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Inorganic co-ordination polymer—I General considerations
J. Inorg. Natl. Chem., 1962, Vol. 24, pp. 365 to 369. Perllamon Presa Ltd. Printed in England
INORGANIC CO-ORDINATION POLYMERSml GENERAL CONSIDERATIONS B. P. BLOCK and G. BARTH-WEHRENALP Research and Development Department, Pennsalt Chemicals Corporation, Wyndmoor, Pa. (Received 14 September 1961 ; in revised form 30 October 1961)
Abstract--Synthetic approaches to co-ordination polymers are classified as condensation polymerizations, addition-type polymerizations involving co-ordinatively-unsaturated intermediates, and degradation polymerizations. The first two classes are further subdivided into homocondensations and heterocondensations and into elimination-additions, substitution-additions, oxidation-additions, redistribution-additions and rearrangementadditions, respectively. These various classes are discussed. IN our Laboratories we are investigating the synthesis of inorganic co-ordination polymers, where we define co-ordination polymers as substances with repeating units containing atoms in the backbone acting as acceptors in co-ordinate covalent bonds. This definition is more restrictive than that used by SOWERBY and AUDRIETH,(1) "polymeric materials containing co-ordinated metal ions", for it excludes polymers which do not contain co-ordination centres in the backbone such as [--CH2CH(CsH4FeCsHs)--]x: 2) We do not include the latter type in our definition, for the co-ordinated portion is not an integral part of the polymer backbone, but rather an adjunct to a substance which is a polymer in its own right. To systematize our synthetic approaches to co-ordination polymers we have formulated a classification scheme which we wish to present in this initial communication in order to provide the framework for succeeding papers in this series, in which we will report the results of our studies. SOWERBY and AUDRIETH(1) and BERLIN and MATVEEVA(3) have also presented such schemes for somewhat different situations. Their treatments were, however, less detailed because of the nature of their reviews. Broadly speaking, organic polymerization reactions are classified as condensation or addition reactions. These two types of reactions involve the elimination of small molecules between units in the first case and the union of unsaturated molecules in the latter. In general, the central element of a co-ordination compound does not form double or triple bonds leading to the chemical behaviour classed as unsaturation in organic chemistry, so that addition polymerization in the foregoing sense is not expected for co-ordination compounds. There is another kind of unsaturation conceivable in co-ordination compounds, co-ordinative unsaturation, in which there are not sufficient ligands present to meet the co-ordination number requirements of the co-ordination centre. In the classification that follows the addition-type polymerizations can be pictured as involving the creation of co-ordinative unsaturation and its satisfaction by polymerization. I*) D. B. SOW~ReYand L. F. AUDaIEX,, J. Chem. Educ. 37, 134 (1960). ~21 F. S. ARIMOTO and A. C. HAVEN, JR., J. Amer. Chem. Soc. 77, 6295 (1955). ~3) A. A. BERLINand N. G. MA'rV~EVA,Russ. Chem. Revs. 29, 119 (1960). 365
366
B.P. BLOCKand G. BARTH-WEIRRENALP
The term "condensation polymerization" will be used in the same sense that it is for organic polymerization, i.e., to designate those reactions in which small molecules are split out between molecules. We propose two classes of inorganic condensation polymerizations: "homocondensations" and "heterocondensations", referring to reactions in which the elimination takes place between like molecules and between different kinds of molecules, respectively. Examples of both these classes have been reported in the literature. JOYNERand KENNEYfound(4) that the compound Ge(C32HIrNs)(OH)2, where C32HI6N~ is the phthalocyanine anion, loses water when heated, presumably forming (--Ge(CseH16Ns)O--)x. This reaction, by definition, is a homocondensation. There do not seem to be any simple examples of heteroeondensation polymerizations leading to co-ordination polymers known, although several reactions which probably should be so classified have been reported. For illustrative purposes a reaction which has been attempted in our Laboratories will be fi~ed. The desired reaction was to take place between [Pt(NH3)zCI2(OH)2] and (C6Hs)2SiCI2 by