UNIT 16 POLYMERS
Syllabus
· Classification of polymers
· General methods of Polymerisation
· Molecular mass of polymers
· Biopolymers
· Some Commercially Important polymers
POLYMERS
A polymer is a high molecular mass compound ranging from 5000 to one million and they are formed by the combination of a large number of one or more low molecular weight compounds. The unit substance or substances from which the polymer is obtained is called a monomer. The process by which the polymers are formed is called polymerisation. For example, polythene is obtained from its monomer ethylene as a result of polymerisation. The repeat unit is derived from the monomer, ethylene.
In certain cases, the repeat units are derived from two different monomers (e.g. nylon- 66, carbohydrates, proteins). Thus polymers obtained from only one type of monomers are called homopolymers ( e.g. polyethene) while those obtained from two or more different types of monomers are called copolymers ( e.g., nylon, bakelite). Polymers are obtained from both organic and inorganic molecules. Some inorganic polymers are metaphosphoric acid (HPO3)n, silicates and silicones.
Classification of polymers
Classification based on source of availability
1. Naturally occurring polymers
These occur in plants and animals and are very essential for life. These include starch cellulose, proteins, nucleic acids and natural rubber. Starch is a polymer of glucose, cellulose is also a polymer of glucose, proteins are polymers of amino acids. Natural rubber consists of repeat units of isoprene (2-Methyl-1,3-Butadiene).
2. Semi synthetic polymers
These are mostly derived from naturally occurring polymers by chemical modifications. Cellulose on acetylation with acetic anhydride in presence of sulphuric acid forms cellulose diacetate used in making threads and materials like films, glasses etc. Vulcanised rubber has much improved properties and is used in making tyres etc. Gun-cotton , which is cellulose nitrate, is used in making explosives.
3. Synthetic polymers
The polymers which are prepared in the laboratory are called synthetic polymers. These are also called man made polymers . These include fibres, plastics and synthetic rubbers and
find diverse uses as clothing, electric fittings, eye lenses, substitute for wood and metals.
Classification based on Mode of polymerisation
1. Homopolymers and Copolymers
Polymers when made by polymerisation of a single monomeric chemical species are known as homopolymers. Polythene formed from ethylene is a homopolymer.
When the polymers are synthesised by polymerisation of two or more than two different monomers, they are called copolymers. When styrene and butadiene are polymerised, it gives the copolymer called styrene-butadiene rubber.
2. Addition and condensation polymers
(a) Addition polymers
A polymer formed by the direct repeated addition of monomer is called addition polymerisation. In this type of polymers, the monomers are unsaturated compounds and are generally derivatives of ethene. For example , the addition polymers polyethene and polypropylene are obtained as :
(b) Condensation polymers
Condensation polymerisation involves a series of condensation reactions generally involving two different monomers. Each monomer normally contains two functional groups. During these reactions, there is a loss of small molecules usually water. Some of the condensation polymers are ; nylon, terelene, alkyd resins, bakelite. The formation of nylon-66 is shown.
Recent method of classification
In many polymerisation processes, it is difficult to find out whether the polymerisation has occurred through addition or condensation. Therefore the polymers has been classified according to the mode of addition of monomers. The polymerisation has been divided into two types.
(i) Chain growth polymerisation.
(ii) Step growth polymerisation.
i. Chain growth polymerisation
Chain growth polymerisation involves a chain reaction. Therefore it requires an initiator like organic peroxide to form free radical. These free radicals are added to another monomer forming another free radical. The process results in the formation of chain and ultimately a polymer is formed. This is known as chain growth polymerisation. The initiators are added in small quantities. Examples :
Monomer | Polymer |
ethylene | polyethylene |
butadiene | polybutadiene |
tetrafluoroethylene | teflon (PTFE) |
Vinyl chloride | poyvinyl chloride (PVC) |
ii) Step growth polymerisation
This type of polymerisation is due to the condensation process which takes place in several steps. The condensation process may or may not be accompanied by elimination of smaller molecules such as water etc. Examples,
Monomer | polymer |
Adipic acid and hexamethylene diamine | Nylon 66 |
Terephthalic acid and ethylene glycol | Terelene or dacron |
Phenol and formaldehyde | Bakelite |
Classification based on structure
On the basis of structure of polymers, these can be classified as :
(i) Linear chain polymers
(ii) Branched chain polymers
(iii) Crossed linked polymers.
