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Electroplating of Plastics in Theory and Practice.

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peratures at which the composites may be used will be
further raised by the use of glasses or metals as matrix
materials 154,551. Carbon fiber reinforced carbons are
stable up to 3000 “C in inert o r reducing atmospheres.
[54] D . H . Bowen, R . A . J . Sambell, K . A . D. Lambe, and N. J.
Mattingley, DOS 1925009 (1968), U.K.A.E.A.
I551 A . A . Baker and R. J. Bache, Brit. Pat. 1177301 (1968)
Rolls Royce.
However, their production is still in the experimental
The further development of these new fiber-reinforced
high-temperature materials will open up wide fields of
application for carbon fibers.
Received: March 1 1 , 1970
[A 754 IE]
German version: Angew. Chem. 82, 400 (1970)
Translated by Express Translation in Service, London
Electroplating of Plastics in Theory and Practice
By Kurt Heymann, Wolfgang Riedel, and Gunter Woldt [*I
Electroplated plastics combine many advantages of plastics and of metals; they have the
low weight and ease of shaping ofplastics, together with the luster, hardness, and electrical
conductivity of metals. An important part of any process for the electroplating of plastics
is a pretreatment to ensure good adhesion of the metal film. Etch activation in the straightthrough method requires only six steps for pretreatment.
1. Introduction
The rapid growth of the consumption of plastics in
the last 20 years is primarily due to the ease with which
they can be shaped to obtain useful objects. This was
greatly appreciated by industrial users, who required
versatility of types and shapes with no great increase
in production costs, such as arises e.g. in the processing
of metals.
For this reason the users accepted disadvantages in
color matching, the attraction of dust by static electricity, and the relatively soft surface of injection molded
These “made-to-measure materials” offer considerable scope for the work of the chemist. The polymers
can be suited to a wide range of uses by modification
of the nature of the monomers, the polymerization
conditions, and pigmentation, and because of the
possibility of reinforcement with glass fibers. Nevertheless, plastics have to compete with the traditional
touch. Electroplated plastic feels warm and is much
lighter. Because of its lightness, it is easier to transport
and handle, and this is a further argument in favor of
metalized plastics.
Plastics can be shaped more easily, more cheaply, and
in greater variety than metals. To obtain shiny surfaces, the mold into which the plastic is injected is
polished instead of each individual article. Without
knowing the market, one might think that the electroplating of plastics is a passing fashion. However, this
practice has many technical advantages, apart from
its decorative value.
Many plastics are damaged by ultraviolet radiation.
This damage is prevented by a film of metal. The
flammability of plastics is also reduced by metalization.
Use is often made in the electrical industry of chassis
having complicated shapes. To produce a metal
chassis of this kind, several individual parts have to
be fitted together and welded. It has been found in
It seemed natural, therefore, to seek a good combination of plastic and metal, i.e. a genuine composite
of plastic and metal.
An important factor here is undeniably the wish to
give a plastic surface a metallic luster for decorative
reasons, since large sections of the market like the
glitter of metals. It is in fact impossible to tell by appearance whether a chromium-plated object is made
of metal or plastic; the difference can be felt only by
[ * ] Dr. K . Heymann, Dip1.-Phys. W. Riedel, and Dr. G. Woldt
Schering AG, Galvanotechnik
1 Berlin 65, Postfach 59 (Germany)
Angew. Chem. internal. Edit.
1 Vof. 9 (1970) 1 No. 6
Fig. 1.
Chassis of ABS plastic (Bosch Elektronik GmbH. Berlin).
practice that the same result can be achieved by injection molding of the chassis in one operation, followed by electroplating (Fig. 1). The metal deposit is
electrically conductive. Contacts can be made as
before if soldering is carried out quickly and without
too much heat.
copper or nickel reduction bath is possible only if
electrons can be transferred from the reducing agent
to the metal ions to be reduced via the solid phase. For
this to happen, the solid phase must be a good electronic conductor and allow fast electron exchange.
This condition is satisfied by many metals; however,
noble metals are preferred since they can be deposited
particularly readily in the cavities in the plastic by
reduction of their ions. It is also possible to apply
colloidal noble metals to the surface of the plastic.
In other cases a metal deposit is desirable to prevent
static build-up. Other uses are discussed in Section 4.
