Chapter 4 - Inorganic Polymers.pdf

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1. INORGANIC POLYMERS

2. Polymer
Apolymerisaverylongchainmacromoleculesinwhichhundredsorthousandsofatoms
arelinkedtogethertoformaone-dimensionalarray.
Oligomer:(2-10repeatingunit).
ThewordpolymerisderivedfromtheGreektermpoly,meaningmany,andmeros,which
meanspart.
Polymercontains-
(i)highmolecularweight;
(ii)repeatingunitorstructuralunitormonomer(Monomerisdefinedasasimplemoleculewith
twoormorebindingsitesthroughwhichitformscovalentlinkageswithothermonomer
moleculestoformthemacromolecule);
(iii)InorganicPolymersarenotcombinedwithrepeatingunithavingcarbonatoms.

3. Carbon
Silicon
C is organic (plant-based) Si is inorganic (mineral-based)
C forms covalent bonds Si forms hybrid ionic/covalent bonds
CO2 is a simple gas SiO2 is a complex solid polymer
CCl4 is a reasonably stable fluid
SiCl4 is highly reactive to water and some
organic substances





Supplemental

4. Some important differences between silicon and
carbon
•The silicon atom is larger than the carbon atom. Its bonds
are longer and more flexible with wider bond angles.
•Silicon is less electronegative than carbon (1.8 for silicon vs.
2.5 for carbon); it is able to give up more of its electrons to
form strong, energetic bonds with other elements.
•Silicon only forms single bonds, not multiple bonds (single
bonds are more stable and harder to break than double or
triple bonds).

5. The Si-O bond –the key to silicone’s unique
properties
❖The Si-O bond has higher bond energy than the C-O bond.
❖The Si-O bond is longer and flatter than the
C-O bond.
❖The Si-O bond has a lower barrier to rotation than the C-O
bond and higher free volume.

8. Inorganic polymer
Aninorganicpolymerisapolymerwithaskeletalstructurethatdoesnotincludecarbon
atomsinthebackbone.Polymerscontaininginorganicandorganiccomponentsare
sometimescalledhybridpolymers,andmostso-calledinorganicpolymersarehybrid
polymers.
Oneofthebestknownexamplesispolydimethylsiloxane,otherwiseknowncommonlyassiliconerubber.
Inorganicpolymersoffersomepropertiesnotfoundinorganicmaterialsincluding:
❑low-temperatureflexibility:Theabilityofamembraneorothermaterialtoresistcrackingwhenflexedafterit
hasbeencooledtoalowtemperature.
❑electricalconductivity:Conductivityisthemeasureoftheeaseatwhichanelectricchargeorheatcanpass
throughamaterial
❑Non-flammability:incombustible,noncombustible.notcapableofignitingandburning.
Theterminorganicpolymerrefersgenerallytoone-dimensionalpolymers,ratherthantoheavily
crosslinkedmaterialssuchassilicateminerals.

9. Terminology Introduction

10. Network Polymer
A network polymer networkis a polymer comprising two or more networks
which are at least partially interlaced on a polymer scale but not covalently
bonded to each other.
The network cannot be separated unless chemical bonds are broken.
Structureofcadmiumcyanide
(Cd(CN)
2),highlightingthe
interpenetratedstructure.
Blue=oneCd(CN)
2substructure,
red=otherCd(CN)
2substructure.

12. Homochain polymers have only one kind of atom in the main chain.
One member is polymeric sulfur, which forms reversibly upon melting any of the
cyclic allotropes, such as S
8.
The gray allotrope of selenium
consists of helical chains of Se atoms.
S8 octasulfurmolecule vectorimage

13. Heterochain polymers have more than one type of atom in the main chain.
Typically, two types of atoms alternate along the main chain.
Si-based
Thegreatcommercialinterestarethepolysiloxanes,wherethemainchainfeatures
SiandOcenters:−Si−O−Si−O−.EachSicenterhastwosubstituents,usuallymethyl
orphenyl.Examplesincludepolydimethylsiloxane(PDMS,(Me
2SiO)
n),
polymethylhydrosiloxane(PMHS(MeSi(H)O)
n)andpolydiphenylsiloxane(Ph
2SiO)
n).
Relatedtothesiloxanesarethepolysilazanes.Thesematerialshavethebackbone
formula−Si−N−Si−N−.OneexampleisperhydridopolysilazanePHPS.Suchmaterials
areofacademicinterest.
Heavieranaloguesofpolysilanesarealsoknowntosomeextent.Theseinclude
polygermanes,(R
2Ge)
n,andpolystannanes,(R
2Sn)
n.

