POLYMERS
POLYMERS: Polymers are compounds of very high molecular masses formed by the combination of a large number of simple molecules through chemical bonds. n(c 2 =C 2 ) -(-C 2 -C 2 -) n - Polyethane (polymer) Ethane (monomer)
Due to their large size they are also sometimes called macromolecules. Small molecules which combine with each other to form polymer molecules are known as monomers. Monomer Polymer C 2 C 2 C 2 C 2 C 2 CCl C 2 C 2 O 2 C C 2 Cl C 2 C 2 O OC 2 C 2 O C 2 C 2 O O O CO 2 O C
Polymers may be classified on the basis of structure: Linear chain polymers: These are polymers in which monomeric units are linked together to form linear chains. They possess high densities, high tensile strength and high melting point. E.g., polyethylene, nylons and polyesters. Branched chain polymers: These include polymers in which the monomer units are joined to form long chains with side chains or branches of different lengths. They have low tensile strength and low melting point. E.g., glycogen, Amylopectin etc.
Crosslinked polymer: These are polymers in which monomer units are crosslinked together to form a three dimensional network. These are hard, brittle and rigid. E.g., melamine, bakelite etc. (a) linear (b) branch (c) network
Polymers may be classified on the basis of origin: Biopolymers/Natural polymers: The polymers obtained from nature are called natural polymer (biopolymers). E.g., starch, cellulose, protein, natural rubber etc. Synthetic polymers: These include polymers that have been synthesized from low molecular mass starting materials. E.g., Plastics, synthetic fibres, rubber, paints etc.
They are also classified on the basis of their functional characteristics (molecular forces) and end use applications, into three groups namely, (i) PLASTICS: Plastic can be broadly classified as: Thermoplastics: These have either linear or branched structure. They can be amorphous or semi crystalline materials. Neighbouring polymeric chains are held together by weak van der waals forces or dipole-dipole forces or hydrogen bonding. There are no crosslinks.
On heating, they soften very readily but on cooling they stiffen again. They can be remoulded, reshaped and reused. ence, they can be recycled. These polymers are usually soluble in suitable solvents. E.g., Polyethylene, Nylons, Polyesters, Polyvinyl alcohol etc.
Thermosetting plastics: They have three dimensional, cross-linked networked structure. Neighbouring polymeric chains in thermosets are held together by crosslinks (strong covalent bonds). eating does not soften them, since softening would require breaking of covalent bonds. E.g., vulcanized rubber, epoxy, Polyurethene foam etc.
APPLICATIONS: Plastics find use as structural materials for fabricating a wide variety of products such as telephones, instrument boards, electrical instruments, insulators, optical instruments, automobile parts, lamps and household appliances.
(ii) FIBERS: These are polymers which have strong inter-molecular forces (hydrogen bonds or dipole-dipole interactions) between the chains and exhibit high tensile strength and sharp melting point. Various intermolecular forces hold the linear chains together resulting in their regular alignment. The different types of fibres may include: Natural fibres: Natural fibres are of either plant origin (e.g., cotton and jute) or animal origin (e.g., silk and wool). Regenerated fibres: eg cellulose fiber, rayon Synthetic fibers: nylon
(iii) ELASTOMERS: These polymers have rubber-like or elastic properties capable of undergoing reversible deformation and elongation. The polymeric chains in elastomers are held together by weak intermolecular forces (besides occasional crosslinks) so that the original conformation is recovered easily on being deformed. E.g., natural rubber, synthetic rubbers, etc. (A) is an unstressed polymer; (B) is the same polymer under stress. When the stress is removed, it will return to the A configuration. (The dots represent cross-links)
Classification on the basis of tacticity (spatial arrangement): Isotactic polymers: Polymers in which all the asymmetric carbon atoms have the same (d or l) configuration. Atactic polymers: Polymers having random sequences of d- or l- configurations are termed as atactic polymers. Syndiotactic polymers: Polymers having regular alternation of d-or l- configurations in the molecular chains are called syndiotactic polymers.
1. Polyethylene terephthalate (Dacron) - POLYESTER Monomer Polymer O 2 C Terephthalic acid CO 2 O Ethylene glycol O O Poly(ethylene terephthalate O Dacron O O Ester 2 C 2 C O (polyester) n It is used in fibers for clothing, containers for liquids and foods, thermoforming for manufacturing, and in combination with glass fiber for engineering resins.