As long as there have been plastics, there have been
efforts to electroplate them 111. Plastics, like other
nonconductors, clearly require petreatment to make
them electrically conductive for the plating operation.
The surface must first be made wettable and active,
and then covered with a thin film of metal in electroless
baths. Though promising methods were known [I 21,
all efforts remained unsuccessful for years, since an
adhesive bond could not be produced between the
plastic and the electrochemically applied metal film.
Conducting lacquers seemed for a time t o offer a
suitable bonding substance, but this plan failed
because the conducting lacquer rounded the contours
of the surfaces, and the metal pigments could not be
uniformly exposed and built up by electrodeposition.
Around 1964, it was found by the work of Narcus r31,
Saubestre [41, and investigators a t Marbon Chemical 1 5 ~ 5 1 that ABS plastics (acrylonitrile-butadienestyrene graft polymers) are excellently suited for electroplating. On this basis and with steady improvement of the pretreatment methods, the electroplating
of plastics was developed in only five years into an
industrially recognized process 171.
2. The Plating Processes
The sequences of operations involved in the electroplating of plastics are summarized in Table 1. For the
first step of the pretreatment, i.e. etching, the ABS
objects are mounted on jigs and immersed in chromosulfuric acid at 65 “C. This makes the surface of the
plastic hydrophilic, and the butadiene units are oxidatively degraded. Cavities are formed at these points,
and these later provide mechanical anchorage for the
Table 1 . Sequence of operations in varlous methods for the electroplating of ABS and zinc die castings.
Etch activation
Zinc die castings
Cleaning in
Activation Sn2
Activation Pd2+
Chem. deposition
of Ni
Ag+ or Pd2+
Reduct ion
9. Chem.
of Cu or Ni
of Cu or Ni
The production of the noble metal particles that
catalyze the chemical deposition of metal is known in
the plating industry as “activation of the plastic”, or
as “nucleation”. This can be achieved by the following
a) Immersion in stannous chloride solution and then,
after rinsing with water, in silver nitrate, paIladium(1r)
chloride, or gold(m) chloride solution.
metal coating [ 8 9 . To make the plastic conductive,
metal is deposited on it by electroless plating. However, deposition of metal on objects immersed in a
b) Immersion in a solution of a palladium salt and then
in hydrazine hydrate or sodium tetrahydroborate
c) Immersion in a colloidal palladium solution.
[I] S . Wein: Metallizing Non-Conductors. Finishing Publ.,
Inc., Westwood, USA, 1945.
[2] S . Wein: Metal Finishing 58/60, 69 (1948).
f3] H. Narcus: Metalloberflache 17, 284 (1963).
[4] E . B. Saubesrre, C . J . Durney, J . Hajdu, and E . Bastenbeck,
Plating 52, 982 (1965).
[S] J . B. McNamara, Metal Progr. 86, 112 (1964).
[6] C. R. Holt and B. J . Jensen, Plastverarbeiter 2, 72 (1965).
[7] K. Heymann and G. Woldt, Galvanotechnik 6I,221 (1970).
[8] K . Heymann, W. Riedel, and G . Woldt in H. Wiegand:
Metallische Uberzuge auf Kunststoffen. Hanser Verlag, Miinchen, 1966, p. 48.
[9] K . Wiebusch, H . Hendus, and E. Zahn in H . Wiegand
Metallische oberzuge auf Kunststoffen. Hanser Verlag, Miinchen 1966, p. 9.
d) Etch activation in chromosulfuric acid containing
noble metal.
Method (a) was chemically dependable, but required
too many workers for its execution. While the parts
are being rinsed with water after activation with stannous chioride solution, hydrolysis leads to the formation of gel-like oxide chloride, which remains adsorbed
not only on the ABS but also on the insulated jig. The
result is that noble metal nuclei, and later copper o r
nickel, are deposited on the insulation of the jig. To
confine the electrodeposition of metal to the ABS
plastic, the parts are then transferred to different inAngew. Chem. internat. Edit.
Vol. 9 (1970) No. 6
sulated jigs that have not been in contact with the
pretreatment baths. The efforts to overcome this
disadvantage and to ensure selective deposition of the
metal on the ABS led first t o the introduction of activation methods (b) and (c).