splitting out HCI to yield (--Pt(NH3)2CI2OSi(C6H6)20--)x. It will be noted that in this reaction the co-ordination sphere about the metallic element would not be disturbed. The second broad class is that of "addition-type polymerizations." This term will be applied to reactions which can be pictured as involving co-ordinatively unsaturated intermediates in the formation of the backbone. Obviously all homocondensations as well as those heterocondensations in which the co-ordination sphere of the metal is disturbed must involve co-ordinatively unsaturated intermediates, but in deference to established usage these classes will be arbitrarily maintained for reactions obviously analogous to organic condensations. There are different additiontype polymerizations conceivable: accordingly, we have made the following subdivision based on different ways of achieving co-ordinative unsaturation; eliminationaddition, substitution-addition, oxidation-addition, redistribution-addition and rearrangement-addition. An elimination-addition polymerization is a reaction in which a polymer is formed from a fully co-ordinated monomer by the loss of neutral ligands. For example, ELVIDGE and LEVER have found(5~ that (CsHsN)Mn(Ca2H16Ns)O another phthalocyanine-containing co-ordination compound, loses pyridine when heated at reduced pressure to give the polymer (--Mn(C32HI6Ns)O--)~. This reaction differs from a homocondensation in that the molecule eliminated from the monomer is not split out from between two units but originates from only one molecule, suggesting that a co-ordinatively unsaturated intermediate, presumably pentaco-ordinate Mn(C32HI6Ns)O in the example, is formed at least transitorily. We will arbitrarily base the distinction between elimination-addition and homocondensation on this difference, i.e., the small molecule is eliminated from a single unit in the former and from between two units in the latter. The difference between the two is schematically shown in Fig. 1. The second addition-type polymerization, substitution-addition, designates reactions in which a catenating group which cannot act as a multidentate ligand replaces either a multidentate ligand or an appropriate combination of unidentate ligands. There are several polymerization reactions in the literature which appear (4) R. D JOYNERand M. E. KENNEY,J. Amer. Chem. Soc. 82, 5790 (1960). (5) j. A. ELVlDOEand A. B. P. LEVER,Proc. Chem. Soc. 195 (1959).
Inorganic co-ordination polyn~'s--I
367
to fall into this classification, but, as with heterocondens~tion, no simple example has been reported. Another reaction that has been investigated in our Laboratories was designed to be of this type and can serve as an illustration of the basic concept. It is the reaction of diphenylphosphinic acid with beryllium acetylacetonate in oneto-one molar ratio in order to displace one acetylacetonate io t'rom each beryllium /'~ f'~ f'~ f'% f'N /'N
Y f~W ¥ . , l .. . ; ,, .. • W . X ,, .W
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• 'M" 'M" "M'~'N'N':' " " " {z!.!}z ". . . . . ' ,;z~Y.Y,Z "-'" "'~"" .y.;z ~zY • " Z " t,
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1,
"Z" IZS
1,
b FIG. l.--Polymerization by (a) elimination-addition and (b) homocondensation. and produce (--Be(C5H?Oz)OP(C6Hs)20--)x, a composition which could correspond to a high molecular weight polymer. Here the replacement of the bidentate acetylacetonate ion with the diphenylphosphinate ion is expected to lead momentarily to a trico-ordinated intermediate which then polymerizes in order to give beryllium(lI) its usual co-ordination number of four, it being assumed that the diphenylphosphinate ion will not act as a chelating agent with beryllium(II). The example just cited is not a heterocondensation because all new bonds are not formed by elimination of molecules: instead, the process is unsymmetrical, forming both co-ordinate covalent and covalent bonds to beryllium. Although there are reactions which are definitely either substitution-additions (Fig. 2a) or heterocondensations (Fig. 2b), some reactions reported in the literature involving his(chelates) can be classified as either, on the basis of the preceding criteria. Thus, the reaction of
A B
A B
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"M'
--
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t,
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b FIG. 2.--Polymerization by (a) substitution-addition and (b) heteroc ondensation.
