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Methods for the Synthesis of Uniform Polymers.

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host tissues and of the parasites showed that the radioactivity was concentrated largely in the mature flukes
and in the eggs. Females absorbed more of the radioactive substance than did males. The level of radio-
3
m
-
activity was appreciably higher in the deposited eggs
than in the surrounding tissues, as is shown in the
micro-autoradiogram of a liver section reproduced in
Figure 2. There was a high concentration also in the
mucous lining of the stomach and in the intestines.
In the light of our findings, it may be assumed that the
mode of action is as follows. Owing to the presence of
the imidazolidinone ring [or of the perhydrodiazinone
ring in (24)], the nitrothiazole derivative is selectively
absorbed by the schistosomes and their eggs; the nitro
group then exerts its influence on metabolism, perhaps
by blocking a redox enzyme. This assumption is all the
more plausible in view of the fact that metabolites and
synthetic substances without the nitro group displayed
no activity.
[ A 536 I € ]
Received: June 7th, 1966
German version: Angew. Chem. 78, 850 (1966)
Fig. 2. Micro-autoradiogram ( 5 4 0 ~ of
) schistosome ova in the liver of a
mouse 24 hours after treatment with 1 g/k2 p.0. of Ambilhar".
Methods for the Synthesis of Uniform Polymers
BY DR. J. H. WINTER
FARBWERKE HOECHST AG., FRANKFURT-HOCHST (GERMANY)
The dif$culties encountered in the preparation of polymers that are uniform chemically,
structurally, and in their molecular weight are due mainly to the wide variety of possible
polymers. This article gives a review of attempts to synthesize homogeneous polymers by
chain reactions and by stepwise synthesis. Syntheses on templates are extremely important in
living cells; methods have recently beenfound for ryntheses of this type that are independent of
natural processes. Uniformpolymers can also be obtained by reactions on existing polymers.
In replicating polymers, this in principle requires the modification of only one polymer
molecule.
1. Introduction
a) The Problem
Polymers are substances formed by the linkage of low
molecular weight compounds (monomers) and containing continuous sequences of covalent bonds as in
long-chain hydrocarbons (but all the atoms need not
be C atoms). The number of monomer units present
in a polymer molecule (i.e. its degree of polymerization)
may be as large as desired. Polymers are characterized
by the many possibilities regarding the nature and
number of monomer units, the manner in which they
are combined, and the side reactions accompanying the
synthesis of the polymer.
Owing to this variability, it is very difficult to prepare monodisperse polymers, i.e. polymers whose molecules are identical
in structure and size. Any side reactions occurring during
the preparation of the polymer will affect a much larger
proportion by weight of the reaction mixture than in reactions
involving only small molecules. Finally, it has been found
that the difficulty of isolation and purification increases with
the molecular weight of the polymer. For a long time, the
chemistry of synthetic polymers was concerned almost
862
exclusively with ill-defined, heterogeneous, amorphous
substances. On the other hand, many uniform polymers occur
in Nature, some with very high degrees of polymerization,
the correct structure of which is essential to the life processes.
It was expected that homogeneous synthetic polymers would
have better mechanical properties and thermal stability.
This expectation was confirmed when a number of structurally identical, crystallizable polymers had been prepared
in the laboratory. Methods have now been developed for
the preparation of polymer molecules of uniform length.
Even very complex polymers similar to those in Nature and
exhibiting biological activity can now be synthesized uniformly with or without the aid of natural substances [I].
b) The Concept of Uniformity
It is usual to speak of chemical uniformity, structural
uniformity, and uniformity of molecular weight. A
polymer is chemically uniform if each molecule has the
same composition. Homopolymers, by definition, contain only one type of monomer unit (apart from the
ends of the chains), so that these polymers satisfy the
[l] Cf. J. H . Winter: Die Synthese von einheitlichen Polymeren.
Springer, Berlin-Heidelberg-NewYork, in press.
Angew. Chem. internnt. Edit.
/ VoI. 5 (1966) J No. I0
above condition. However, if a polymer contains more
than one type of monomer unit (copolymers), it can be
chemically uniform only if the monomer units are
present in the same ratio in all molecules and are joined
together in a regular manner. (Random distribution of
the different monomer units merely presents an illusion
of chemical uniformity.) In most polymer syntheses,
each monomer unit is linked to two other monomer
units; the resulting chains should therefore be unbranched. This linearity of the chains is an important
criterion of chemical uniformity. Another criterion is
the uniformity of the end groups.
Jn connection with the structure of the polymers, let us
consider first their microstructure. The mode of linkage
of monomer units of the type - (A-A’) - in the polymer
chain is important. An arrangement often encountered is
-(A
-
A’) - (A --A’) -(A- A’) -.
The measured molecular weight of a polymer generally
is an average value. Depending on the method used,
the value obtained may be the number-average molecular weight M,, or the weight-average M,, the latter
~always being greater. The closer the ratio M,/Mn to
unity, the more uniform is the molecular weight of the
polymer.
2. Syntheses Based on Classical Methods
a) Chain Reactions
The synthesis of a polymer by a chain reaction begins
with a reaction (initiation) between an activated substance R* (free radical or ion) as the initiator (sometimes as part of a “catalyst”) and a cyclic or unsaturated
monomer molecule. As a result, two valences are made
available in the monomer by the opening of the double
bond or of the ring. The initially activated substance
In polyvinyl compounds and polyolefins, this is known
as the head-to-tail arrangement (the substituted C atom
being the head). Another factor is the spatial arrangeF r e e - r a d i c a l polymerization:
ment of the substituents on the C atoms in the chain
(e.g. the methyl group in polypropylene). These C atoms R’ + n CH2=CHC1
R-CH2-C’HC1 + (n- 1) CHZ=CHCl
Initiation
are asymmetric centers. Following Nutta, the simplest
regular arrangements (with uniform head-to-tail linkage)
of the monomer units are known as the isotactic
* R-(CH2-CHCl),-l-CH2-C‘HCl
Propagation
(d,d,d,d..