+ CuO+ HCOO-+ 2 H20
Ni2++ HzP02+ H2O -+ N i o t H2P03+ 2 H
C d + + HCOH+ 3 OH-
Unlike the deposition of silver, which takes place
spontaneously in silver salt-formaldehyde solutions,
the deposition of copper and of nickel is strongly inhibited and occurs only in the presence of catalytically
active nuclei.
The carry-over of small quantities of noble metal activator
into a metalization bath was a t first a great danger to practical operation, since these activators caused decomposition
of the metalization bath. It was only with the development
of efficient stabilizers, which prevent the spontaneous precipitation of copper or nickel (compounds of sulfur, selenium,
and some heavy metals, as well as cyanides and oxygen, are
suitable for use as stabilizers), that this danger was eliminated and the chemical deposition of metals became economicalI1ol.
Once the plastic objects have been made conductive
by the deposition of metal, the metal film can be built
up by electrodeposition. The first step is the electrodeposition of copper from a sulfuric acid electrolyte.
Any metal capable of being electrodeposited can
then be applied. Electrolytes having a smoothing
effect are preferred, since the deposition of the metal
then proceeds in such a way that any depressions
already present in the surface are preferentially filled
up (Fig. 3).
Fig. 2. Jigs with ABS components. Left: conventional process (deposition of metal o n jig). Right: straight-through process (metal deposited o n ABS only).
Processes involving n o change of jig are known as
“straight-through” processes, since the jig passes
continuously through the plants from etching to
plating with the object (Fig. 2).
Further rationalization of the pretreatment has recently
been achieved by etch activation [71. In this case some
steps are no longer necessary, as can be seen in Table 1,
so that the electroplating of plastics is now as simple
as the electroplating of metals. Noble metals are added
t o the etching solution, and are carried into the plastic
by the chromosulfuric acid.
It can be verified by experiments with radioactive isotopes
that the noble metal is very firmly fixed to the plastic under
these conditions, and is not washed out again. In these experiments, the noble metal compound was first activated by
neutrons in a nuclear reactor, and then added to the etching
solution. Several ABS samples were etched in the resulting
activated etching acid for 4, 8, or 12 minutes at 65 O C , and
were then vigorously rinsed for 1 or 10 minutes with introduction of air. After the samples had been dried, the radioactivity was monitored with a Geiger-Muller counter.
Despite the vigorous rinsing, the activity of the samples
decreased by only about 10%. This small decrease is of no
importance to the subsequent chemical deposition of metal [71.
The next step after activation is the chemical deposition of metal from copper or nickel reduction baths.
The principal components of these baths are the metal
salts together with complexing agents and reducing
agents. The reductions for copper-formaldehyde and
nickel-hypophosphite solutions can be represented by
the following simplified equations:
Angew. Chem. infernat. Edit.
Vol. 9 (1970) 1 No. 6
Fig. 3. Smoothing of a rough base material by electroplating with
A copper base about 20pm thick ensures that the
composite withstands thermal shocks between +80
and -40°C despite the difference in the expansion
coefficients of the plastic and of the metal.
The thermal shock test devised by the “Electroplated
Plastics” work group of the German Standards Committee for quality control 1111 specifies the following
105 rnin
15 min
105 min
15 rnin
room temperature
-40 or -30 or -20 “C, as required
room temperature
This cycle is repeated three times. The metal must not
blister or crack. If the plastics processor and the electroplater have followed the correct procedures for injection molding and electroplating, the production
parts will pass the test. If on the other hand highly strained plastics articles are electroplated without
following the rule that the copper film should be twice
as thick as the nickel-chrome film deposited on it, the
quality standard will generally not be satisfied.
G. Woldt,Jahrbuch der Oberflachentechnik25,178 (1969).
[ l l ] Galvanotechnik 58, 669 (1967).
3. Theory of Adhesion
ABS plastics are thermoplastics prepared from acrylonitrile, butadiene, and styrene either by copolymerization or by graft polymerization. Chemically speaking, they are ternary materials, but they consist morphologically of two phases, i.e. the finely dispersed,
soft butadiene rubber phase and the relatively rigid
styrene-acrylonitrile copolymer phase [121. Figure 4
shows a transmission electron micrograph of an ABS
graft polymer L131. The polybutadiene phase, which is
in the form of globular particles, is visible as a result
of material contrast. Depending on the type of plastic,
the rubber particles have diameters of 10-5 to lO-4cm.