368
B.P. BLOCK and G. BARTH-WEHRENALP
beryllium acetylacetonate with terephthaloyldiacetone to yield the polymer (> BeCH3COCHCOC6H4COCHCOCH 3 <)x(6) could be classified either as a substitution-addition (replacement of two bidentate ligands by a bis(bidentate) catenating agent) or a heterocondensation (splitting out of acetylacetone). We prefer the first classification. Consequently, our criterion for distinguishing between substitution-addition and heterocondensation polymerization in such cases will be that in the former co-ordinate covalent bonds are formed in the polymer backbone.
Oxidation-addition polymerization (short for oxidation-number-change-addition) is a unique process in which the co-ordination number of an element is increased by a change in oxidation state in an environment such that there are not sufficient ligands present to satisfy the new co-ordination number without sharing of ligands and polymerization. No examples of this type have yet been reported, so a hypothetical system will be used as the example. The reaction of [Pt(NH3)2CI2] with sulphur to yield (--Pt(NH3)2(CI)2S--)x would involve the oxidation of four-coordinate platinum(II) to six-co-ordinate platinum(IV). For the latter to have a co-ordination number of six, platinum atoms must share sulphur or chlorine atoms. Although, in principle, an increase in co-ordination number could be achieved by reduction of the central atom, e.g., by conversion of four-co-ordinate chromium(Vl) to Six-co-ordinate chromium(III), we do not believe that at this time such reactions warrant separate classification as reduction-addition polymerizations. The fourth of the addition-type processes, redistribution-addition polymerization, has not been reported either. The reaction of zinc(II) cyanide with zinc(II) acetylacetonate to form (--Zn(CsH~O2)CN--)x would be an example of this class. Superficially, this process seems to be a low probability method since it is the reverse of disproportionation, which frequently appears to be spontaneous. It should be noted, however, that in many of the formation constant data(T) there is sufficient difference between Kl and K2 to indicate that redistribution-addition polymerizations arc possible. The last addition-type class, which we term "rearrangement-addition polymerization", has recently been used to prepare beryllium(II) bis-fl-diketone polymers. (a) The basic principle of this method is the conversion of a cyclic compound to a linear polymer by opening the ring under such conditions that the radicals recombine into chains. In the work cited macrocyc[ic beryllium chelates with bis-fl-diketones were converted to polymers by heating them above their melting points. Although this seems to be the only example of the use of such a reaction for the preparation of a co-ordination polymer it is, of course, also the kind of reaction involved in the formation of plastic sulphur and elastomeric phosphonitrilic ha[ides. Another classification which we wish to use is that of "degradation polymerization". This category embraces those reactions in which a co-ordinating agent converts a two- or three-dimensional polymeric structure into a less-cross-linked system. We have found no examples of this class in the literature, so we will again cite a reaction which we have investigated. In this case the plan was to displace one half the cyanide from zinc(ll) cyanide, a three-dimensional co-ordination polymer,(9) by acetylacetone (6) j. p. WU.KINSand E. L. WITTm~CKER,U.S. Pat. 2659711 (1953). (7) J. BJERRUM, G. SCHWARZENBACH and L. G. SILLRN, Special Publication No. 6. The Chemical Society, London (1957). (s) R. W. KLUmER and J. W. LEwis, J. Amer. Chem. (Joe. 82, 5777 (1960). (~) G. S. ZHDANOV, Dokl. Akad. Nauk SSSR, 31, 352 (1941).
Inorganic co-ordination polymers--I
369
to give polymeric (--Zn(CsHTO2)CN--)x. Although hydrogen cyanide was liberated, the reaction produced a mixture of Zn(CsHTO2)2 and Zn(CN)2 rather than the desired polymer. Degradation polymerizations are not of the addition-type because, even though co-ordinative unsaturation may be involved in the reaction, it is not essential to the formation of the backbone, which already existed in the original polymer. We have investigated and are investigating many reactions from this point Of view and will report on them in succeeding papers in this series. Part II, which follows, is an investigation of the product formed either by an elimination-addition polymerization or by a redistribution-addition polymerization. Acknowledgeraent--We are greatly indebted to Dr. L. W. Btrrz and our colleaguesat Pennsalt
for valuable discussion of the preceding concepts. Our thanks are due to the Officeof Naval Research for partial support of our studies in the area of co-ordination polymers.