2. Bakelite CO + N 4 O
Properties and Uses of bakelite Moldings are smooth, retain their shape and are resistant to heat, scratches, and destructive solvents. It is also resistant to electricity, and has low conductivity. It is not flexible. Phenolic resin products may swell slightly under conditions of extreme humidity or perpetual dampness. When rubbed or burnt, Bakelite has a distinctive, acrid, sickly-sweet or fishy odor. Used in making Jewellery boxes, lamps, desk sets, clocks, radios, telephones, kitchenware, tableware, and a variety of game pieces such as chess sets, billiard balls, and poker chips
3. Nylon-6,6 O Cl 4 O Cl Adipoyl chloride 2 N N 2 4 1,6-Diaminohexane NaO O O Cl N N 4 4 Nylon 66 has high mechanical strength, rigidity, good stability under heat and/or chemical resistance. It is used in fibers for textiles and carpets and molded parts. O O O N N 6 carbon diacid 4 4 6 carbon diamine Nylon-6,6 n
4. Nylon 6 Properties and Uses Nylon 6 fibres are tough, possessing high tensile strength, as well as elasticity and lustre. They are wrinkle-proof and highly resistant to abrasion and chemicals such as acids and alkalis. The fibres can absorb up to 2.4% of water, although this lowers tensile strength. It is widely used for gears, fittings, and bearings, in automotive industry for underthe-hood parts, and as a material for power tools housings. Nylon 6 is used as thread in bristles for toothbrushes, surgical sutures, and strings for acoustic and classical musical instruments, including guitars, sitars, violins, violas, and cellos. It is also used in the manufacture of a large variety of threads, ropes, filaments, nets, and tire cords, as well as hosiery and knitted garments.
5. Poly(methyl methacrylate) or Acrylic glass or Plexi glass PMMA is a strong and lightweight material. PMMA swells and dissolves in many organic solvents; It has poor resistance to many chemicals. Uses For rear-lights and instrument clusters for vehicles, appliances and lenses for glasses. PMMA in the form of sheets is used in panels for building windows, skylights, bullet proof security barriers, signs & displays, sanitary ware (bathtubs), LCD screens, furniture and many other applications
6. Polytetrafluoroethylene(Teflon) PTFE is hydrophobic PTFE has one of the lowest coefficients of friction against any solid. PTFE is used as a non-stick coating for pans and other cookware It is used in containers and pipework for reactive and corrosive chemicals PTFE is used for applications where sliding action of parts is needed: plain bearings, gears, slide plates, etc.
7. Polystyrene Or Styrene Polystyrene is clear, hard, and brittle Uses- Uses include protective packaging (such as packing peanuts and CD and DVD cases), containers, lids, bottles, trays, tumblers, and disposable cutlery
8. Polyvinyl chloride (PVC) PROPERTIES PVC is a white, brittle solid. It is insoluble in alcohol but slightly soluble in tetrahydrofuran USES PVC comes in two basic forms: rigid (sometimes abbreviated as RPVC) and flexible or placitized RPVC is used in construction for pipes, doors and windows. For making bottles, other non-food packaging, and cards (such as bank or membership cards). It can be made softer and more flexible by the addition of plasticizers (phthalates). Placitized PVC is used in plumbing, electrical cable insulation, imitation leather, signage, inflatable products, and many applications where it replaces rubber.