Fig. 4. Electron micrograph of ABS graft polymer (after 1131. magnification 3700 x).
During the etching of ABS plastics, the double bonds
of the polybutadiene are oxidized, and this phase is
selectively dissolved out of the ABS plastic. Figure 5
shows an ABS film that has been cast from solution,
etched, and activated with silver nuclei 181. The preparation of a film of this type from a solution naturally
leads to relatively good homogenization of the macromolecular components. The etching has produced
spherical holes wherever the polybutadiene component was present in the film.
The activation and the chemical deposition of metal
take place preferentiaily on the inside walls of the
holes, as is shown by Figure 6, which was taken after a
copper-plating time of 45 s. After 1.5 min, all the
cavities have already been filled with copper.
Fig. 6. Electron micrograph of a n ABS film with incipient deposition
of copper (after [81, magnification 6000 %).
The chemical attack of the etching solution on the
ABS graft polymer thus leads to dissolution of the
elastomer out of the surface of the plastic, as was
clearly recognizable. The resulting spherical cavities
are more or less completely filled with metal during
the chemical metalization. The good adhesion of the
metal to the ABS plastic could thus be explained by
mechanical anchoring of the metal in the ABS matrix
polymer [8,141. Figure 7 shows a model of mechanical
anchoring (“snap fastener theory” [179.
Fig. 7. Schematic diagram of the adhesion of metal to ABS plastic.
a) Ideal model, b) real structure.
Fig. 5. Electron micrograph of cast, etched, and activated ABS plastic
after Wiesenberger (from 181, magnification 6000 x).
T o check whether this hypothesis could explain
adhesions of about 200 kg/cm2 in a tensile test and
peel strengths of about 4 kg/inch, an attempt was
made to estimate the adhesion [8,15].
1121 K . Kato, Kolloid-Z., Z . Polymere 220, 24 (1967).
1141 K . Heyrnann, W . Riedel, R . Blaschke, and G . Pfefferkorn,
Kunststoffe 58, 309 (1968).
[13] E . Zahn and K . Wiebusclt, Kunststoffe 56, 173 (1966).
[15] W. Riedel, Galvanotechnik 57, 579 (1966).
Angew. Chem. internat. Edit. J Vol. 9 (1970)
No. 6
The force required t o pull a metal sphere of radius r out of
the matrix polymer AS when the center of the sphere is
situated a t a distance a under the surface (a < r ) is:
KL =22(r--l/r2-,2)S
tensile strength of the AS matrix.
< <
For random variation of the depth a with 0
r, the
force required t o separate the bonded surfaces over a n area
of 1 cm2 in the tensile test is:
film. Figure 8 shows the measured and estimated dependence of the peel strength on the thickness of the
copper coating. For a film thickness of 40pm, one
can expect a peel value of 3.5 f 1.5 kg/inch if adhesion
is assumed to be due to mechanical anchoring in accordance with the snap fastener theory. Good agreement is found between the experimental and the estimated values. It can thus be concluded that the bonding between the plastic and the metal may be of a
mechanical-physical nature. There is therefore no
need to postulate the existence of chemical, dipole, or
van der Waals forces between the materials in contact,
as was done by Saubestre et al. 141 and by Logie and
Rantell [181.
If the butadiene spheres form a three-dimensional trivial
lattice with a lattice constant e, then
400 x r3
e3 = ___
3 8
volume fraction of butadiene in the ABS copolymer (yo).
Thus the adhesion in the tensile test is
K~ = 3.22 x 1 0 - 3 SB
Taking into account a number of second order factors such
as the compressibility of the metal, the polydispersity and the
shape factor of the holes, and the microroughness, we obtain
Ks values of the order of 150 kg/cmz with S !z 7 5 0 kg/cm*
and B w 25 %. This almost agrees with the adhesions found
experimentally [8,161.
In practice, the adhesion is usually taken to be not the tensile
strength but the peel strength in accordance with DIN 40802.
The peel strength is defined as the force required to pull the
coating away from a strip of plastic 1 inch wide with a pull
parallel to the normal t o the surface.
The peel strength P consists of two components, one taking
the deformation of the metal into account (Pv), and the other
the force needed t o overcome the adhesion (PH)1151:
Fig. 8. Dependence of the peel strength P on the thickness h of the
our experimental values.
copper film (after [15]; - calculated.