INORGANIC CO-ORDINATION POLYMERSml GENERAL CONSIDERATIONS B. P. BLOCK and G. BARTH-WEHRENALP Research and Development Department, Pennsalt Chemicals Corporation, Wyndmoor, Pa. (Received 14 September 1961 ; in revised form 30 October 1961)
Abstract--Synthetic approaches to co-ordination polymers are classified as condensation polymerizations, addition-type polymerizations involving co-ordinatively-unsaturated intermediates, and degradation polymerizations. The first two classes are further subdivided into homocondensations and heterocondensations and into elimination-additions, substitution-additions, oxidation-additions, redistribution-additions and rearrangementadditions, respectively. These various classes are discussed. IN our Laboratories we are investigating the synthesis of inorganic co-ordination polymers, where we define co-ordination polymers as substances with repeating units containing atoms in the backbone acting as acceptors in co-ordinate covalent bonds. This definition is more restrictive than that used by SOWERBY and AUDRIETH,(1) "polymeric materials containing co-ordinated metal ions", for it excludes polymers which do not contain co-ordination centres in the backbone such as [--CH2CH(CsH4FeCsHs)--]x: 2) We do not include the latter type in our definition, for the co-ordinated portion is not an integral part of the polymer backbone, but rather an adjunct to a substance which is a polymer in its own right. To systematize our synthetic approaches to co-ordination polymers we have formulated a classification scheme which we wish to present in this initial communication in order to provide the framework for succeeding papers in this series, in which we will report the results of our studies. SOWERBY and AUDRIETH(1) and BERLIN and MATVEEVA(3) have also presented such schemes for somewhat different situations. Their treatments were, however, less detailed because of the nature of their reviews. Broadly speaking, organic polymerization reactions are classified as condensation or addition reactions. These two types of reactions involve the elimination of small molecules between units in the first case and the union of unsaturated molecules in the latter. In general, the central element of a co-ordination compound does not form double or triple bonds leading to the chemical behaviour classed as unsaturation in organic chemistry, so that addition polymerization in the foregoing sense is not expected for co-ordination compounds. There is another kind of unsaturation conceivable in co-ordination compounds, co-ordinative unsaturation, in which there are not sufficient ligands present to meet the co-ordination number requirements of the co-ordination centre. In the classification that follows the addition-type polymerizations can be pictured as involving the creation of co-ordinative unsaturation and its satisfaction by polymerization. I*) D. B. SOW~ReYand L. F. AUDaIEX,, J. Chem. Educ. 37, 134 (1960). ~21 F. S. ARIMOTO and A. C. HAVEN, JR., J. Amer. Chem. Soc. 77, 6295 (1955). ~3) A. A. BERLINand N. G. MA'rV~EVA,Russ. Chem. Revs. 29, 119 (1960). 365
366
B.P. BLOCKand G. BARTH-WEIRRENALP
The term "condensation polymerization" will be used in the same sense that it is for organic polymerization, i.e., to designate those reactions in which small molecules are split out between molecules. We propose two classes of inorganic condensation polymerizations: "homocondensations" and "heterocondensations", referring to reactions in which the elimination takes place between like molecules and between different kinds of molecules, respectively. Examples of both these classes have been reported in the literature. JOYNERand KENNEYfound(4) that the compound Ge(C32HIrNs)(OH)2, where C32HI6N~ is the phthalocyanine anion, loses water when heated, presumably forming (--Ge(CseH16Ns)O--)x. This reaction, by definition, is a homocondensation. There do not seem to be any simple examples of heteroeondensation polymerizations leading to co-ordination polymers known, although several reactions which probably should be so classified have been reported. For illustrative purposes a reaction which has been attempted in our Laboratories will be fi~ed. The desired reaction was to take place between [Pt(NH3)zCI2(OH)2] and (C6Hs)2SiCI2 by splitting out HCI to yield (--Pt(NH3)2CI2OSi(C6H6)20--)x. It will be noted that in this reaction the co-ordination sphere about the metallic element would not be disturbed. The second broad class is that of "addition-type