. o r l,l,l,l. . .) and syndictactic (d,l,d,l,d,l.
. .)
arrangements of the substituents (Fig. 1)[21.
Cationic polymerization :
-
3
‘a’ H
4
H O - C H ~ - C H & ( B F , ~ H ) @ + (n- 1) H2CqFH2
0
Propagation
H(O-CH2-CH2)n_1-O-CH2-CH24..(BF30H)0
Anionic polymerization:
rn
Fig. 1 . Model of tactic polymers with head-to-tail monomer units;
it is arbitrarily assumed that the chains are extended linearly. (a) isotactic, (b) syndiotactic arrangement of the substituents.
The presence of carbon-carbon double bonds offers the
possibility of cis-trans isomerism. Configurations of this
type in a polymer may again be uniform or nonuniform.
The mobility of the chain atoms with respect to one
another (e.g. “free” rotation about C-C single bonds)
enables polymers to assume different conformations,
and this is another structural factor. Whereas polymers
in solution and in the amorphous state are mainly in
the form of random coils, they form certain preferred
chain conformations on crystallization. Special mention
should be made of the helix structure, which is encountered in many natural macromolecules and in
synthetic polymers with regular microstructures.
[2] G . h’urm, J . Polymer Sci. 16, 143 (1955).
Angew. Chem. internat. Edit. 1 Vd. 5 (1966)
1 No. I0
863
usually becomes linked to the monomer by one of these
valences, while the other is used for linkage with
another monomer unit, and this process continues
(chain growth or propagation).
Whether a chemically uniform polymer can be formed
depends on the choice of initiators or catalysts, the
monomers, and the other substances present in the
reaction mixture. For example, organic peroxides give
free radicals that tend to cause side reactions (this
tendency is much smaller for azo compounds). Ionic
catalysts often react with functional groups in the
monomer. As a general rule, the smaller the number of
monomer types and of other reacting substances
(including impurities) and the higher the activation
energies of possible side reactions relative to that of the
propagation steps, the easier will be the formation of a
chemically homogeneous polymer.
Reactions with high activation energies are relatively
unimportant at low temperatures. Reactions of the
growing polymers with one another are suppressed by
dilution and by polymerization to low conversions.
Thus essentially linear homopolymers and copolymers
can be obtained by all types of polymerization.
In copolymerization it is not unusual for a monomer
unit A at the active end of the polymer to react only
with difficulty or not at all with the monomer A, but to
react readily with the monomer B, and vice versa. The
resulting polymer, irrespective of the monomer concentrations, then has an alternating monomer sequence
. . .A-B-A-B-A-B
. . . An example is the free-radical
copolymerization of olefins with sulfur dioxide to form
polysulfones[31. However, the same result can be obtained if only one of the two monomers does not react
readily with itself. The concentration of this monomer
must then be kept high relative to the other during
polymerization.
Thus dimethylketene and acetone, which by itself does not
polymerize under these conditions, react alternately at
-60°C in toluene with butyl-lithium as catalyst to give a
highly crystalline polyester ( I ) [41.
CH3
CH,
n
Dimethylketene and benzaldehyde react similarly [51. Alternating copolymerization of ethylene with cis-2-butene 161 and
cyclopentene 171 (and possibly also butadiene [a]) can also be
achieved by the use of Ziegler-Natta catalysts. In order to
prevent homopolymerization of the ethylene, the other
component must always be present in excess.
[3] For recent work see R. E. Cook, F. S . Dainton, and K . J . Jvin,
J. Polymer Sci. 26, 351 (1957); 29, 549 (1958).
[4] G. Natta, G. Mazzanti, G . Pregaglia, and M. Biriaghi, J.
Amer. chem. SOC.82, 5511 (1960).
[5] G. Natta, G. Mazzanti, G . Pregaglia, and G . Pozzi, J. Polymer
Sci. 58, 1201 (1962).
[6] G. Natta, G. Dall'Asta, G . Mazzanti, and F. Ciampelli,
Kolloid-Z. u. Z . Polymere J82, 50 (1962).
[7] G. Natta, G. Dall'Asta, G . Mazzanti, I. Pasquon, A . Valsassori,
and A . Zambrlli, Makromolekulare Chem. 54, 95 (1962).
181 G. Natta, A . Zambelli, I. Pasquon, and F. Ciampelli, Makromolekulare Chem. 79, 161 (1964).
864
The activation energy for the incorporation of monomers depends on the mode of addition (i.e. head-to-tail,
head-to-head). The uniform head-to-tail addition is
generally favored on steric grounds, since this is the
position in which the substituents present the least
hindrance to one another. However, this also means
that the monomer unit at the active end of the polymer
has a directing influence on the incoming monomer. In
polymers in which the substituents on the centers of
asymmetry can assume d or I arrangements, the
activation energy for head-to-tail polymerization, for
example, in the isotactic position is different from that
for the syndiotactic position. In free chain growth, as
generally occurs in free-radical polymerization, syndiotactic steps have the lower activation energy. Polymethyl methacrylate prepared by free-radical polymerization contains 75 % of syndiotactically arranged
substituents when prepared at room temperature [91,
and almost 100 % when prepared at -78 "C.