Classification on the basis of mode of synthesis: Addition polymers (Chain growth) : A polymer formed by direct addition of monomers without the elimination of any by product molecules is called addition polymers. The monomers are unsaturated. The polymers bear the same empirical formula as their monomers. E.g., polyethene and polypropylene, Polyvinyl chloride, Teflon (CF 2 -CF 2 ) etc. (i) (ii) Free radical Ionic polymerization (a) Anionic Polymerization (b) Cationic Polymerizatio
Free Radical Polymerization Usually, many low molecular weight alkenes undergo rapid polymerization reactions when treated with small amounts of a radical initiator. For example, the polymerization of ethylene
1. Combination 2. Disproportionation
Free-Radical AdditionPolymerization of Ethylene 2 C C 2 200 C 2000 atm O 2 peroxides C 2 C 2 C 2 C 2 C 2 C 2 C 2 polyethylene
.. Mechanism RỌ.2C CC3
.. RO: Mechanism 2 C CC 3
.. RO: Mechanism 2 C CC 3 2 C CC 3
.. RO: Mechanism 2 C CC 3 2 C CC 3
.. RO: Mechanism 2 C CC 3 2 C CC 3 2 C CC 3
.. RO: Mechanism 2 C CC 3 2 C CC 3 2 C CC 3
.. RO: Mechanism 2 C CC 3 2 C CC 3 2 C CC 3 2 C CC 3
Likewise... 2 C=CCl 2 C=CC 6 5 F 2 C=CF 2 polyvinyl chloride Teflon polystyrene
Ionic Polymerization Whereas free radical polymerization is non-specific, the type of ionic polymerization procedure and catalysts depend on the nature of the substituent (R) on the vinyl monomer Anionic initiation, requires the R group to be electron withdrawing in order to promote the formation of a stable carbanion (ie, -M and -I effects help stabilise the negative charge). Cationic initiation is therefore usually limited to the polymerization of monomers where the R group is electrondonating.this helps stabilise the delocation of the positive charge through the p orbitals of the double bond
Ionic Polymerization
Ionic Polymerization
Anionic Polymerization Involves the polymerization of monomers that have strong electron-withdrawing groups, eg, acrylonitrile, vinyl chloride, methyl methacrylate, styrene etc. The reactions can be initiated by methods (b) as shown in the sheet on ionic polymerization Example : for mechanism (b) Initiators- KN 2, n-buli, and Grignard reagents such as alkyl magnesium bromides
Anionic Polymerization of Styrene
Anionic Polymerization of Styrene
Cationic Polymerization
Cationic Polymerization
Cationic Polymerization (iii) Termination Termination of cationic polymerization reactions are less well-defined than in free-radical processes. Two possibilities exist as follows:
Cationic Polymerization
Types of Addition Polymerizations Anionic C 3 7 Li Radical Ph C 4 9 Ph Li + n Ph C 4 9 n Ph Ph Li + PhCO 2 Ph PhCO 2 Ph n Ph PhCO 2 n Ph Ph Cationic Ph Cl 3 Al O 2 Ph OAlCl 3 n Ph Ph n Ph OAlCl 3
Condensation polymers: Polymers formed by the condensation of two or more than two monomers with the elimination of simple molecules like water, ammonia, hydrogen chloride, alcohol etc., are called condensation polymers. Each monomer generally possess two functional groups. E.g., Nylon 66, terylene, bakelite etc.
Condensation or Step-growth Polymerization Step-polymers are made by allowing difunctional monomers with complementary functional groups to react with one another Condensation between two molecules O MeO terephthalic acid O OMe + O O ethylene glycol O C O C OC 2 C 2 O Poly(ethylene terephthalate) n This is an example of a poly(ester) The reaction is a transesterification PET Recyclable plastic bottles and textile fabrics Using a condensation reaction
Step-growth Polymerization These are poly(amides) bristles of toothbrishes, s t o c k i n g s, r o p e, t i r e s, c a r p e t f i b r e - 2 O Molten nylon spun into fibres 260-280 C 250 psi MW = 10,000, m.pt. 250 C, fibres stretched (to increase strength) to 4 times their length
Ziegler-Natta Addition Polymerization or Coordination polymerization TiCl 4 / AlR 3 1-4 atm, rt R n TiCl 4 + AlR 3 Cl 3 Ti Cl R Cl 3 Ti R + Cl AlR AlR 2 2 s-complex R Cl 3 Ti R Cl 3 Ti Cl 3 Ti R p-complex Cl 3 Ti R Cl 3 Ti R Cl 3 Ti R Cl 3 Ti n R
Ziegler-Natta Chain (Addition) Polymerization Termination reaction Cl 3 Ti + Cl Ti Cl Cl
CONDUCTING POLYMERS: A polymer which can conduct electricity is termed as conducting polymers. e.g., Polyaniline (used in rechargeable batteries), polypyrrole etc. Two types of conducting polymer may be distinguished: Ionically conducting polymers or solid polymer electrolytes/extrinsically conducting polymers: These may be defined as solid ionic conductors formed by the dissolution of inorganic salts in suitable polymer solution and evaporating the solvent. They owe their conductivity to the externally added ingredients. The polymer suitable for solid polymer electrolyte should have a) atoms with electron donor capability to form coordinate bonds with cations of simple inorganic salts (e.g., lithium or sodium salts), b) repeat units forming segments with sufficient rotation or motion within the segment of the polymer and c) facilitate multiple intrapolymer ionic bonds.