(i) Linear chain polymers
These are polymers in which monomeric units are linked to form linear chains. These linear polymers are well packed and therefore have high densities, high tensile strength and high melting points. For example, polythene, nylon and polyesters are linear chain polymers.
(a) Linear chain polymer
(ii) Branched chain polymers
These are polymers in which monomers are joined to form long chains with branches of different lengths. Fig (b) These branched chain polymers irregularly packed and therefore, they have low tensile strength and melting points than linear polymers. For example, low density polyethene , glycogens etc.
Branched chain polymer
(iii) Cross-linked polymers
These are polymers in which monomer units are cross- linked together to form a three-dimensional net work. These polymers are hard, rigid and brittle because of network structure. These are shown in Fig (c) . For example melamine formaldehyde resin bakelite etc.
Fig c. Crosssed linked polymer
Classification based on molecular forces
The intermolecular forces present in polymers are van der waal’s forces and hydrogen bonding. Although these are weak forces but in macromolecules (polymers) , these forces have accumulative effect all along the chain of polymers. Polymers are thus classified into four categories on the basis of magnitude of these intermolecular forces in them.
(i) Elastomers
In these polymers, the chains are held together by weakest intermolecular forces which permit the polymers to be stretched. A few ‘cross-links’ are introduced between the chains which help the polymers to regain its original position, when the force is released. e.g., vulcanised rubber.
(ii) Fibres
These polymers possess high tensile strength and are used in making fibres. This is due to the strong inter nuclear forces like hydrogen bonding which operate, for example in nylon-66 (a polyamide) These forces also lead to close packing of chains and thus give crystalline nature. Because of that, these polymers show sharp melting points.
(iii) Thermoplastics
These are polymers in which the intermolecular forces of attraction are mid-way between those of elastomers and fibres. Due to that, these can be easily moulded by heating. In thermoplastic polymers, there exists no cross-linking between chains. Examples of these polymers are polyethylene, polystyrene etc.
(iv) Thermosetting polymers
These are prepared from low molecular mass semifluid substances. When heated in a mould, they get highly cross-linked to form hard infusible and insoluble products. The common example is bakelite.
General Methods of Polymerisation
Two major methods generally used for preparing polymers are – addition and condensation polymerisation.
Addition Polymerisation
When the molecules of the same monomer or different monomers simply add together to form a polymer, this process is called addition polymerisation. The monomers used here are unsaturated compounds such as alkenes, alkadienes and their derivatives. This mode of polymerisation can take place through formation of either radicals or ionic species such as carbanions and carbocations. The process is also chain growth polymerisation because it takes place through stages leading to increase of chain length and each stage produces reactive intermediate for use in the next stage of the growth of the chain. Such free radical and ionic addition polymerisations are discussed below.
Free radical Addition polymerisation
A variety of unsaturated compounds, alkenes or dienes and their derivatives are polymerised by this process. Polymerisation of ethylene takes place through radicals, which are generated by an initiator. These initiators are molecules , which decompose to provide radicals easily. t-Butyl peroxide is a commonly used initiator because it decomposes under mild conditions to form t-butoxide radical.
Here the reactive intermediate is a free radical that adds to a monomer molecule to form a new free radical of larger size. Such repeated addition is depicted below to form the polymers.
a) Vinyl Polymerisations
Most of the commercial addition polymers are vinyl polymers obtained from alkenes and there derivatives CH2=CH-G.