14. P, N and S-based
Arelatedfamilyofwellstudiedinorganicpolymersarethepolyphosphazenes.They
featurethebackbone−P−N−P−N−.Withtwosubstituentsonphosphorus,theyare
structurallysimilarrelatedtothepolysiloxanes.Suchmaterialsaregeneratedbyring-
openingpolymerizationofhexachlorophosphazenefollowedbysubstitutionoftheP−Cl
groupsbyalkoxide.Suchmaterialsfindspecializedapplicationsaselastomers.

18. Polymerization methods
Inorganic polymers are formed, like organic polymers, by:
❑Step-growth polymerization: Polysiloxanes;
❑Chain-growth polymerization: Polysilanes;
❑Ring-opening polymerization: Poly(dichlorophosphazene)

20. Step-growth polymerization
Step-growthpolymerizationreferstoatypeofpolymerizationmechanismin
whichbi-functionalormultifunctionalmonomersreacttoformfirstdimers,
thentrimers,longeroligomersandeventuallylongchainpolymers.
Manynaturallyoccurringandsomesyntheticpolymersareproducedbystep-
growthpolymerization,e.g.polyesters,polyamides,polyurethanes,etc.
Theeasiestwaytovisualizethemechanismofastep-growthpolymerizationis
agroupofpeoplereachingouttoholdtheirhandstoformahumanchain—
eachpersonhastwohands(=reactivesites).Therealsoisthepossibilityto
havemorethantworeactivesitesonamonomer:Inthiscasebranched
polymersproductiontakeplace.

21. Step-growth polymerization

22. Chain-growth polymerization
Chain-growthpolymerizationisapolymerizationtechniquewhereunsaturated
monomermoleculesaddontotheactivesiteonagrowingpolymerchainoneata
time.
Therearealimitednumberoftheseactivesitesatanymomentduringthe
polymerizationwhichgivesthismethoditskeycharacteristics.
Manycommonpolymerscanbeobtainedbychainpolymerizationsuchas
polyethylene(PE),
polypropylene(PP),
polyvinylchloride(PVC),
poly(methylmethacrylate)(PMMA),
polyacrylonitrile(PAN),
polyvinylacetate(PVA).

23. Reaction steps
Chain initiation
Chaininitiationistheinitialgenerationofachaincarrier,whichisanintermediatesuch
asaradicaloranionwhichcancontinuethereactionbychainpropagation.Initiation
stepsareclassifiedaccordingtothewaythatenergyisprovided:thermalinitiation,high
energyinitiation,andchemicalinitiationetc.Thermalinitiationusesmolecularthermal
motiontodissociateamoleculeandformactivecenters.Highenergyinitiationrefersto
thegenerationofchaincarriersbyradiation.Chemicalinitiationisduetoachemical
initiator.
Forthecaseofradicalpolymerizationasanexample,chaininitiationinvolvesthe
dissociationofaradicalinitiatormolecule(I)whichiseasilydissociatedbyheatorlight
intotwofreeradicals(2R°).EachradicalR°thenaddsafirstmonomermolecule(M)to
startachainwhichterminateswithamonomeractivatedbythepresenceofanunpaired
electron(RM
1°).

