Inorganic Polymers

Inorganic Polymers

Polymers are gaint molecules or macromolecules. Generally monomers and polymers have the same emperical formula but the molecular weight of the polymers is usually whole number multiple of that of monomer. When two, three and four monomers unite, the polymer so formed is called dimer, trimer and tetramer and the process of their formations as dimerization, trimerization and tetramerization respectively.
All the covalent macromolecules, which do not have carbon in their back bones, are considered to be inorganic polymers. Covalently-bonded crystals (e.g. oxides and halides), condensed phosphates etc., are the examples of inorganic polymers. These polymers possess distinctive physico-chemical characteristics and unique physical, mechanical and electrical properties. These polymers are of extensive utility in everyday life, particularly in the area of engineering and technology.

The important point of similarity between inorganic and organic polymers is, that both can be prepared by the addition and condensation methods. The former method is used when polymers of higher molecular weight and greater mechanical strength are needed.

General Properties of Inorganic Polymers

Followings are the properties of inorganic polymers-
1. Most of the inorganic polymers do not burn, but only soften or melt at high temperatures. Inorganic polymers, containing sulphur etc. are exceptions.
2. Inorganic polymers, having cross-linked structures with many covalent bonds, are generally stiffer and harder than the organic polymers.
3. Since most of the inorganic polymers are built up of highly polar repeat units, these polymers dissolve only in polar solvents. These polymers react with solvent molecules.
4. Inorganic polymers are generally less ductile than the organic polymers.
5. Inorganic polymers have structures which are purely crystalline or purely amorphous while organic polymers have structures which are partly crystalline and parlty.
6. Inorganic polymers are generally stronger, harder and more brittle than the organic polymers.

Classification of Inorganic Polymers

There are different ways of classifying inorganic polymers-
1st classification
Depending on whether the polymer contains the atoms of only one element or of different elements in its backbone, the polymers are classified into the following two groups.
1. Homo-atomic polymers
These polymers contain the atoms of only one element in their back bones. Silicon, phosphorus, sulphur, germanium and tin form homo-atomic inorganic polymers. For example, sulphur has a tendency to form chain or rings in its elemental form (S₈) and in several compounds, like persulphides (H-S-S-H, H-S-S-S-H, H-S-S-S-S-H etc.), polythionic acids etc. Single bond strengths in homo-atomic inorganic polymers are 54 to 60 Kcal for sulphur, about 53 Kcal for silicon, 48-53 for phosphorus, about 45 for germanium and about 39 for tin.
2. Hetero-atomic polymers
These contain the atoms of different elements in their backbones.
2nd classification
Inorganic polymers can also be classified in another way, which is based the type of reaction by which the polymers are formed. On this basis, inorganic polymers may be of the following types-
1. Condensation polymers
Condensation polymers are those, which are formed by condensation process. In this process, two or more simple molecules of the same substance polymerise together and form the condensation polymer. One or more H₂O, NH₃, H₂, HCl etc. molecules are also eliminated.
Examples-
a. Cross-linked silicone is obtained by the polymerisation of many RSi(OH)₃ molecules. Similarly, when many molecules of R₂Si(OH)₂ undergo polymerisation, a straight chain (linear) or cyclic (ring) silicone is obtained. When two molecules of R₃Si(OH) undergo polymerisation, a straight chain silicone (dimer) is obtained.
b. When PCI₅, is partially hydrolysed by water, dichloro phosphoric acid, PO(OH)CI₂ is obtained. When PO(OH)CI₂ is heated, many molecules of this substance get polymerised and give to the formation of a condensation polymer. In this process HCl is eliminated.