This type of polymerisation is performed by heating the monomer with only a small amount of the initiator or by exposing the monomer to light. The process of polymerisation starts with the addition of the radical formed from the initiator to the alkene double bond generating a new radical and these steps are called chain initiating steps. As this radical reacts with alkene, another bigger sized radical is generated. The repetition of this sequence with new and bigger radicals propagates the reaction and these steps are termed as chain propagating steps. Ultimately, at some stage the product radical thus formed reacts with another radical to form polymeric product. This step is called chain-terminating step. This general mode of radical polymerisation of vinyl monomers is depicted below :
Chain initiation step
Chain propagating step
For termination chain , these free radicals can combine in different ways to form the polymer. One mode of termination of chain is shown as under :
Chain terminating step
In vinyl polymerisations, various other reactions of free radicals with some other compounds present may compete with the parent addition chain reactions. One such reaction takes place with molecules that can react with the growing chain to interrupt the further growth of the original chain. But again the product of such a reaction may initiate its own chain growth. This leads to the lowering of the average molecular mass of the polymer. Such reagents are called chain transfer reagents and include carbon tetrachloride , carbon tetrabromide etc. For example, in the presence of carbon tetrachloride, styrene polymerises to form polystyrene of lower average molecular mass which also contains some chlorine. What happens here is that growing polystyrene radical which normally would add on a monomer reacts with chlorine transfer reagent to end the original chain and produces a new radical. The latter initiates a new polymerisation chain and thereby forms a new polymer as depicted below.
If the chain transfer agent forms a radical, which is highly unreactive, the reaction chain gets terminated. Such a compound thus inhibits polymerisation. Many amines, phenols, quinones act as inhibitors. So even traces of certain impurities , which can act as chain transfer agent or inhibitor can interfere with the original polymerisation chain reaction. Hence, monomers should be free from inhibitors.
Vinylic polymerisation is exemplified by the formation of polythene.
Polyethylene or Polythene Formation
It is obtained by polymerisation of ethylene under high pressure of 1000 to 2000 atmospheres at a temperature of 350 to 570 K in presence of trace of oxygen or a peroxide which initiates the polymerisation. The polymer obtained by this procedure of free radical addition and H-atom abstraction has highly branched structure and is called low density polyethene. Its mode of formation is depicted below :
Low density polyethene is chemically inert, tough but flexible and poor conductor of electricity. Hence it is used in the insulation of electric wires, manufacturing of squeeze bottles, toys and flexible pipes.
When polymerisation of ethylene is carried out in the presence of a catalyst at 330 to 350 K at atmospheric pressure, the polymer obtained has a linear structure and is called high density polythene. It is also chemically inert but relatively tough and hard with high tensile strength and is used in the manufacture of containers, house wares , bottles, pipes etc.
b) Conjugated diene Polymerisation
1,3-Butadiene, a conjugated diene can be polymerised like a simple alkene but there are two modes by which this process can take place.
i. 1,4-Polymerisation
When the polymerisation takes at C1 and C4 of butadiene, an unbrached polymer is formed. This product is different from that formed from alkene in having a double bond which at each of its carbons is substituted by different groups and hence can exist either as trans-polybutadiene or cis-polybutadiene or a mixture as shown below :
ii) 1,2-Polymerisation
1,3-Butadiene can undergo polymerisation at C1 and C2 to yield the polymeric product , polyvinyl polythene.
The double bonds in these initial polymers can be linked by further treatment with chemicals to modify the properties of the polymers. These reactions form the basis of formation of rubbers.
Ionic addition polymerisation
Vinylic monomers can undergo addition polymerisation through the formation of ionic intermediates instead of the free radicals. Here initiator instead of a free radical source would be an ion source. The general modes of both these ionic polymerisations are discussed below:
a. Cationic Addition Polymerisation
When the initiator is cationic in nature, on the addition to the double bond, it would generate a cationic intermediate for propagating the addition chain process and is termed as cationic addition polymerisation. The process is initiated by an acid. In the chain initiation step, the acid adds on the double bond to form a cation. On further addition to the double bond, bigger cation is formed and sequence of steps propagates the chain to the polymeric cation. The chain termination can take place by loss of the proton. The stages of polymerisation are depicted below :
Chain initiation step
Chain propagating step
Chain termination
The chain reaction may be terminated by combination of carbocation with negative ion or by loss of a proton.