24. Chain propagation
IUPACdefineschainpropagationasareactionofanactivecenteronthegrowing
polymermolecule,whichaddsonemonomermoleculetoformanewpolymer
molecule(RM
1°)onerepeatunitlonger.
Forradicalpolymerization,theactivecenterremainsanatomwithanunpaired
electron.Theadditionofthesecondmonomerandatypicallateradditionstepare
•RM
1°+M→RM
2°
•.............................
•……………… .....
•RM
n°+M→RM
n+1°
Forsomepolymers,chainsofover1000monomerunitscanbeformedin
milliseconds.

25. Chain termination
In a chain termination step, the active center disappears, resulting in the termination
of chain propagation. This is different from chain transfer in which the active center
only shifts to another molecule but does not disappear.
For radical polymerization, termination involves a reaction of two growing polymer
chains to eliminate the unpaired electrons of both chains.
Recombination is the reaction of the unpaired electrons of two chains to form a
covalent bond between them.
The product is a single polymer molecule with the combined length of the two
reactant chains:
•RM
n°+ RM
m°→ P
n+m

26. Chaintransfer
Insomechain-growthpolymerizationsthereisalsoachaintransferstep,inwhichthe
growingpolymerchainRM
n°takesanatomXfromaninactivemoleculeXY,terminating
thegrowthofthepolymerchain:
RM
n°+XY→RM
nX+Y°
TheYfragmentisanewactivecenterwhichaddsmoremonomerMtoformanew
growingchainYM
n°.
ThiscanhappeninfreeradicalpolymerizationforchainsRM
n°,inionicpolymerization
forchainsRM
n
+
orRM
n
–
,orincoordinationpolymerization.
Inmostcaseschaintransferwillgenerateaby-productanddecreasethemolarmassof
thefinalpolymer.

28. Ring-opening polymerization
Ring-openingpolymerization(ROP)isaformofchain-growthpolymerization,inwhich
theterminusofapolymerchainattackscyclicmonomerstoformalongerpolymer(see
figure).
ROPisaversatilemethodforthesynthesisofbiopolymers.
Ring-openingofcyclicmonomersisoftendrivenbythereliefofbond-anglestrain.Thus,
asisthecaseforothertypesofpolymerization,theenthalpychangeinring-openingis
negative.

29. Anionic ring-opening
polymerization (AROP)
Cationic ring-opening
polymerization (CROP)

30. Ring-opening
metathesis
polymerization
Ring-opening
metathesis
polymerization
(ROMP)
produces
unsaturated
polymers from
cycloalkenesor
bicycloalkenes.
It requires
organometallic
catalysts.

31. Welcome to Fascinating Silicone Chemistry
Silicone is not a product, but an entire field of chemistry
Whatissilicone?
Thename“silicone”wasgivenin1901byKipping.Siliconeisa
genericnameforawidevarietyofpolymericchainsand
networksconstructedaroundabackboneofSi-O-Si.
Technicallyknownas“polysiloxanes,”siliconesarethe“mixing
link”betweenorganicandinorganicchemistry.(Si-O-)n
R
R

32. Bond energy –kJ/mole
Bond length –angstroms (Ǻ)
Barrier to rotation –kJ/mole

33. Preparation: From Sand to Silicones
•Silicone polymers. Silicone polymers are obtained by a three-
step synthesis:
1. Chlorosilanesynthesis
2. Chlorosilanehydrolysis
3. Polymerisationand polycondensation

34. 1. Chlorosilanesynthesis.

35. 2. Hydrolysis of chlorosilane
(to produce HCland siloxanediol)
(CH
3)2SiCl
2+H
20------------->HCl+(CH
3)
2Si(OH)
2
Dehydration of siloxanediolto cyclomethiconeand silanols
x(CH
3)
2Si(OH)
2--> H
2O + y HO-(CH
3)
2SiO)
nH + z {(CH
3)
2SiO)m} cylclomethicone
Linear Cyclic
n = 20-50 m = 3, 4, 5 mainly 4
(CH
3)
2Si(OH)
2condenses,withHClactingasacatalyst,togivea
mixtureoflinearorcyclicoligomersbyinter-orintramolecular
condensation.