2. Addition polymers
These polymers are obtained, when many simple molecules (monomers) combine together.
Examples-
a. Many molecules of sulphur trioxide may be polymerised by the addition of a small amount of water. This gives addition polymer. b. When SO₂ reacts with propylene, CH₃-CH = CH₂, in presence of benzoyl peroxide, an addition polymer is obtained.
3. Coordination polymers
These are formed by the addition of saturated molecules to each other or by combining a ligand with a metal atom. These polymers contain chelated metal atoms or ions.
Types of coordination polymers
a. In these polymers, the chelated metal atom (ion) is an integral part of the polymer framework.
b. In these polymers the metal atom (ion) remains bound to a polymeric ligand, which has donor groups in its framework.
3rd classification
According to this classification, the inorganic polymers can be classified into the following catagories-
1. Polymers containing two bridging bonds per units, e.g., homo-atomic sulphur, selenium and teturium polymers.
2. The alternating silicone-oxygen polymers. Examoles are Silicones and related compounds.
3. The alternating phosphorus-nitrogen polymers. Examples are phosphonitrilic halides, (NPX₂)_n.
4. The alternating phosphorus-oxygen polymers. Examples are metaphosphates polyphosphates and cross-linked phosphates.
5. The alternating sulphur-nitrogen polymers. Examples are-
a. Polymeric nitrides of sulphur (e.g S₂N₂, S₄N₄, S₅N₂, etc.)
b. Thiazyl halides, [e.g., (NSF)₃, (NSF)₄,(NSCI)₃]
c. Imides sulphur (e.g., S₇(NH), S₆ (NH)₂, S₅(NH)₃, and S₄(NH)₄].

4th classification
This classification is based on the element which forms inorganic polymers. Thus, we have
1. Polymers containing boron
Examples are
a. Borazine, (BH)₃(NH)₃ or B₃N₃H₆.
b. Substituted borazines like-
i. B-trimethyl borazine, [B(CH₃)₃ (NH)₃]
ii. Boroxine, (BH)₃O₃
iii. N-trimethyl borazine, (BH)₃ [N(CH₃)]₃
c. Boron nitride, (BN)_n.
2. Polymers containing silicon
These are called silicones. 3. Polymers containing phosphorus
Examples are-
a. Metaphosphates
b. Polyphosphates
c. Cross-linked phosphates
d. Phosphonitrilic halides, [PNX₂]_n.
4. Polymeric compounds of sulphur
Examples are nitrides of sulphur, thiazyl halides imides of sulphur.

Silicone Based Polymers

Silicone polymer is also called polysiloxane and is a type of inorganic polymer that is mainly made up of polymerized siloxane (−R₂Si−O−SiR₂−). Silicone polymer has a silicon-oxygen backbone instead of carbon as the backbone structure.

Preparation of Silicone Polymers

Silicone Polymers are prepared by the hydrolysis of alkyl or aryl derivatives of SiCl₄ like RSiCl₃, R₂SiCl₂ and R₃SiCl and polymerisation of alkyl or aryl hydroxy-derivatives obtained by hydrolysis. When many molecules of dialkyl dihydroxy-silane, R₂Si(OH)₂ undergo polymerisation, a straight chain (linear) or cyclic (ring) silicone is obtained.
Since an active OH group is present at each end of the chain, polymerisation continues on both the ends and hence the length of the chain increases and gives rise to the formation of long chain silicone as shown below-

R₂Si(OH)₂ can also undergo cyclic polymerization as shown below-

When many molecules of alkyl trihydroxy-silane, RSi(OH)₃ undergoes polymerisation, a cross-linked two dimensional silicone is obtained.

Types of silicone polymer (Polysiloxane)

Depending on the crosslinking density, polysiloxanes can be categorized into three types-
1. Silicone fluids
2. Silicone Elastomers
3. Silicone Resins
1. Silicone fluids
These are low molecular weight polymers produced by the hydrolysis of chlorosilanes with agitation. Silicone fluids are typically straight chains of poly(dimethylsiloxane), with the repeating structure. In many cases, the cyclic tetramer predominates in the resulting mixture. They usually have trimethylsilyl groups, Si(CH₃)₃ at each end of the chain.
Silicones with short chains are fluids which, compared to hydrocarbons, have a more or less constant viscosity over a wide temperature range (200 to 450 K). They also have very low vapour pressures.
2. Silicone Elastomers
Silicone elastomers are high molecular weight linear polymer usually polydimethylsiloxanes. This type of silicones can be prepared-
i. By free radical (benzoyl peroxide as initiators) crosslinking through the formation of ethylenic bridge between chains
ii. By crosslinking of vinyl or allyl group attached to silicon through the reaction of silyl hydride groups.
Silicone elastomers are outstanding materials in low temperature flexibility (upto - 80⁰C) and stability at high temperature (upto 200⁰C). It has resistance to weathering and also to lubricating oils.
3. Silicone Resins
Silicone resins contains Si-atoms with no or only one organic substituents, hence it crosslinked to a harder and stiffer compounds than the elastomers. It is prepared form the desired chlorosilane blend in the presence of solvent (mineral spirit, butyl acetate, toluene or xylene). These materials are usually treated with metal soaps or amines.