0 Saubebrre’s values).
The surfaces of bulk-injected or extruded ABS articles
after etching naturally differ in morphology from the
surfaces of cast ABS films. Figures 9-11 show electron-scan micrographs of an ABS plastic after etching
for 2 min, 8 min, and 32 min, respectively. The morphological similarity of the specimen exhibiting optimum etching (Fig. 10) to that of Figure 5 is unmistakable.
E = elasticity modulus of the metal, ii = average “immersion
depth” of the snap fasteners, b = width of the metal film
(2.5 cm), h = thickness of the metal film, y = contact length.
The unknown contact length y , i.e. the length of the zone in
which stresses are acting to break the contact, can be eliminated by optimization:
The peel strength is therefore:
= 1.3 x 106 kg/cmz, 2 = 0.2 x lO-“cm, KL = 1.65 x
10-6 kg, d = 1.26 x 10-4 cm, h (in cm) gives
222 x h314
Thus the peel strength of a copper coating is theoretically strongly dependent on the thickness of the metal
Fig. 9. Electron-scan micrograph of ABS plastic after etching for
2 min (magnification 6000 x).
[ 1 6] N . F. Murphy and E. F. Swansey, Plating 56, 371 (1969).
[I71 K. Heymann, Galvanotechnik 56, 413 (1965).
[18] G . R . Logie and A . Rantell, Trans. Inst. Metal Finishing
46, 91 (1968).
Angew. Chem. internat. Edit. J Vol. 9 (1970)J No. 6
A question of practical importance that now arises is
whether plastics other than ABS can give strongly
bonded composites on electroplating. On the basis of
the theory of mechanical anchorage, it is possible to
predict the composition required of a plastic if it is to
be capable of being electroplated.
The plastic must consist of at least two phases, one of
which must have a cotton-wool like structure with
colloidal dimensions 1141. This phase must dissolve
much more rapidly than the other phases in an etching
medium. The phases that are less soluble in the etching
solution must be coherent and mechanically stable.
It must also be possible to deposit nuclei for chemical
metalization on the etched surfaces, particularly in
the holes.
Other plastics have in fact been electroplated; the peel
values were similar to those found for ABS, and in
some cases even higher. Figures 1 3 to 15 show three
Fig. 10. Electron-scan micrograph of ABS plastic after etching for
8 min (magnification 6000 x).
Fig. 13. Electron-scan micrograph of etched modified
magnification 6000 x).
PPN (after 1191,
Fig. 1 1 . Electron-scan micrograph of ABS plastic after etching for
32 min (magnification 6000 x ).
Figure 12 shows a cross section through the ABSmetal composite after Zahn and Wiebusch[l3l, in
which the mechanical anchorage of the metal film in
the plastic is visible.
Fig. 14. E!ectron-scan micrograph of etched modified PPO (after 1191,
magnification 6000 x).
FiS. 12. Electron micrograph of a cross section of the ABS-metal
interface (after [131, magnification 9250 Y ).
[19] C . Woldr, Trans. Inst. Metal Finishing 47, 236 (1969).
Angew. Chem. internat. Edit. / Vol. 9 (1970) 1 No. 6
contents. Lead-lined sheet steel tanks are used for
strongly acidic etching solutions, PVC-polyester
laminate tanks for the more dilute solutions, and glass
fiber reinforced polyester tanks for rinsing water. The
decontamination of the waste water is carried out
separately. Accessories required are immersion heaters
in some places, filter pumps, fume-extraction equipment, and (in the electroplating section) rectifiers. The
jigs carrying the articles are passed in succession into
the baths, where they remain for fixed times.
Fig. IS. Electron-scan
magnification 6000 ).
micrograph of etched adhesive (after I201
electron-scan micrographs of electroplatable plastics
that have received optimum etching: modified polypropylene (PPN), modified polyphenylene oxide
(PPO), and an adhesive for the production of printed
circuits by an additive process 1201. These plastics were
modified by copolymerization and by introduction of
organic or inorganic fillers. The etched surfaces show
a distinct similarity to that of optimally etched ABS;
they are relatively uniformly roughened, and the light
borders of the holes clearly show the presence of
undercuts in which the metal can anchor.