36. 3. Polymerisationand polycondensation
Thelinearandcyclicoligomersobtainedbyhydrolysisofthe
dimethyldichlorosilanehavetooshortachainformostapplications.
Theymustbecondensed(linears)orpolymerised(cyclics)togive
macromoleculesofsufficientlength.
Cyclics, (R
2SiO)
m, can be opened and polymerisedto form long linear
chains
x (Me
2SiO)
4+ KOH (Me
2SiO)
y+ KO(Me
2SiO)
zH

37. Me
Me
SiMe O
Me
3SiO
1/2 Me
Me
SiOO
Me
2SiO
2/2 O
O
SiMe O
MeSiO
3/2 O
O
SiO O
SiO
4/2 M D
T Q
“M unit” is monosubstituted(one oxygen atom on silicon)
“D unit” is disubstituted(two oxygen atoms on silicon)
“T unit” is trisubstituted(three oxygen atoms on silicon)
“Q unit” is tetrasubstituted(four oxygen atoms on
silicon)
Which one
Is chain
stopping
Unit ?

38. Chlorosilane
Precursor
Product of
Hydrolysis
Silicone
"Shorthand"
Forms, Functions

Trimethylchlorosilane



End-blockers


Dimethyldichlorosilane
The primary building
block used in the
polymerization of
polydimethylsiloxanes
(classic linear silicones)



Linear (straight
chain)




Silicone fluids,
elastomers, and gels

Methyltrichlorosilane


Branched
(crosslinkers)




Tetrachlorosilane


Three-dimensional
resinous networks

Silicone resins

40. Silicone oil
•Asiliconeoilisanyliquidpolymerizedsiloxanewithorganicsidechains.
Theyareformedwithabackboneofalternatingsilicon-oxygenatoms
(...Si-O-Si-O-Si...)–i.e.siloxane–ratherthancarbonatoms.
•AtypicalexampleisHexamethyldisiloxane,wherethreemethylgroups
attachtoeachsiliconatomtoformSi(CH
3)
3OSi(CH
3)
3.Thecarbon
analoguewouldbeanalkyl-alkane,thatis2,2,4,4-tetramethylpentane
C(CH
3)
3CH
2C(CH
3)
3.

41. Applications
•used as lubricantsor hydraulic fluids,
•electrical insulators, freeze-dryersas refrigerants, anti-foaming
agents
•Silicone oil is also one of the two main ingredients in Silly
Putty

42. Silicone grease
•Silicone greaseis a waterproof grease made by combining a
silicone oil with a thickener.
•Most commonly, the silicone oil is polydimethylsiloxane and
the thickener is amorphous fumed silica.

43. Application
•lubricating and preserving rubber parts, such as O-rings
•Silicone grease is widely used as a temporary sealant and a
lubricant for interconnecting ground glass joints, as is typically
used in laboratory glassware
•Silicone-based lubricants are often used by consumers in
applications where other common consumer lubricants, such
as petroleum jelly,
•Used as dielectric grease in electrical connectors,

44. Silicone rubber
•Silicone rubberis an elastomer (rubber-like material)
composed of silicone.
•Itself a polymer—containing silicon together with carbon,
hydrogen, and oxygen.

45. Boron Compounds with Nitrogen Donors
The“B-N”unitisisoelectronic(3+5valenceelectrons)tothe“C-C”unit(4
+4valenceelectrons).Thetwomoietiesarealsoisolobal,andassuch
therearemanyofthecompoundtypesformedbycarbonhaveanalogous
derivativesinthechemistryofboron-nitrogen.
Boroncompounds,BX
3,arestrongLewisacidsandassuchformstable
additioncompoundswithLewisbases,inparticularthosewithnitrogen
donorligands.
BF3+ NMe3 → F3B-NMe3

46. Aminoboranes
R2N-BR
2
isisoelectronicandisolobaltotheolefinsub-unitR’
2
C=CR
2
,
andthereisevenappreciableπ-bondingcharacter.
Ameasureofthemultiplebondcharactercanbeseenfroma
comparisonofthecalculatedB-NbondinH
2
NBN
2
(1.391Å)as
comparedtoatypicalolefin(1.34Å).Itisinterestingthata
considerationofthepossibleresonanceformsuggestthedipoleintheσ-
bondisintheoppositedirectionofthatoftheπ-bond