"Free chain growth" means that the monomer can
approach the active polymer ends and can combine
directly with the free valences of the terminal monomer
units. If the chain growth is not free, the activation
energies of the propagation steps change, so that the
probabilities of the various steps may change. For
example, highly crystalline polyvinyl chloride (crystallinity 80 %) is obtained even at 50 "C by free-radical
polymerization in butyraldehyde, whereas the crystallinity is only 10 % in the absence of butyraldehyde "01. It
may be assumed that the butyraldehyde becomes
attached to the polymer radical and influences the
addition of monomer. Effects of this type are more
pronounced in ionic polymerization, where the oppositely charged ion (counterion) can influence the conditions at the active polymer ends. At least two states
are possible at the active polymer ends, i.e. the state of a
dissociated ion pair and that of an undissociated ion
pair 111,121.
e
e e
-CH2
M" +
-CHzM
In the dissociated state, relatively free, fairly rapid
growth occurs at the active chain end. As in free-radical
polymerization, this leads to a sterically irregular
arrangement of the monomer units, which becomes
increasingly syndiotactic as the temperature is lowered.
At undissociated ion pairs, on the other hand, the
probabilities of the different types of linkage are reversed,
so that isotactic addition occurs.
An interesting example is the polymerization of methyl
methacrylate with lithium catalysts. At -60°C in a nonpolar solvent such as hexane, the product is predominantly
isotactic"3V141. In this system the ion pair at the growing
191 W. G. Galland N . G. McCrum, J. Polymer Sci. 50,489 (1961).
1101 I. Rosen, P . H . Burleigh, and J. F. Gillespie, J. Polymer Sci.
54, 31 (1961).
[ l l ] C . Geacintov, J. Smid, and M. Szwarc, J. Amer. chem. SOC.
83, 1253 (1961).
[12] H . Hostalka, R. V. Figini, and G . V . Schulz, Makromolekulare Chern. 71, 198 (1964).
[13] T. G. Fox, B. S. Garrett, W. E. Goode, S. Gratch, J . F. Kinraid, A . Spell, and J. D . Stroupe, J . Amer. chem. SOC.80, 1768
(1 958).
[14] D . Braun, M. Herner, U. Johnsen, and W. Kern, Makromolekulare Chem. 51, 1 5 (1962).
Angew. Chem. internat.
Edit. Vol. 5 (1966) No. 10
polymer end is undissociated. In polar solvents such as
dimethoxyethane, on the other hand, the ion pair is
strongly dissociated; the influence of the counter-ion is
therefore practically eliminated, free growth can occur, and
predominantly syndiotactic polymer is formed.
The tendency towards dissociation of the ion pairs is
determined by the polarity of the solvent and the
degree of solvation of the ions 1151. The tendency towards
solvation increases with decreasing ionic radius and
with increasing polarity of the solvent, sometimes also
of the monomer.
In growth o n the undissociated ion pair, the incoming
monomer is probably first coordinately bound [161. I t is
therefore oriented with respect t o the polymer end, and
can add t o the chain stereospecifically. A process of this
type is particularly likely with monomers containing
functional groups that serve merely t o enhance the
coordination and orientation. This is probably why
'monomers containing ether, carbonyl, amino, or similar
groups or with a second double bond are particularly
suitable for stereospecific polymerization. Examples are
vinyl alkyl ethers, alkenyl alkyl ethers, alkoxystyrenes,
vinylcarbazoles, and P-chlorovinyl ether, which can be
polymerized cationically, and butadiene, isoprene,
styrene, 2-vinylpyridine, esters of sorbic acid, acrylates,
and methacrylates, which can be polymerized anionically t o give stereoregular polymers.
An energetically favored six-membered ring in the cisposition may be initially formed with the monornerr171 as
the activated coordination complex [(2)], (e.g. in the ionic
polymerization of isoprene).
for the incorporation of the two isomers are different
(asymmetric induction).
D
The first generally convincing stereospecific polymerizations
were carried out by Natta et al.[*O1 using Ziegler catalysts.
Stereoregular polymers can be prepared with these catalysts
even at 100 "C. The mixed catalysts contain a compound of
a transition metal in one of its lower oxidation states and an
organic compound of a metal from Groups I to 111 of the
periodic systemr211. A typical example is the system
TiC13-Al(C2H&CI. The titanium compound forms a
crystalline phase of colloidal dimensions, and complexes are
formed on the crystal surfaces with the aluminum compound.
The active centers from which the polymer grows, generally
by an anionic mechanism, are located here. The monomer is
again assumed to be coordinately bonded to the active
centers (221. The presence of crystal surfaces undoubtedly
contributes to the high stereospecificity of these systems.
Important examples are the essentially linear polyethylene
and the predominantly isotactic polypropylene obtained by
this method at high temperatures [231.
The catalysts with the greatest specificity are produced
by Nature in the form of the enzymes. An example of a n
enzyme-catalysed chain reaction is the synthesis of
natural rubber, which was elucidated by Lyvnen et al. [*41.
I n the chain initiation, the monomer 3-methyl-3-butenyl
pyrophosphate (3) is isomerized t o 3-methyl-2-butenyl
(dimethylallyl) pyrophosphate (4) by the enzyme isomerase. Another enzyme catalyses the 1inkage:with the
next molecule of substrate (3), the pyrophosphate group
being eliminated.
y
[15] S. S. Medvedev and A . R . Gantmakher, J. Polymer Sci. C4,
173 (1963).
[16] G . Natta, Angew. Chem. 71, 205 (1959).
[I71 R . S. Stearns and L . E. Forman, J . Polymer Sci. 41, 381
(1959).