The complete reaction may be written as :
Cationic polymerisation is facilitated in monomers containing electron releasing groups. Thus, isobutylene undergoes cationic polymerisation easily as it has two electron releasing –CH3 groups that will stabilise the intermediate cation.
b. Anionic Addition Polymerisation
An anionic initiator will similarly generate carbanionic intermediate and the resulting polymerisation is categorised as anionic addition polymerisation. Here active centre of the propagating species is negatively charged. Hence it occurs easily with monomers containing electron-withdrawing groups such as phenyl, nitrile etc., which is able to stabilize the propagating species. Initiation can be brought about by reagents such as n-butyl lithium or potassium amide. In the initiation step, the base adds to double bond to form a carbanion. In the chain propagation, this carbanion adds to the double bond and the process goes on getting repeated to form a polymeric carbanion. The chain reaction can be terminated by addition of an acid. The formation of polystyrene from styrene in the presence of potassium amide is an important example of this category of polymerisation. The mode of anionic polymerisation is depicted below :
Chain initiation step
Chain propagating step
Chain terminating step
Copolymerisation
If a mixture of more than one monomeric species is allowed to polymerise, a copolymer is formed and it contains multiple units of each monomer used in the same polymeric chain . For example, a mixture of styrene and methyl methacrylate can form a copolymer.
Generally, the composition of polymer depends not only on the proportion of the monomers but also on their reactivity. Some monomers as such do not polymerize at all but copolymerize. Maleic anhydride does not polymerize as such but copolymerises with styrene in a highly unsymmetrical manner to form styrene maleic anhydride copolymer.
Copolymers have properties quite different from homopolymers. Polystyrene, a homopolymer obtained from styrene is an electrical insulator and is moulded into toys , combs, radio and television parts. When styrene is copolymerized with butadiene in 1 : 3 ratio, a copolymer of styrene-butadiene rubber (SBR) is obtained. It is very tough and is a good substitute for natural rubber. It possesses high abrasion resistance, high load bearing capacity and is used for the manufacture of auto tyres. Its other uses include floor tiles, footware components, cable insulations etc.
Natural Rubber
It is a natural polymer and possesses remarkable elasticity. It undergoes long range reversible extension under relatively small applied force. This elasticity makes it valuable for a variety of uses. It is manufactured from rubber latex which is a colloidal suspension of rubber in water and is obtained by making incisions in the bark of rubber trees found in tropical and semitropical countries such as India (southern part), Indonesia , Malayasia , Ceylon
Structure
From the structural view-point, natural rubber may be considered as a linear 1,4-polymer of isoprene. In this polymer, the residual double bonds are located between C2 and C3 of isoprene units in the polymer. All these double bonds have cis-configurations and thus rubber is cis-1,4-po;yisoprene.
In the structure of rubber, there are no polar substituents. Hence, intermolecular attractions are largely limited to van der Waal’s interactions. These interactions are again weak because all cis configurations about double bonds do not let the polymer chains come close enough for effective attraction. Thus, the cis-polyisoprene molecule is not a straight chain but has a coiled structure. Consequently, it can be stretched like a spring. On stretching , the molecules become partially aligned with respect to each other and on releasing the force , the chain reverts back to its original coiled state.
Vulcanization of Rubber
Rubber as such is used in the temperature range 283 to 335 K , and it becomes soft and at higher temperatures and brittle at low temperatures. It has large water absorption capacity , non-resistant to nonpolar solvents and is attacked by oxidising agents. Accidently, in 1893, Charles Goodyear discovered that addition of sulphur to hot rubber causes changes that improve its physical properties in a spectacular manner. This process is called vulcanisation. It was origibnally performed by heating a mixture of raw rubber and sulphur at 373 to 415 K. This process is slow and additives such as zinc oxide etc are added to accelerate the rate of vulcanization. The vulcanised rubber has excellent elasticity, low water – absorption tendency, resistance to oxidation and organic solvents. The double bonds in the rubber molecules, in addition to determining their configurations act as reactive sites. The allylic –CH2-, alpha to double bond is also very reactive. On vulcanisation sulphur establishes cross-links at these reactive sites. Thus, the rubber gets stiffened and intermolecular movement of rubber springs is prevented resulting in change of physical character of rubber. The extent of stiffness of vulcanized material depends upon the amount of sulphur added. Thus, about 5% sulphur is used for making tyre rubber and 30% of it for making battery case rubber. The detailed mode of vulcanization process may be difficult to visualize, but probable structures of vulcanized rubber molecules are depicted below.