48. Structure of borazine, the
inorganic analog to benzene.
Probability distributions for the π-bond
orbitals in (a) borazine and (b) benzene.
Structure of h-BN Structure of c-BN

58. Hydrolysis
Borazine hydrolyzes readily, yielding boric acid, ammonia, and hydrogen.
•B(OH)
3
+ H
2
O → B(OH)
3
(OH
2
)
•B(OH)
3
(OH
2
) + H
2
O + HO
−
→ [B(OH)
4
]
−
+ H
3
O
+
Polymerization
Heating borazine at 70°C expels
hydrogen with formation of
polyborazylene:
n B
3
N
3
H
6
→ [B
3
N
3
H
4
]
n

59. With hydrogen halides and halogens
With hydrogen chloride it forms an adduct.
B
3
N
3
H
6
+ 3 HCl → B
3
N
3
H
9
Cl
3
Addition reaction of borazine with hydrogen
chloride
B
3
N
3
H
9
Cl
3
+ NaBH
4
→ (BH
4
N)
3

60. Ceramic precursor
Boron nitride can be prepared by heating polyborazyleneto 1000°C
Borazinesarealsostartingmaterialsforotherpotentialceramicssuchasboron
carbonitrides.Borazinecanalsobeusedasaprecursortogrowhexagonal
boronnitride(h-BN)thinfilmsandsinglelayersoncatalyticsurfacessuchas
copper,platinum,nickel,ironandmanymore,withchemicalvapordeposition
(CVD).

62. Sulphur-nitrogen polymers
Polythiazyl(polymericsulfurnitride),(SN)
x,isanelectricallyconductive,
gold-orbronze-coloredpolymerwithmetallicluster.Itwasthefirst
conductiveinorganicpolymerdiscoveredandwasalsofoundtobea
superconductoratverylowtemperatures(below0.26K).

63. Phosphazenes/Phosphonitrilic
•Phosphazeneswereinitiallytermedphosphonitrilicpolymers.
•Phosphazenesareaclassofchemicalcompoundsinwhichaphosphorus
atomiscovalentlylinkedtoanitrogenatombyadoublebondandto
threeotheratomsorradicalsbysinglebonds.
•Thenewtermwasusedtorepresentphosphorus,nitrogen(=azo)andP=
Ndoublebonds(=ene)whicharealwayspresentinthesepolymers.They
arethus‘unsaturatedPNcompounds’containingphosphorus,mostlyin
+Vstate.Asisusualwithpolymers,theymayhavecyclicorchain
structure.

64. The nitrogen in groups A
isin2-coordinationandphosphorusin4-coordination. -N= P-
R
R -O- Si-
R
R
They contain the
group which is isoelectronic with the
A

65. Poly(dichlorophosphazene),alsocalleddichlorophosphazinepolymeror
phosphonitrilechloridepolymer,isachemicalcompoundwithformula(-
PNCl
2-)
n.Itisaninorganic(hencecarbon-free)polymer,whosebackbone
isachainofalternatingphosphorusandnitrogenatoms,connectedby
alternatingsingleanddoublecovalentbonds.
Thecompoundcanbepreparedbypolymerizationof
hexachlorophosphazene((PNCl
2)
3)byheatingtoca.250°C.Itisan
"inorganicrubber"andthestartingmaterialformanyotherpolymers
withthe-P=N-backbone(polyphosphazenes),whichhaveimportant
commercialuses.