[18] T. Higashimura, T . Watanabe, K . Suruoki, S. Okatnura, and
I. Iwasn, J. Polymer Sci. C4, 361 (1963).
[19] T . Tsuruta, S . Inoue, M . Ishimori, and N . Yoshida, J. Polymer Sci. C4, 267 (1963).
Angew. Chem. internat. Edit.
i Vol. 5 (1966) / No.
10
0"
3
0"
-
C H ~ - C = C H - C H ~ - OI- P - O1- P - O ~ (3)
Even in orientation of the monomer on the ion pair, the
terminal monomer unit, and the polymer chain as a whole,
still has some directing effect [181. On the whole, the stereospecificity of a catalyst depends on the interaction of all the
substances taking part in the reaction.
The syntheses of optically active polymers are particularly impressive. F o r example, propylene oxide
generally is a racemic mixture, which gives a n inactive
product on polymerization with potassium hydroxide as
catalyst. However, if diethylzinc that has previously
been treated with a little water and (+)-borne01 is used
as the catalyst, the D-( +)-propylene oxide is polymerized
preferentially t o give a n optically active polymer [191. The
remaining monomer mixture is optically active, since
the L-isomer predominates. Owing t o the asymmetry in
the catalyst (due t o the borneol), the activation energies
or L
(4)
8
b
Since a new ally1 pyrophosphate group is simultaneously
introduced, further molecules of the substrate can be
introduced in the same manner. The double bond
assumes the cis-configuration.
Polymers of uniform molecular weight generally are
difficult t o obtain by chain reactions, since the reactivity
of the active polymer end is practically independent of
the chain length. Further, chain termination and chain
transfer also frequently take place independently of the
[20] G . Natta, P . Pino, P . Corradini, F. Danusso, E. Marttica,
G . Mazzanti, and G. Moraglio, J. Amer. chem. SOC.77, 1708
(1955).
[21] K . Ziegler, E. Holzkamp, H . Breil, and H . Martin, Angew.
Chem. 67, 541 (1955).
[22] Cf. G . Natta, J. Polymer Sci. 48, 219 (1960).
[23] Cf. G. Natta, A . Zanibelli, I. Pasquon, G . Gatti, and D . DeLuca, Makromolekulare Chem. 70, 206 (1964).
1241 F. Lvnen and U. Henning, Angew. Chem. 72, 820 (1960).
865
molecular weight. In free-radical polymerization, the
principal termination reactions are the combination of a
polymer radical with an initiator radical or with another
polymer radical
-CH2+
R ‘ + -CH2R
and the disproportionation of polymer radicals
However, the polymer radical may also become saturated
by abstraction of an atom or group from another molecule, so that the reaction chain is carried on by the
new radical formed (chain transfer) :
-CH2-CH2
+ R-H
+
-CH~-CHJ
+ R‘
All these processes occur randomly. In ionic polymerization and in polymerization on Ziegler-Natta
catalysts, chain transfer also occurs, mainly to the
monomer, as well as catalyst decomposition, side
reactions at the active polymer ends, and depolymerization, which also proceed randomly.
Polymers of uniform molecular weight are therefore
formed only if none of these reactions take place.
Moreover, the growth of all polymer chains must begin
as nearly as possible simultaneously and proceed at the
same rate. Several cases are now known in which these
conditions are approximately satisfied.
“living”) 1281. However, these processes are very sensitive to impurities. The reaction conditions must be
such that either all the polymer anions grow freely or all
the ion pairs at the polymer ends are undissociated. For
example, in the case of a polystyrene anion with a
sodium cation, the dissociation of the ion pairs can be
suppressed by the addition of sodium salts that are
soluble in the reaction medium (291. The polymerization
then proceeds more slowly, but uniformly. Rapid mixing
of the catalyst and the monomers is important, particularly when initiation is slow [301. In pre-initiation, the
whole of the catalyst is first allowed to react with small
quantities of monomer[311. In this way all the chains
are initiated, but the degrees of polymerization remain
low. The actual polymerization then takes place simultaneously for all polymer molecules as the monomer is
added. The “living” polymers must be deactivated by
termination when the desired degree of polymerization
has been reached; the latter is given by the ratio of the
number of monomer molecules to the number of catalyst
molecules.
The enzyme-catalysed synthesis of amylose (8), which
proceeds in vitro in the presence of phosphorylase, can also
be carried out in accordance:with the above principle [321.
The substrate is’glucose-1-phosphate (7), which’polymerizes
in a chain reaction with elimination of phosphate.
r
For example, in the polymerization of sarcosine N-carboxylic
anhydride (5), C02 is eliminated, and a polypeptide (6 ) [251
having a uniform molecular weight is formed. The chain
reactions are initiated with protonic substances. Each: propagation step leads to the formation of a peptide that is also
(8)
Initiators, such as maltosaccharides containing more than
two glucose units, are required. The initiation can be carried
out rapidly, and amyloses with very uniform degrees of
polymerization (up to 500) can be obtained.
protonic, but very stable in this form in the absence of the
monomer. No side reactions occur in acetophenone with
secondary amines as initiators. The chains grow at the same
rate, so that the molecular weight of the product is fairly
uniform 1261.
The polymerization of ethylene oxide with protonic substances proceeds similarly, and can also lead to polymers
with narrow distributions of molecular weight (Mn = 44000,
M ~ / M , ,= 1.1)[271.
Anionic polymerizations with metal cations are other
examples. Many organo-alkali metal compounds, when
used as catalysts, give such stable ion pairs that the
polymer ends remain active (the polymers remain
[25] S. G. Waley and J. Watson, Proc. Roy. SOC. (London) A199,
499 (1949).