Thus, on the basis of present knowledge, one can say
that, t o give good adhesion of electrodeposited films,
a plastic must contain at least two phases, though the
difference between the phases may be merely that one
is crystalline while the other is not. In accordance with
the mechanical adhesion model (snap fastener theory),
the phase that can be dissolved out chemically must be
uniformly distributed in the surface zones of the injection molded article.
Fig. 16. F u lly automated continuous plant for plating plastics with
copper, nickel, and chromium located a t the Overpelt-Plascohel S. A.
The electroplating of plastics has already found applications in the automobile, audio, furniture, clockmaking, photographic, household goods, fancy goods,
and textile industries. We shall now describe a few
examples selected from the many uses.
The number of operations and tanks required is shown
in Table 1. A large Plant should have a fully automated
transport system. The construction of the containers,
which have capacities of up to 3000 1, depends on their
Figure 1 7 c shows an outside mirror for an automobile.
The mechanical polishing required when the part is
made of metal is unnecessary when plastic is used; the
same is true of the armrest fixture (Fig. 17b) and the
miniature radio cover (Fig. 17h). The possibilities
available to the mold designer are illustrated by the
door knob (Fig. 17d) and the clock case (Fig. 17i). In
this connection, there is greater variety in the design
of clothes buttons (Fig. 17f). The camera front plate
in Figure 1 7 g is completely electroplated, though only
the chromium plated edges are visible after the application of a black film and assembly. Since there is no
need for polishing, this construction is cheaper than
production of a front plate from zinc die castings or
light metal. The exposure meter housing shown in
Figure 1 7 a can be injection molded cheaply in one
piece. It is dull nickel plated and chromium plated.
The processing of bathroom fittings made of plastic
has also proved particularly economical (Fig. 17e).
Other applications are navigation light housings for
motor boats, radiator grills, automobile rear light
housings, thermos flasks, perfume bottles, and mixing
valves for bathroom fittings, to name but a few.
1201 H.-J. Ehrich, Galvanotechnik 61, 14 (1970).
[21] P. Spiess, Ingenieur Digest 7, No. 9 (1968).
The applications are not confined t o large objects,
which must be fixed to jigs. Small articles can also be
If these requirements are satisfied, a number of further simple
conditions must be observed in the injection molding process.
The nature and number of gates must be such as t o ensure
that the articles produced will be as free as possible from
strains and orientations. Care should be taken to follow the
raw material producer’s recommendations o n predrying of
the granulate, mass and mold temperatures, injection pressure, and injection rate. The article must be ejected from the
mold without the aid of mold release agents. The objects must
naturally be of a suitable design for electroplating 1211.
4. Applications
Angew. Chem. internat. Edit.
1 Vol. 9 (1970) 1 No. 6
43 1
Flg. 17.
Examples of electroplated piastics articles (for a-i, see text).
Angew. Chem. internat. Edit.
1 YO^. 9 (1970)1 NO. 6
prevents the light plastic parts from floating away
from the contacts.
These examples illustrate the magnitude to which the
electroplating of plastics has grown within a few years.
By rationalization of the process and improvement of
reliability, even in the operation of the electroless
metalizing baths, which has not until now been much
used in industry, the confidence of the user was won,
and a new field of application was opened up both for
plastics processing and for electroplating.
electroplated. Bulk material is first pretreated in sieve
baskets. After being made conductive, the articles are
copper plated, nickel plated, silver plated, or gold
plated electrochemically in barrels. However, this
requires a special barrel in which reliable current
transfer is ensured by several cathode contacts. This
Angew. Chern. internat. Edit. Vol. 9 (1970)1 NO. 6
As a readily obtainable and cheaply producible composite material, metalized plastic will be a material of
the future, if only because of the rapidly expanding
production of plastics. The continuous rapid growth
of the plastics industry has now been in progress for
30 years. The consumption of plastics in the USA per
head of population has increased by 413 % since 1953.
The consumption of plastics in the Federal German
Republic is at present higher (34.1 kg/head of population) than in any other country. In 1980, German
industry will process mOre than 5 million tonnes of
plastics. we believe that the future will belong t o the
composite of plastic and metal.
[A 757 IE]
Received: March 1 1 , 1970
German version: Angew. Chem. 82, 412 (1970)
Translated by Express Translation Service, London
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