Properties of Silicone Polymers

1. Silicones are typically colorless, oils, or rubber like substances.
2. It's structure may be linear, cyclic or crosslinked.
3. It has low thermal conductivity, chemical reactivity, and toxicity.
4. It has very high thermal stability and are also called high-temperature polymers.
5. It has low surface tensions and are capable of wetting most surfaces.
6. It does not support microbiological growth and can repel water.
7. It has high gas permeability and good electrical insulation properties.
8. They are chemically inert and are resistant to water and oxidation.

Applications of Silicone Polymer

1. They are used in the manufacturing of sealants, surfactants, pressure-sensitive tapes, anti-foam agents, silicone rubber molds, etc.
2. They are used in medicinal and cosmetic implants due to low toxicity.
3. They are used as lubricants in both high and low temperatures.
4. They are used as grease, varnishes and these can be used even at low temperatures.

Boron Based Polymers

Boron-based polymers constitute an important class of inorganic polymers. Amongst these the most important are polycarboranes and polymeric boron nitride.

1. Polycarboranes

These are mainly linear polymers in which meta or para carboranes are linked through a variety of hetero atom bridges such as P-O-P.
When acetylene is added to decacarborane, B₁₀H₁₀, it forms a closed icosahedral molecule C₂B₁₀H₁₂ with a diameter of about 40Å in which the two carbon atoms are adjacent to each other. This product is named ortho (or 1,2)-di carbaciosododecacarborane.
B₁₀H₁₀ + C₂H₂ → C₂B₁₀H₁₂
When the ortho product is heated to 475°C, it undergoes an intramolecular rearrangement to give the isomeric meta carborane in which the two carbon atoms are separated by one boron atom. On further heating above 630°C. the material isomerises to para carborane in which the carbon atoms lie at opposite vertices of the cage. The skeletal structures of ortho, meta and para carboranes are shown below-

In these compounds, the H atoms, attached to carbon atoms can be easily replaced by Li.
C₂B₁₀H₁₂ + 2Li → C₂B₁₀H₁₀Li₂

Polymeric Boron Nitride (BN)_n

The polymeric boron nitride (BN)_n exist in two forms, i.e. as a layer polymer and as a three dimentional network polymer.
A. The Layer Polymer
This form resembles graphite and was first discovered by Balmain in 1842. It can be obtained by a variety of methods. A common method involves heating of boric acid with urea in an atomsphere of nitrogen at 600° C followed by the treatment of the product with ammonia at 950° C. The final product is an amorphous powder.
The structure of boron nitride resembles that of graphite. It thus consists of infinite sheets of six-membered rings of alternating boron and nitrogen atoms stacked in layers.

There is, however, an important difference between the structures of polymeric boron nitride and graphite. In boron nitride, the sheets are stacked over one another in such a manner that the atoms in each successive layer are directly superimposed whereas in graphite, the atoms in one layer lie opposite to the centres of the six-membered rings in the crystallographic unit cell constants of boron nitride are very similar to those of graphite, the B-N distance in (BN)_n, being 1-45A compared to C-C distance of 1.42A in graphite. The interlayer separation in (BN)_n is 3-3Å compared to 3-5Å in graphite.
There is, however, a distinct difference between the two polymers. Unlike graphite, the polymeric boron nitride is an electrical insulator. This is because the nitrogen atom in one layer of (BN)_n is always adjacent to a boron atom so that the lone pair of electrons on each N atom in a layer is bound to the electron-deficient B atom of the adjacent layer. In other words, the electrons in boron nitride are highly localised making the polymer non-conducting.
Boron nitride is soft like graphite and can be easily machined. The articles fabricated from this polymer, either by sintering or by machining, can be safely exposed to atmosphere upto a temperature of about 800°C. The melting point of polymeric boron nitride (under an atmosphere of nitrogen) is about 3000°C.
B. The Three-Dimensional Network Polymer
If the layer form of polymeric (BN)_n is heated at 1500-2000°C under extremely high pressure in the presence of a catalyst, it gets converted into a three-dimensional netwok polymeric form. This form is isomorphic with diamond and is comparable with the latter in hardness. It shows signs of surface oxidation in air around 2000°C. It is, therefore, used as a substitute for diamond in jewellery as also for the fabrication of cutting tools.

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