[26] Cf. W . J. Ritschard, Makrornolekulare Chem. 29,141 (1959);
B. Hargitay, A. J . Hubert, and R . Buyle, ibid. 56, 104 (1962);
R . D. Lundberg and P. Doty, J. Arner. chern. SOC.79, 3961 (1957).
[27] E. Wojtech, Makromolekulare Chern. 66, 180 (1963).
866
However, polymers of perfectly uniform molecular
weight can never be obtained by the above methods.
Nevertheless, narrow distributions (Poisson distributions) of the molecular weights are obtained.
b) Stepwise and Stagewise Synthesis of Polymers
In many polymer syntheses, the two sites of linkage of
the monomer react practically independently. Thus
[28] M . Szwarc, M . Levy, and R. Milkovitch, J . Arner. chem. SOC.
78, 2656 (1956).
[29] H. Hostalka, R. V . Figini, and G. V . Schulz, Makromolekulare Chem. 71, 198 (1964).
[30] R. V . Figini, H. Hostalka, K . Hurm, G. Lohr, and G. V .
Schulz, Z . physik. Chem. N.F. 45, 269 (1965); F. Wenger, Makromolekulare Chern. 64, 151 (1963).
[31] Cf. also D . J. Worsfoldand S. Eywater, Canad. J. Chern. 36,
1141 (1958).
[32] E. Husemann, E. Fritz, R. Lipperf, E. Pfannemiiller, and
E. Schupp, Makromolekulare Chem. 26, 181 (1958).
Angew. Chem. internat. Edit.
1 Vol. 5 (1966) / No.
10
these are not chain reactions in the sense described. A
typical example is the polycondensation of a hydroxy
acid to a polyester:
n HO-R-COOH
--f
H(O-R-C0)-0-R-COOH
n- 1
+ (n-1)HzO
The process is a mixture of stepwise polycondensation
(in which one monomer unit is introduced at a time)
and sfagewise polycondensation, in which two polymer
fragments combine. This type of monomer will be
denoted by a-R-b. Similarly, monomers of the types
a-R-a and b-R-b, each containing two identical
functional groups a or b per molecule, combine in an
alternating sequence
a--R a b -R’--ba-R-ab
~
-R‘~-b.
The position of the monomer units of the type a-R-b
is fixed by the functional groups. The monomer also
determines the centers of asymmetry if these can be
introduced unchanged into the polymer, i.e. if no
racemization or cis-trans rearrangement takes place
during the synthesis. This can be achieved by the use
of mild reaction conditions, in particular low temperatures, by the use of very pure materials, and by
catalysis of the linkage reactions.
If it is desired to obtain polymers of this type with
uniform molecular weights, the stepwise and stagewise
syntheses must not occur randomly, but must proceed
in a definite manner. In the case of monomers of the
types a-R-a and b-R’-b, one may be used in such a
large excess that on grounds of probability only a single
linkage step (in both directions in this case) takes place:
-f
2 b~ R’--ba-R-ab-R’-b
i 9)
or
b--R’-b+ 2 a-R-a
--f
a-R-ab-R’-ba-R-a
(10)
If the products (9) or (10) are now separated from the
excess of monomer and then allowed to react with a
large excess of the other monomer, the next (double)
step also takes place in a controlled manner. Polymer
fragments can be joined in accordance with the same
principle (“duplication process”) ‘331 :
19) - I - 2 110)
--t
(10)-(9)-(10)
In the cases described, two functional groups can react
in each monomer molecule and in each polymer fragment. If only one functional group were present per
fragment, it would be easier to obtain uniform polymers,
but these would be unable to react further:
R- a -1- b -R’
--f
R-ab- R’
However, it is possible to chemically alter the activities
of functional groups, to block them, or to remove them
altogether, as well as to introduce new functional groups.
[33] R . Fordyce, E. L . Lovell, and H . Hibbert, J. Amer. chem.
SOC.61, 1905 (1939); W. Kern and W. Thoma, Makromolekulare
Chem. 16, 89 (1955); W. Kern, Angew. Chem. 71, 585 (1959);
W. Kern, H . Kalsch, K. J. Rauterkus, and H . Sutter, Makromolekulare Chem. 44-46, 78 (1961).
Angew. Chem. internat. Edit.
/ Vol. 5
a’-R
(1966) J No. 1 0
a + b R’-b’
--t
a’-R-ab-R’-b’
(11)
After conversion of b’ into b or of a’ into a in ( I I ) , the
process can be continued as follows:
a’-R-ab-R‘-b+
a-R-a’
-+ a’--R-ab-R’-ba
-R -a’
(12)
or
a’-R-ab..R’-b&
What has been said about polycondensations is also
true when the functional groups combine by addition.
a--R- a + 2 b--R’-b
It is therefore also possible to use effectively monofunctional compounds. In the bifunctional monomers
a-R-a and b-R’-b, for example, some of the functional
groups a may be chemically changed (a’) in such a way
that they no longer react with b, and some of the groups
b (b’) so that they no longer react with a. The resulting
monomers can give only one dimer :
a R-ab .R’-b’
a’-R-ab-R’
+
...ba -R-ab-R’-b’
(131
This procedure may be continued as long as the required
isolation and purification can still be carried out. It is
possible in this way to produce very complex aperiodic
polycondensates, e.g. polyesters [341, polypeptides, and polynucleotides [351. The method has very often been used for the
synthesis of peptides [361. The 20 most important naturally
occurring amino acids, of which all but glycine are optically
active, are used. To prevent racemization, polymerization
must be carried out under particularly mild conditions. For
example, an amino acid whose amino group is protected by
the t-butoxycarbonyl (BOC) group is allowed to react with
another amino acid whose amino group is unprotected, but
whose carboxyl group is esterified with ethanol. The compounds can be condensed in the presence of dicyclohexylcarbodiimide (DCCI):
BOC -NH--R-COOH
DCCI
+ H~N-R’-COOC~HS
~-4
BOC-HN-R-COO-NH-R’-C~OCZH~
f HzO
After removal of e.g. the BOC group, the reaction can be
continued. The yields obtained in such syntheses are generally
about 80 % per step. Consequently, in a synthesis involving
20 steps, the final yield of uniform material is only about 1 %.