Synthetic Rubbers
To improve the qualities of natural rubber and to meet the ever increasing demands of mankind, a number of forms of synthetic rubber have been prepared. Some important forms of synthetic rubber are : cis-polybutadiene, Buna-S , Buna-N and neoprene.
Condensation Polymerisation
This type of polymerisation occurs through a series of independent reactions. Each such reaction involves the condensation ( bond formation) between two bifunctional monomer molecules to produce dimers which in turn , produce tetramers and so on with loss of simple molecules like H2O, NH3, HCl etc. These molecules of moderate size then combine together to form the polymer. Since in this process, the polymer is formed in a stepwise manner, it is called step-growth polymer and the process is called step-growth polymerization.
Some typical examples are :
Monomers | Polymer |
(i) Hexamethylenediamine and adipic acid (ii) Phenol and formaldehyde (iii) Terephthalic acid or its methyl ester and ethylene glycol. | Nylon – 66 Bakelite Polyester ( or Terelene) |
Molecular mass of Polymers
During the process of synthesis of polymers, the degree of polymerisation or length of polymer chain depends upon the availability of monomer molecules near the growing polymer chain. Since the number of monomer molecules differs from one place to another in the reaction mixture, therefore, a particular sample of a synthetic polymer contains a number of species of varying chain lengths. Since each species have a different molecular mass and a given sample of a polymer contains a number of such species, therefore the polymer as a whole has an average molecular mass. In contrast, natural polymers such as proteins , contain chains of identical length and hence their molecular masses are singular in nature.
Types of average molecular mass
There are two types of average molecular masses of polymers, i.e.
Number average molecular mass
If N1, N2, N3 ….. are the number of macromolecules with molecular mass M1, M2, M3 …. respectively then the number average molecular mass of the polymer is given by
where Ni is the number of macromolecules of ith type with molecular mass Mi.
The number average molecular mass is determined by using methods which depend upon the number of molecules present in the polymer sample, viz., colligative properties such as osmotic pressure, depression in freezing points and elevation in boiling points. The latter two methods are not usually used since the depression in freezing points and elevation in boiling points are too small to be measured accurately.
Problem
01. Calculate, the average molecular mass of a polymer sample in which 30% molecules have a molecular mass 20,000 ; 40% have 30,000 ; and the rest have 60,000.
Weight average molecular mass
If m1, m2, m3 …. are the masses of macromolecules with molecular masses M1, M2, M3 …. respectively, then weight average molecular mass of the polymer is given by
But mi = Ni Mi where Ni is the number of macromolecules of ith type with molecular mass Mi.
The weight average molecular mass is determined using methods which depend upon the masses of individual molecules, viz., light scattering, ultra-centrifuge, sedimentation etc.
Poly Dispersity index (P D I )
The ratio of weight average molecular mass and number average molecular mass is called poly dispersity idex (PDI).
PDI is used to determine the homogenity of a polymer. On the basis of value of PDI , polymers have been classified into two categories :
(i) Monodisperse polymers : Polymers whose molecules have same or narrow range of molecular masses are called monodisperse polymers. For these polymers , `Mw = `Mn and hence their PDI is equal to 1 (unity). Natural polymers usually have PDI equal to one and hence are more homogeneous.
(ii) Polydisperse polymers : Polymers whose molecules have wide range of molecular masses are called polydisperse polymers. For these polymers, `Mw > `Mn and hence their PDI > 1. Synthetic polymers usually have PDI > 1 and hence are less homogeneous. Thus, in general, monodisperse (natural) polymers are more homogeneous than polydisperse (synthetic ) polymers.
Biopolymers and Biodegradable polymers
Polymers such as polysaccharides (starch, cellulose etc.) proteins and nucleic acids which control the various life processes are called biopolymers.
All these biopolymers disintegrate by themselves during certain period of time and hence are biodegradable. As a result they do not cause any pollution.
