHF to form polymers which are branched at their chain
ends. If the end groups of the polymer are diethylene triamine units, four to five living polymer
chains are attached to each side of the original chain. This reaction is shown in Scheme 8
Scheme 8
I
living polyTHF
These reactions work well if the grafting chains are the same as the main chain. If different
polymers are used, the grafting becomes more difficult. For example, it was not possible to graft atosylateii poly(oxyethy1ene) on amino-terminated polyTHF.
9
POLYMER NETWORKS AND PNs BY CROP.
Different approaches are possible for the synthesis of polymer networks by CROP. Several
methods have been reviewed recently13. In the present review, the use of a,w-acrylate terminated
prepolymers ("bis-macromonomers") for the synthesis of homo-networks and segmented networks
is reported.
As described above (Scheme 6), a,o-(meth)acrylate terminated polyDXL can be prepared. When
this bis-macromonomer is polymerized by a free radical mechanism, the corresponding polyDXL
network is formed. When the bis-macromonomer is copolymerized with another "vinyl" monomer
(VM), a segmented polymer network consisting of polyVM chains connected to each other by
polyDXL chains is obtained. For example with methyl methacrylate (Mh4A) :
Analysis of the thus formed material by dynamic mechanic thermal analysis (DMTA) showed an
almost single maximum for the tan 6, situated at S O T , i.e. 60" below the Tg of polyMMA, indicating a high degree of compatibility between the two segments. This is ascribed to the fact that
the network structure prevents the phase separation duririg the polymerization. In contrast, interpenetrating networks (PNs) having the same combination of polymers, prepared by sequential
homo-network formation (swelling of the polyDXL network in MMA + crosslinker and initiator,
followed by polymerization at 70 "C) and containing the same ratio of the two polymer chains as
the segmented network described above, shows a clear phase separation. If the polyMMA network
was produced in the presence of a polyMMA-containing segmented network, the DMTA showed a
very broad peak for the tan 6 which can be ascribed to the presence of a variety of morphologies
ranging from homogeneous to co-continuous phases. However, there appears no maximum at or
near the glass temperature of polyMMA. This indicates that in IPNs, like in linear polymer blends,
the presence of a segmented copolymer network consisting of segments of the two components,
has a compatibilizing effect s.
10
REFERENCES
1 H.Jacobson, C. 0. Beckmann, W. H. Stockmayer, J. Chem. Phys. (1950) 1600
2 S. Penczek, P. Kubisa, K. Matyjaszewski, Adv Polym. Sci. 68/69 (1985)
3 E.J. Goethals, Comprehensive Polymer Science, 1st ed , Pergamon Press, Oxford 1989, Vol. 3,
p. 831
S.Kobayashi, T.Saegusa, Encyclopedia of Polymer Science and Engeneering, 2nd ed., John
Wiley & Sons, New.York 1986, Vol. 4, p. 525
S.Kobayashi, Prog. Polym. Sci. (1990) 751
I. Yilgor, J. S. Riffle, J. E. McGrath, Reactive Oligomers,ACS Symp. Ser. No. 282, F. W. Harris,
H. J. Spinelli, Eds., American Chemical Society, Washington, D. C. 1985, p.161
R. De Clercq, E. J. Goethals, Macromolecules 25 (1992) 1109
M.Vande Velde, E. J. Goethals, Makromol. Chem., Macromol. Symp. 6 (1986) 271
9 S. Kobayashi, H. Uyama, E. Ihara, T. Saegusa, Macromolecules22 (1990) 1586
lo P. Van Caeter, V. Goncheva, E. J. Goethals, unpublished results
J. Geeraert, unpublished results
l 2 E. Franta, L.Reibel, J Lehmann, I. Polym. Sci., Symp. 56 (1976) 139
l3 A.Munir, E.J. Goethals, J. Polym Sci., Polym. Chem Ed 19 (1981) 1985
l4 E. J. Goethals, Makromol. Chem., Macromol. Symp. 3 (1992) 217
l 5 F. Du Prez, E. J. Goethals, submitted to Macromol. Chem. Phys
11
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