Merrifiefd[371 therefore linked the carboxyl group of the first
amino acid of the peptide to a n insoluble carrier, e.g. a crosslinked polymer
BOC-NH -R -COO-carrier.
All reagents can then be used in excess, so that yields of
approximately 100 % per step are obtained. After each
reaction, the cross-linked polymer bearing the peptide chains
can be filtered off and washed if necessary. When the synthesis is complete, the peptide chains are carefully released
from the cross-linked polymer. Other polycondensates can,
in principle, also be prepared in this way. The main problem
is to find a suitable cross-linked polymer.
A question that arises here is how Nature produces
uniform polypeptides and proteins, many of which have
[34] Cf. H . Zahn, C . Borsrlap, and G. Valk, Makromolekulare
Chem. 64, 18 (1963).
[35] For example, see S. A. Narang, T . M. Jacob, and H . G.
Khorana, J. Amer. chem. SOC.87, 2988 (1965).
[36] K . Hofinanrz and P . G . Katsoyannis in H . Neurath: The
Proteins. 2nd Ed., Vol. 1, p. 53, Academic Press, New YorkLondon 1963; Th. Wieland and H . Determann, Angew. Chem. 75,
539 (1963); Angew. Chem. internat. Edit. 2,358 (1963); R. Schwyzer, W. Rittel, H . Kappeler, and B. Iselin, Angew. Chem. 72, 915
(1960); H . Zahn, J. Meienhofer, and H . Klostermeyer, Z. Naturforsch. 196, 110 (1964).
[37] R . B. Merrifield, Biochemistry 3, 1385 (1964); Science 150,
178 (1965); R. B. Merrifield and J . M . Stewart, Nature (London)
207, 522 (1965).
867
extremely high molecular weights. The syntheses themselves take place in the cell cytoplasm, and exhibit
certain similarities to “stepwise” laboratory syntheses.
However, one fundamental difference is that the peptide
chains are produced in special centers of synthesis, the
ribosomes, one chain being synthesized per ribosome.
The information concerning the synthesis, i.e. the synthesis program, is carried from the cell nuclei to the
ribosomes by nucleic acids, the messenger RNA’s
(m-RNA). Other nucleic acids, the transfer (soluble)
RNA‘s (s-RNA), are linked by their carboxyl groups to
the monomeric amino acids. Special s-RNA‘s correspond to the various amino acids. The synthesis program
is fixed in the m-RNA by the sequence of the nucleotide
units, a sequence code (the “genetic code”) consisting of
a combination of three out of four fundamental units
(“codons”) being assigned to the amino acids[381. In
accordance with this key system, the amino acids”are
carried to the growing peptide chains by their s-RNA,
which becomes attached for a short time to the m-RNA.
The linkage itself is brought about by enzymes, and
proceeds in accordance with the following scheme:
A,, A?, e f c . : amino-acid residues.
3. Syntheses on Templates
In the syntheses described so far, the contact of a
substance with the active polymer ends favors certain
reaction steps, and so leads to a more controlled synthesis. Contact of this nature can be extended to
include the whole of the polymer molecule being formed.
Such processes are known as syntheses on templates.
a) Crystals of Low Molecular Weight Compounds as
Templates
Many studies have been carried out in recent years on
polymerizations of crystalline monomers. The monomer
molecules rearrange in the crystal lattice and form the
polymer. The polymer occupies a smaller volume than
its monomers. If the contraction on polymerization is
not too great, the crystal lattice of the monomer should
impose order on the polymer, i.e. it should act as a
template.
However, only a few examples of this nature are known:
Solid methyl methacrylate (mixed with magnesium) undergoes free-radical polymerization at -100 “C to form isotactic
polymethyl methacrylate, though it normally gives a syndiotactic polymer at low temperatures c391. This observation
can only be explained by the influence of the solid monomer.
Solid trioxane gives a crystalline polymer whose chains are
arranged parallel to the trigonal axis of the monomer
crystal [401. Crystalline polymers of high structural uniformity are also formed in the crystalline monomers di-
ketene,P-propiolactone,and3,3-bis(chloromethyl)oxetane [411.
Whereas the templates in these cases are the crystalline
monomers themselves, this function can also be taken
over by foreign substances. Clasen 1421 made use of the
fact that crystalline urea and thiourea, when powdered
with many low molecular weight compounds (in the
presence of a little methanol) and shaken, change their
crystal structure. They form tubular tunnels, in which
the added compounds are deposited. Clasen, and later
other authors, prepared tunnel inclusion compounds of
polymerizable substances such as 2,3-dimethylbutadiene,
1,3-cyclohexadiene, and vinylidene chloride 1431. These
monomers could be converted into polymers of high
structural uniformity by heating or by irradiation with
electrons, y rays or X rays. The space available in the
tunnels forces the monomers into an arrangement that
is reproduced in the polymers (Fig. 2).
c
/ aI
A t mplate may be described as a polymeric substance
or a crystal of a low molecular weight compound, which
exerts a contact effect on the polymer being formed;
under favorable conditions, the latter assumes the dimensions of the available contact area. The polymer
formed may be related or unrelated to the template
substance, and in the extreme case the two are interchangeable or even identical, so that replication of
polymers becomes possible. The contact effect is
exerted by bonding of the entire polymer being formed
to the matrix for a limited time; all types of bonds are
possible, and they may differ widely in strength. It must
be possible for the polymer molecule to be released
intact from the template. Thus a template is simply a
catalyst, though this does not mean that no other
catalysts and (for chain reactions) initiators are required.