In contrast , synthetic polymers – a major proportion of which being used and thow away containers and packing materials do not disintegrate by themselves ( i.e., are non-biodegradable) over a period of time. This durability is not altogether an advantage. Rather it has presented mankind a serious waste disposal problem. With ever increasing use of plastics, soon the entire civilization will be buried under a pile of plastic debris. In order to meet this challenge, chemists are currently doing research to evolve such plastics which will disintegrate spontaneously (biodegradable) after a suitable length of time.
Aliphatic polyesters are one important class of biodegradable polymers as several of them are commercially potential biomaterials. Some examples are described below.
Poly –Hyydroxybutyrate –co-b-Hydroxyvalerate (PHBV)
It is a copolymer of 3-hydroxybutanoic acid and 3-hydroxypentanoic acid, in which the monomer units are connected by ester linkages.
The properties of PHBV vary accordingly to the ratio of both the acids , 3-hydroxybutanoic acid provides stiffness and 3-hydroxypentanoic acid imparts flexibility to the copolymer. It is used in specialty packing , orthopaedic devices and even in controlled drug release. When a drug is put in a capsule of PHBV, it is released only after the polymer is degraded. PHBV also undergoes bacterial degradation in the environment.
Poly (Glycolic acid) and Poly(Lactic acid)
They constitute commercially successful biodegradable polymers such as sutures. Dextron was the first bioabsorbable sutures made from biodegradable polyesters for post-operative stiches.
Nylon-2-Nylon-6
It is an alternating polyamide copolymer of glycine and amino caprioic acid and is biodegradable.
SOME COMMERCIALLY IMPORTANT POLYMERS
ADDITION POLYMERS
1. Polyolefins
These are generally obtained from ethylene or its derivatives. The polymerisation normally takes place at temperature between 473-673 K under high pressure and in presence of traces of oxygen. Some common examples are :
(i) Polyethylene
The monomer units are ethylene molecules. It is manufactured by heating ethylene to 465-485 K under pressure in presence of 0.03 to 0.1% of oxygen.
It is a white transluscent material. Low density polythene is flexible, but high density polyethene is harder , tough and chemically inert. Its major uses are as packing films, containers, laboratory apparatus, bottles, buckets, mould articles and insulators.
(i) Polypropylene
It is generally manufactured by passing propylene through n-hexane (inert solvent) containing Ziegler-Natta catalyst (a mixture of triethyl aluminium and titanium chloride).
It is harder, stronger and lighter than polyethylene. It is used for making ropes, fibres, packing textiles and foods, shrinkable wrap for records and other articles.
(ii) Polystyrene or Styron
The monomer units are styrene molecules. It is prepared by free radical polymerisation of styrene in the presence of benzoyl peroxide.
It is a white thermoplastic material which is transparent and floats on water. It is used for making toys, combs, model construction kits, packing for delicate articles and lining material for refrigerators and TV cabinets. Polystyrene is sold under the name styrofoam or styron.
2. POLYDIENES
(i) Neoprene synthetic rubber
Neoprene is a synthetic rubber, which resembles natural rubber in its properties. Natural rubber is a polymer of isoprene ( 2-Methyl-1,3-butadiene).
Synthetic rubber is obtained by the polymerisation of chloroprene (2-chloro-1,3-butadiene) in the presence of potassium persulphate.
Neoprene is superior to natural rubber in its stability to aerial oxidation and also in its resistance to oils and other solvents. It is generally used for making ; hoses, shoe heals, stoppers and belts.
ii) Buna - S
It is a co-polymer of 1,3-butadiene and styrene. It is obtained by the polymerisation of butadiene and styrene in the ratio of 3 : 1 in presence of sodium. It is also known as styrene butadiene rubber (SBR)
In Buna-S, Bu stands for butadiene and na stands for sodium which is polymersising agent and S stands for styrene.
SBR has slightly less tensile strength than natural rubber. It is used in the manufacture of automobile tyres, rubber soles , belts , hoses etc.
3. Polyacrylates
(i) Polyacrylonitrile (PAN)
Polyacrylonitrile is obtained by polymerisation of acrylonitrile (vinyl cyanide). It is commercially known as acrilan or orlon.
It is hard and high melting material. It is used as synthetic fibre for cloth, carpets and blankets.