66. Monophosphazenes
•Monophosphazenesare compounds of the
type X3P = NR, (X and R = Cl, OR, NR2, Aretc.)P(C
6H
5)
3 + C
6H
5-N
3
(C
6H
5)
3- P = N - C
6H
5
-N
2
(C
6H
5)
3PCl
2 + C
6H
5–NH
2 (C
6H
5)
3 – P = N – C
6H
5
–2HCl
(a) Reacting an azide (R – N
3) with PR
3 (R = Ar, OR, Cl, NR
2) :
(b) Reacting dihalotriphenylphosphenes with aromatic amines

67. Diphosphazenes
•These contain two P = N –groups and the P = N
and the P –N bonds are equivalent.
•These are usually prepared by reacting PCl
5
with NH
4Cl under mild conditions:
PCl
5+ NH
4Cl [Cl
3P = N –PCl
2= N –PCl
3]
+
Cl
–

68. Polyphosphazenes
These contain more than two P = N –groups. PCl
5 + NH
4Cl (NPCl
2)
3 + 4HCl
120-150
o
C
‘n’ value upto10
4
.

69. ThePhosphazenesarecyclicorchaincompoundsthatcontain
alternatingphosphorusandnitrogenatomswithtwosubstituentson
eachphosphorusatom.
Thethreemainstructuraltypesarecyclictrimer,cyclictetramerandthe
oligomerorhighpolymer.
Afewcyclicpentamersandhexamersarealsoknown.Thealternating
singleanddoublebondsinthefiguresarewrittenforconveniencebut
shouldnotbetakenliterally.
Hexachlorocyclotriphosphazene(NPCl2)3,isakeyintermediateinthe
synthesisofmanyotherphosphazenes.

71. The chlorinesin the product may be substituted by nucleophilesand
varieties of substituted polyphosphazenesare obtained.

73. Nature of Bonding in Triphosphazenes
•DiphosphazenecontainsoneP=NandoneP–Nbondsbothofwhichare
equivalentandthebondlengthisshorterthansinglebond.Thisiseasily
explainedbyresonance.
•In case of trimerand higher phosphazeneshowever, things are complex.
•Atfirstsight,thereseemstobealternatesingleanddoublebondconjugate
systemsthroughoutP–N–Pskeleton.PNP

74. Suchconjugatesystemsareexpectedtoleadto
delocalizationofthesortofbenzene.
Thecyclictrimerhasanaccuratelyplanar6-membered
ringstructurebut,unlikearomaticsystems,itisdifficult
toreduceitelectrochemically.
Also,thespectralevidenceofdelocalizedπsystem
(bathochromicultravioletshift)isnotfound.Thisis
explainedbytheuniqueandinterestingstructurethat
triphosphazenespossess.

75. Asinotherphosphazenes,triphosphazeneshave
(i)Verystablering
(ii)Alltheskeletalinter-atomicdistancesareequalifallthesix
substituentgroupsonphosphorusareidentical.
(iii)AshorterP–Nbondlength(~158pm)thansinglebond(~177
pm)
(iv)Allthenitrogenatomsweaklybasichenceprotonable(moreso
ifsubstituentsonPareelectronreleasing).

76. The symmetry of the molecule is like that of
benzene.
Ithasplanarstructurewiththreenitrogenandthreephosphorus
atomssituatedonalternatecornersofthehexagon.Thesubstituents
(X)onPareplacedaboveandbelowtheplane.Ifallthesubstituents
areidenticalthenthreeofthemformtogetheroneplaneaboveand
threeformoneplanebelowthemolecularplane.

77. Some more features are mentioned below:
• Nitrogen atoms are sp2 hybridized:
Two such hybrid orbitals are involved in σ -bonding
• Similarly, each phosphorus is sp3 hybridized. Three such hybrid
orbitals are involved in σ-bonding.

78. •Thereisthenone‘inplane’π-bondinginvolvingthelone
pairofN(sp2orbital)inxyplaneandthevacantdxyordx2–
y2orbitaloftheP—atom.
•Therearetwo‘outofplane’interactions:
(i)Heteromorphicinteractions:Thesinglyoccupiedpz1
orbitalonthenitrogenoverlapswiththedxz
o
ordyz
Oorbital
ofphosphorus(pπ–dπbonding).
(ii)Homomorphicinteractions:Thereisapπ–pπinteraction
ofpzorbitalsoftwonitrogensthroughthedyzorbitalofthe
phosphorusinbetween.