Fig. 2. Channel inclusion (a) and channel polymerization (b), schematic
(after [421).
[38] See, e.g., S. Ochoa, Ber. Bunsenges. physik. Chem. 68, 707
[39] V. A . Kargin, V. A . Kabanov, and V . P. Zubov, Vysokomolekuljarnye Soedinenija 2, 303 (1960).
[40] J. Lando, N . Morosoff, H . Morawetz, and B. Post, J. Polymer Sci. 60, S 24 (1962).
[41] K. Hayashi, Y. Kitanishi, M . Nishii, and S. Okamura, Makromolekulare Chem. 47, 237 (1961).
[42] H . Clasen, 2. Elektrochem. Ber. Bunsenges. physik. Chem.
60, 982 (1956).
[43] J. F. Brown jr. and D . M . White, J. Amer. chem. SOC.82,
5671 (1960); 0. L. Glavati and L. S. Polak, Petroleum Chem. 2,
(1964).
201 (1963).
868
Angew. Chem. internat. Edit.
Vol. 5 (1966)
No. 10
The template effects of urea and of thiourea can be seen
from the fact that only monomers of suitable dimensions can be used in this method. However, the degrees
of polymerization are not uniform. To obtain uniformity in size, the lengths of the tunnels would have to be
the same and chain-terminating reactions would have
to be excluded, and neither of these conditions has so
far been achieved. It should also be noted that in all the
cases described, the only forces between the templates
and the polymers being formed were physical forces.
(2-hydroxy-5-methylbenzyl)phenol],obtained by “stepwise”
and “stagewise” synthesis [461, and from which they prepared
the triacrylate (14) [471.
I
b) Polymers as Templates
Polymers as templates for the formation of polymers
are found in Nature in the replication of deoxyribonucleic acids (DNA), the genetic material of living
organisms. The nucleic acids differ only in their bases.
There are normally only four bases present, and two of
these, a purine and a pyrimidine base, form very strong
hydrogen bonds. The result is that two D N A chains
of equal length in which the corresponding bases are
situated opposite to each other are “complementary”
and fit closely together. The double chains are wound
into right-handed helices. In replication, the hydrogen
bonds are broken, starting at one end. Activated
nucleotides can act as the substrate, and are linked with
the aid of enzymes and in contact with the individual
chains in such a way that they again complete the double
chains. Thus the existing polymers have acted as
templates for the polymers being formed, which will in
turn act as templates in the next replication, and so on.
In this way, identical polymer molecules are built up in
pairs (ultimately from a single molecule). These processes are not confined to living cells. Artificial polynucleotides have been prepared enzymatically in vitro,
and these replicated by the same mechanism [441.
According to Marx-Figini and G . V. Schulz, another
type of template evidently occurs in Nature 1451. These
authors found that the secondary-wall cellulose of
cotton is uniform, not only in its structure, but also in
its degree of polymerization (ca. 14000). This degree of
polymerization is reached after only a short period of
growth, and subsequently remains constant. The templates may be tubular proteins from which the cellulose
grows.
The preparation of identical polymer molecules on
templates without the aid of enzymes has not yet been
accomplished. However, a point of departure for such
syntheses is found in the range of low degrees of polymerization.
Kummerer and his co-workers started with a monodisperse
4-methylphenol-formaldehyde condensate [4-methyl-2,6-bis[44] H . K . Schachman, J . Adler, C . M . Radding, I . R . Lehman,
and A. Kornberg, J. biol. Chemistry 235, 3242 (1960); C. M .
Radding, J . Josse, and A . Kornberg, ibid. 237, 2869 (1962); C. M .
Radding and A . Kornberg, ibid. 237, 2871 (1962).
[45] M . Marx-Figini and G. V . Schulz, Makromolekulare Chem.
62,49 (1963); M . Marx-Figini, ibid. 68,227 (1963); 80,235(1964),
M . Marx-Figini and E. Penzel, ibid. 87, 301 (1965); cf. also G. V .
Schulz, IUPAC Symposium o n Macromolecular Chemistry,
Prague 1965.
Angew. Chem. internat. Edit. / Vol. 5 (1966) 1 No. I0
p
CH,
( C H ~ ) Z C - C H ~ - C H - C H ~ - C H - C H ~I - C H - CI (CH3)2
I
I
I
COOH
COOHCOOH
COOH COOH
f 16)
A dilute solution of (I4) in benzene was polymerized with
about four equivalents of azobisisobutyronitrile as a freeradical initiator and chain-terminating agent. Owing to the
high dilution and the large quantity of primary radicals
present, the oligomers were largely isolated from one another
during the reaction, and the dinitrile (IS) was formed in 70 %
yield. Alkaline hydrolysis of ( I S ) leads to the pentacarboxylic
acid (16), which is uniform in its molecular weight, though
it is a mixture of various stereoisomers.
4. Other Synthetic Routes
a) Defined Conversions of Existing Uniform Polymers
It is not always necessary to prepare a desired uniform
polymer directly from the monomers. It is sometimes
better to convert an existing uniform polymer chemically into the desired polymer. Since polymers generally
contain a large number of reactive sites, the reactions
may be either complete or incomplete with respect to
the individual molecules, but they must take place in
the same positions in all the molecules. If the reactions
are incomplete with respect to all the molecules of a
polymer, it is still possible to isolate the uniformly
transformed components.