polymerizations." This term will be applied to reactions which can be pictured as involving co-ordinatively unsaturated intermediates in the formation of the backbone. Obviously all homocondensations as well as those heterocondensations in which the co-ordination sphere of the metal is disturbed must involve co-ordinatively unsaturated intermediates, but in deference to established usage these classes will be arbitrarily maintained for reactions obviously analogous to organic condensations. There are different additiontype polymerizations conceivable: accordingly, we have made the following subdivision based on different ways of achieving co-ordinative unsaturation; eliminationaddition, substitution-addition, oxidation-addition, redistribution-addition and rearrangement-addition. An elimination-addition polymerization is a reaction in which a polymer is formed from a fully co-ordinated monomer by the loss of neutral ligands. For example, ELVIDGE and LEVER have found(5~ that (CsHsN)Mn(Ca2H16Ns)O another phthalocyanine-containing co-ordination compound, loses pyridine when heated at reduced pressure to give the polymer (--Mn(C32HI6Ns)O--)~. This reaction differs from a homocondensation in that the molecule eliminated from the monomer is not split out from between two units but originates from only one molecule, suggesting that a co-ordinatively unsaturated intermediate, presumably pentaco-ordinate Mn(C32HI6Ns)O in the example, is formed at least transitorily. We will arbitrarily base the distinction between elimination-addition and homocondensation on this difference, i.e., the small molecule is eliminated from a single unit in the former and from between two units in the latter. The difference between the two is schematically shown in Fig. 1. The second addition-type polymerization, substitution-addition, designates reactions in which a catenating group which cannot act as a multidentate ligand replaces either a multidentate ligand or an appropriate combination of unidentate ligands. There are several polymerization reactions in the literature which appear (4) R. D JOYNERand M. E. KENNEY,J. Amer. Chem. Soc. 82, 5790 (1960). (5) j. A. ELVlDOEand A. B. P. LEVER,Proc. Chem. Soc. 195 (1959).
Inorganic co-ordination polyn~'s--I
367
to fall into this classification, but, as with heterocondens~tion, no simple example has been reported. Another reaction that has been investigated in our Laboratories was designed to be of this type and can serve as an illustration of the basic concept. It is the reaction of diphenylphosphinic acid with beryllium acetylacetonate in oneto-one molar ratio in order to displace one acetylacetonate io t'rom each beryllium /'~ f'~ f'~ f'% f'N /'N
Y f~W ¥ . , l .. . ; ,, .. • W . X ,, .W
t
;
X
X
X
f.%
! a
f.%
f.%
f,,%
~
f"%
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AB
A
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• 'M" 'M" "M'~'N'N':' " " " {z!.!}z ". . . . . ' ,;z~Y.Y,Z "-'" "'~"" .y.;z ~zY • " Z " t,
,I,
1,
"Z" IZS
1,
b FIG. l.--Polymerization by (a) elimination-addition and (b) homocondensation. and produce (--Be(C5H?Oz)OP(C6Hs)20--)x, a composition which could correspond to a high molecular weight polymer. Here the replacement of the bidentate acetylacetonate ion with the diphenylphosphinate ion is expected to lead momentarily to a trico-ordinated intermediate which then polymerizes in order to give beryllium(lI) its usual co-ordination number of four, it being assumed that the diphenylphosphinate ion will not act as a chelating agent with beryllium(II). The example just cited is not a heterocondensation because all new bonds are not formed by elimination of molecules: instead, the process is unsymmetrical, forming both co-ordinate covalent and covalent bonds to beryllium. Although there are reactions which are definitely either substitution-additions (Fig. 2a) or heterocondensations (Fig. 2b), some reactions reported in the literature involving his(chelates) can be classified as either, on the basis of the preceding criteria. Thus, the reaction of
A B
A B
"51'
A e
"M'
--
'M'
ABA
~
'M' "M' "t4"
,_e=.A...x;x,' -.e.....i...x, '-' ' x ,.e~X.p.., ' ' .... " x
°-'-"''
I
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.
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.
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,I,
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t,
I,
,t,
t,
b FIG. 2.--Polymerization by (a) substitution-addition and (b) heteroc ondensation.