(ii) Polymethyl methacrylate (PMMA)
Methylmethacrylate on polymerisation gives polymethylmethacrylate.
Commercially it is known by various names like lucite, acrylite, plexiglass and perspex. PMMA is a hard, transparent polymer. It has high optical clarity and good resistance to effect of light and ageing. It is used for lenses, aircraft windows, protective coating and transparent objects.
(iii) Polyethylacrylate (PEA)
It is obtained by polymerisation of ethylacrylate.
Polyethylacrylate ia a tough and rubbery polymer. It has uses almost similar to PMMA.
4. Polyhalo-olefins
The important polymers of this class are :
(i) Polyvinyl chloride ( PVC)
The monomer units are vinyl chloride molecules. It is prepared by heating vinyl chloride in an inert solvent in presence of dibenzoyl peroxide.
PVC is a hard horny material. However, it can be made to acquire any degree of pliability by the addition of a plasticier. It is used for making rain coats, hand bags, hose pipes, gramophone records, electrical insulation and floor coverings.
(ii) Polyetrafluoroethylene (Teflon, PTFE)
It is prepared by heating tetrafluoroethylene in the presence of ammonium peroxosulphate [(NH4)2S2O8].
It is a very tough material and resistant towards heat, action of acids or bases. It is a bad conductor of electricity. It is used in coating utensils to make them non-sticky, making seals and gaskets which can withstand high pressures, insulation for high frequency electrical installations.
(i) 5. Condensation Polymers
Terelene : It a polymer obtained by the condensation reaction between ethylene glycol and terephthalic acid. It is also known as dacron.
Terelene forms strong fibres. It is used for blending with cotton in clothing. It is used in seat belts, conveyer belts and for packing frozen food. Terylene tubes are good substitute for human blood vessels in human heart by-pass surgery.
(ii) Glyptal or Alkyd resion : Glyptal is a general name of all polymers obtained by the condensation of dibasic acids and polyhydric alcohols. The simplest glyptal is poly(ethylene glycol phthalate) which is obtained by the condensation of ethylene glycol and phthalic acid.
Poly(ethyleneglycolphthalate)
These are three dimensional cross-linked polymers (polyethyleneglycolphthalate) dissolves in suitable solvents and the solution on evaporation leaves a tough and non-flexible film. Thus it is used in : adherent paints, lacquers and building materials like asbestoes and cement.
(ii) Polyamides : Such polymers have amide (-CO-NH-
(a) Nylon-66 : It is a polymer of adipic acid (1,6-hexanedioic acid) and hexamethylene diamine (1,6-diaminohexane)
Nylon-66 can be cast into sheet or fibres by spinning devices. Nylon fibres have tensile strength. They are also some what elastic in nature. Nylon finds uses in : making bristeles and brushes, carpets and fabrics in textile industry, elastic hosiery in the form of crinkled nylon.
(b) Nylon-6,10 : It is a polymer of hexamethylene diamine (6 carbon atoms) and sebacoyl chloride (ten carbon atoms).
(iv) Nylon- 6
It is obtained from the monomer caprolactum. Caprolactum is obtained from cyclohexane according to the reaction sequence as given below.
Caprolactum on heating with traces of water hydrolyses to 6-aminocaproic acid which on continued heating undergoes self-condensation and polymerises to give nylon-6.
Nylon-6 is used for the manufacture of tyre cords, fabrics and ropes.
Phenol-formaldehyde resins
(i) Phenol formaldehyde resins (Bakelite)
These are made by the reaction of phenol and formaldehyde in basic medium. The reaction involves the formation of methylene bridges in ortho, para or ortho as well as para positions as shown below.
Bakelite is a crossed-linked thermosetting polymer. Soft bakelite with very low degree of polymerisation are used as bonding glue for laminated wooden planks, in the preparation of varnishes and lacquers.
High degree of polymerisation leads to formation of hard bakelite which is used for combs, fountain pen barrels, gramophone records, electrical goods, formica table tops and many other products. Suphonated bakelites are used as ion-exchange resins for softening of hard water.