In practice, one aims at complete reaction without
degradation of the polymer. It is necessary to use
dissolved or at least solvent-swollen polymers, since the
reagents cannot otherwise reach all the reactive sites.
The classical example is the hydrolysis of polyvinyl
acetate to polyvinyl alcohol[4*]. Most of the reactions
carried out so far also have taken place on functional
[46] H. Kummerer and H.-G. Haub, Makromolekulare Chcm.
59, 150 (1963).
[47] H. Kummerer and Sh. Ozaki, Makromolekulare Chem. 9 1 .
1 (1966).
[48] W. 0. Herrrnann and W. Haehnel, Ber. dtsch. chem. Ges.
60, 1658 (1927); H. Staudinger, K . Frey, and W. Starck, ibid. 60,
1782 (1927).
8 69
groups. Since these are generally close together they
influence one another, and even their steric position can
be of some importance [491, particularly so when the
reactions involve two adjacent groups (bifunctional
reactions). The influence of the chain conformation of a
polymer is shown by the difference in the reactivities of
proteins before and after denaturation [501. Specific reactions can be brought about with the aid of enzymes.
b) Transformation of Replicating Polymers
If replicative polymers are modified in such a way that
the power of replication is retained, only one polymer
molecule need, in principle, be changed. Identical mole[49] See, e.g., G. Smrts, Angew. Chem. 74, 337 (1962); Angew.
Chem. internat. Edit. I, 306 (1962); H . Morawetz and J. Oreskes,
J. Amer. chem. SOC. 80, 2591 (1958).
[50] C. B. Anfinsen, J. Polymer Sci. 49, 31 (1961); G. H . Gun&
luch, W. H . Stein, and S. Moore, J. biol. Chemistry 234, 1754
(1959).
cules will then be formed by replication, provided the
correct substrate is present. The only replicative polymers known at present are nucleic acids and polynucleotides. Detailed studies have been carried out on
changes by mutation in the genetic matter of bacteria,
bacteriophages, and viruses. However, these nucleic
acids usually suffer lethal lesions during chemical
changes. Nevertheless, changes that do not prevent
replication occasionally occur in the bases of the
nucleotides (premutations). The modified bases are
“read” in the same way as a natural, but different base.
This leads to the incorporation of these wrongly “read”
bases into the resulting polymer, and into the complementary polymer of the latter. Reactions of this type
are brought about by X-rays and ultraviolet radiation,
as well as by chemical agents (mutagens) such as
nitrous acid, mustard gas, nitrogen mustards, diazomethane, ethylene oxide, formaldehyde, and hydroxylamine.
Received: June 27th, 1966
[A 538 IE]
German version: Angew. Chem. 78, 887 (1966)
Translated by Express Translation Service, London
Syntheses of Carboxylic Acids from 1,l-Dichloroethylene [‘I
BY DR. K. BOTT AND PROF. H. HELLMANN
FORSCHUNGSLABORATORLEN DER CHEMISCHE WERKE H u L S AG., MARL (GERMANY)
Dedicated to Professor F. Broich on his 60th birthday
Many B-alkyl- and @-arylpropionicacids are readily obtainable by the reaction of secondary
or tertiary alcohols. esters of these, or olefins with the inexpensive 1,l-dichloroethylene in
sulfuric acid. The success of the synthesis depends on the bulk and encrgy of the carbonium
ion intermediate formed from the alcohols or olefins. With carbonium ions having one H
atom attached to the carbonium C atom, electrophilic substitution of the 1,I-dichloroethylene
takes place to a small extent. Dicarboxylic acids and carbaxylic acids with higher molecular
weights are sometimes formed as a result of side reactions.
1. Introduction
(3-Alkyl- and P-arylpropionic acids generally are prepared by syntheses involving several steps. In the course
of attempts to find a convenient route to P,P-dimethylbutyric acid, we discovered a synthesis that can be used
for the preparation of a series of carboxylic acids of the
above type in a one-stage process using 1,l-dichloroethylene (vinylidene chloride) ( I ) .
Two processes are available for the industrial production of 1,l -dichloroethylene [*, 33.
The action of an equimolar amount of chlorine on vinyl
chloride or on 1,2-dichloroethane yields 1,1,2-trichloroethane. Subsequent dehydrochlorination with aqueous
sodium hydroxide [41 or calcium hydroxide 151 leads
almost exclusively to 1,l-dichloroethylene (1). This dichloroolefin is a colorless liquid boiling at 37 OC; it is
stable on storage in the monomeric state only after the
addition of polymerization inhibitors such as triethylamine and 4-t-butylcatechol.
CH2=CHC1
CHZCl-CHzCl
1
-
[l] K. Bott, Angew. Chem. 77, 967 (1965); Angew. Chem. internat. Edit. 4, 956 (1965); Chem. Ber., in press.
[2] French Pat. 804491 (1936), Compagnie de Produits Chimiques et Electrometallurgiques Alais, Froges & Camargue, Chem.
Abstr. 31, 3509 (1937).
[3] Brit. Pat. 627263 (1946), B. F. Goodrich; Chem. Abstr. 44,
2542 (1950).
870
[4] C. J. Strosacker and F. C. Amstutz, US.-Pat. 2322258 (1938),
Dow Chem. Co.
[5] C. Jung and A. Zimmermann, Ger. Pat. 529604 (1929), 1.G.Farben.
Angew. Chem. internat. Edit. Vol. 5 (1966) J No. I0
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