368
B.P. BLOCK and G. BARTH-WEHRENALP
beryllium acetylacetonate with terephthaloyldiacetone to yield the polymer (> BeCH3COCHCOC6H4COCHCOCH 3 <)x(6) could be classified either as a substitution-addition (replacement of two bidentate ligands by a bis(bidentate) catenating agent) or a heterocondensation (splitting out of acetylacetone). We prefer the first classification. Consequently, our criterion for distinguishing between substitution-addition and heterocondensation polymerization in such cases will be that in the former co-ordinate covalent bonds are formed in the polymer backbone.
Oxidation-addition polymerization (short for oxidation-number-change-addition) is a unique process in which the co-ordination number of an element is increased by a change in oxidation state in an environment such that there are not sufficient ligands present to satisfy the new co-ordination number without sharing of ligands and polymerization. No examples of this type have yet been reported, so a hypothetical system will be used as the example. The reaction of [Pt(NH3)2CI2] with sulphur to yield (--Pt(NH3)2(CI)2S--)x would involve the oxidation of four-coordinate platinum(II) to six-co-ordinate platinum(IV). For the latter to have a co-ordination number of six, platinum atoms must share sulphur or chlorine atoms. Although, in principle, an increase in co-ordination number could be achieved by reduction of the central atom, e.g., by conversion of four-co-ordinate chromium(Vl) to Six-co-ordinate chromium(III), we do not believe that at this time such reactions warrant separate classification as reduction-addition polymerizations. The fourth of the addition-type processes, redistribution-addition polymerization, has not been reported either. The reaction of zinc(II) cyanide with zinc(II) acetylacetonate to form (--Zn(CsH~O2)CN--)x would be an example of this class. Superficially, this process seems to be a low probability method since it is the reverse of disproportionation, which frequently appears to be spontaneous. It should be noted, however, that in many of the formation constant data(T) there is sufficient difference between Kl and K2 to indicate that redistribution-addition polymerizations arc possible. The last addition-type class, which we term "rearrangement-addition polymerization", has recently been used to prepare beryllium(II) bis-fl-diketone polymers. (a) The basic principle of this method is the conversion of a cyclic compound to a linear polymer by opening the ring under such conditions that the radicals recombine into chains. In the work cited macrocyc[ic beryllium chelates with bis-fl-diketones were converted to polymers by heating them above their melting points. Although this seems to be the only example of the use of such a reaction for the preparation of a co-ordination polymer it is, of course, also the kind of reaction involved in the formation of plastic sulphur and elastomeric phosphonitrilic ha[ides. Another classification which we wish to use is that of "degradation polymerization". This category embraces those reactions in which a co-ordinating agent converts a two- or three-dimensional polymeric structure into a less-cross-linked system. We have found no examples of this class in the literature, so we will again cite a reaction which we have investigated. In this case the plan was to displace one half the cyanide from zinc(ll) cyanide, a three-dimensional co-ordination polymer,(9) by acetylacetone (6) j. p. WU.KINSand E. L. WITTm~CKER,U.S. Pat. 2659711 (1953). (7) J. BJERRUM, G. SCHWARZENBACH and L. G. SILLRN, Special Publication No. 6. The Chemical Society, London (1957). (s) R. W. KLUmER and J. W. LEwis, J. Amer. Chem. (Joe. 82, 5777 (1960). (~) G. S. ZHDANOV, Dokl. Akad. Nauk SSSR, 31, 352 (1941).
Inorganic co-ordination polymers--I
369
to give polymeric (--Zn(CsHTO2)CN--)x. Although hydrogen cyanide was liberated, the reaction produced a mixture of Zn(CsHTO2)2 and Zn(CN)2 rather than the desired polymer. Degradation polymerizations are not of the addition-type because, even though co-ordinative unsaturation may be involved in the reaction, it is not essential to the formation of the backbone, which already existed in the original polymer. We have investigated and are investigating many reactions from this point Of view and will report on them in succeeding papers in this series. Part II, which follows, is an investigation of the product formed either by an elimination-addition polymerization or by a redistribution-addition polymerization. Acknowledgeraent--We are greatly indebted to Dr. L. W. Btrrz and our colleaguesat Pennsalt
for valuable discussion of the preceding concepts. Our thanks are due to the Officeof Naval Research for partial support of our studies in the area of co-ordination polymers.