(ii) Melamine formaldehyde resin : It is a polymer formed by condensation of melamine (2,4,6-Triamino-1,3,5-triazine) which is a heterocyclic triamine with formaldehyde. The reaction occurs as
Uses
It is used in making crockery. These are used for making cups and plates which are quite hard and durable. They do not break on being dropped.
QUESTIONS
1. Define a polymer. Give two examples.
2. What do you understand by the term polymerisation ? Name two organic polymers.
3. Explain with suitable example, the difference between (i) chain growth polymer (ii) step-growth polymer.
4. What are co-polymers ? Illustrate with two examples.
5. Give the chemical for the preparation of terelene.
6. Explain the difference between addition condensation polymers by giving one example in each case.
7. Give the points of difference between chain-growth (or addition) and step-growth polymerisations.
8. Explain the following terms giving one example each (i) Thermoplastic polymers (ii) Thermosetting polymers (iii) Elastomers.
9. Give the points of difference between thermoplastic polymers and thermosetting polymers.
10. Give the classification of polymers based on their methods of synthesis.
11. Give the classification of polymers based on molecular forces.
12. Comment on the main structural difference between thermoplastics and thermosetting polymers. What effect does this difference have on their properties ?
13. Arrange the following on the increasing order of their forces. Also classify them as addition and condensation polymers.
14. Give the chemical equations for the synthesis of :
(i) Polypropylene (vi) Teflon
(ii) Buna-s rubber (vii) Ployamide
(iii) Nylon-6 (viii) Terylene
(iv) Nylon-66 (ix) Bakelite
(v) Polyvinyl chloride (x) Neoprene
15. Identify the type of polymerisation on the basis of chain growth and step growth of the following polymers.
(a) Teflon (b) Nylon-66
16. What is Nylon ? What is Buna-S ? What is Teflon ?
17. Write the structure of Nylon, GR-S and polymethylmethacrylate.
18. A regular co-polymer of ethylene and vinyl chloride contains alternative monomers of each type. What is the weight percentage of ethylene in this copolymer ?
19. Give the uses of (a) Teflon (b) Nylon-66 (c ) PVC (d) Bakelite.
20. What is the difference between Nylon-6 and nylon-66 ?
21. Explain polydispersity index (PDI)
22. Explain the terms (I) polyamides (b) polyesters.
23. How is polymethyl methacrylate synthesised ? Give its important uses .
24. How is nylon-6 synthesised from cyclohexane. Give equations.
25. How are low density polyethene and high density ethylene manufactured ? How do they differ in their densities ?
26. Describe briefly polyolefins, giving one example.
27. What are polydienes ? Give one example.
28. What is the difference between polyacrylates and polyesters.
29. What is the general name of synthetic polymers containing R2SiO as repeating units ?
30. What is the monomer unit in natural rubber ?
31. Is (-NHCHR-CO-
32. Could a copolymer be formed in both addition and condensation polymerization or not ? Explain with examples.
33. Write structure of a reagent for initiating a free radical chain reaction. How does it act ?
34. Write mode of free radical polymerisation of an alkene ?
35. Write structures of monomers used for getting the following polymers ?
(a) PVC (b) Teflon (PMMA
36. Why should one always use purest monomer in free radical polymerization ?
37. How does the presence of carbon tetrachloride influence the course of vinylic free radical polymerisation ? Explain with an example.
38. Explain how does 1,3-butadiene polymerize by different routes.
39. Why does styrene undergo anionic polymerization easily ?
40. Why is cationic polymerization preferred in the case of vinylic monomers containing electron donating groups ?
41. Will you prefer to polymerize acrylonitrile under anionic or cationic polymerization conditions ? Explain your choice.
42. Elaborate the structure of natural rubber.
43. Depict a free radical mode of addition polymerization of isoprene.
44. How do double bonds in rubber molecules influence their structure and reactivity ?
45. How does vulcanization change the character of natural rubber ?
46. Why are the numbers 66 and 6 put in the name of nylon 66 and nylon 6 ?
47. Illustrate with equations , how dacron obtained from caprolactam ?
48. What is the difference between thermosetting and thermoplastic polymers ?
49. Explain the defference between polyacrylates and polyesters.
50. What is PHBV ?