These are of following two types: Conductive element filled polymers: This type includes polymers that act as a binder to hold the conducting element (such as carbon black, metallic fibers, metallic oxides, etc.) together in the solid entity. These polymers possesses reasonably good bulk conductivity; are generally low in cost; light in weight, mechanically durable and strong and are easily processable in different forms, shapes and sizes. Such polymers find use in hospital operating theatres.
Blending conducting polymers: These polymers are obtained by blending a conventional polymer with a conducting polymer. Such polymers possess better physical, chemical, electrical and mechanical properties and they can be easily processed. They are used in electromagnetic shielding.
Intrinsic electronically conducting polymers: These possess molecular structure with an extensive system of conjugated double bonds and π electrons. The conjugated system has low ionization potential and high electron affinity and hence it is easy to add or remove electrons to create an excess charge by the use of electron donors (reducing agents) or acceptors (oxidizing agents). These polymers have several advantages such as light weight, flexibility, ultra-thin film formation capability, high energy density and ease of fabrication.
They may be further classified into: Conducting polymers having conjugated π electrons in the backbone: Such polymers contain π electrons in the backbone which is responsible for the extensive conductivity. They exhibit electrical conductivity only after thermal or photolytic exposure. The order of conductivity (10-10 S cm-1of these polymers restricts their applicability. e.g., Polypyrrole, Polyacetylene etc.
Doped conducting polymers: These are conjugated polymers having enhanced conductivity induced through oxidation or reduction. They may be classified as: I) p-doped polymers: It is obtained by subjecting conducting polymers (having conjugated π electrons) to oxidation by treating with Lewis acid (A) or iodine vapour or iodine in CCl 4. E.g., (C) x + A (C) x + A - (Oxidation process) (C) x + 2FeCl 3 (C) x +FeCl 4- + FeCl 2 2 (C) x + 3I 2 2 (C) x +I 3 -
II) n-doped polymer: It is obtained by It is obtained by subjecting conducting polymers (having conjugated π electrons) to reduction by treating with Lewis base (B) like sodium naphthalide. (C)x + B (C)x - B+ (C) x + Na + (C 10 8 ) - Na + (C) x- + C 10 8 Such polymers (e.g., Emeraldine salt) have conductivity (103 S cm-1) comparable to that of metals.
APPLICATIONS OF CONDUCTING POLYMERS: In rechargeable batteries: These batteries are small in size (button type), long lasting and can produce current density upto 50 ma/cm2. Moreover, these rechargeable batteries have ecological advantage as they do not involve heavy metals so they do not appear to have any serious toxicological problems In Analytical sensors: Conducting polymers are also used for making sensors for p, O 2, NO x, N 3 and glucose. For making ion exchangers: Membranes made up of them can show boundary layer effects with selectivity permeability for ions, gases, etc. ence, they are useful for ion-exchangers and controlled release of drugs.
In electrochromic displays and optical filters: Ionically conducting polymers can absorb visible light to give coloured products so can be useful for electrochromic displays and optcal filters (windows with adjustable transparency). Thus, conducting polymers can be used as elcetrochromic materials (i.e., the materials which change colour reversibly during the electrochemical processes of charge and discharge). In electronics: Photostructural lacquers used based on ICP s are useful for electron beam lithography, LED s and Data storage.
BIODEGRADABLE POLYMERS: Polymers which are readily decomposed by microorganisms (fungi or bacteria) via enzymatic activity are known as biodegradable polymers. Types of biodegradable polymers: Natural biodegradable polymers: Natural rubber, collagen, lignin, poly (gamma-glutamic acid) are some of the examples of natural biodegradable polymers. Synthetic biodegradable polymers: Polyvinyl alcohol, polyanhydrides, poly-(3-hydroxybutyrate-co-3-hydroxyvalerate) or PBV are some of the examples of synthetic biodegradable polymers.
Biodegradable Polymers Acetal O 2 O R O C O R' R O + C + R' O emiacetal Ether Nitrile Phosphonate Polycyanocrylate O C C O OC O O C C O R C O C R' R C R O C N RO P OR' O 2 2 O O C C O OC O C O + C==O 2 O R C O R C R O 2 + 2 O R' C O R C R C O C O 2 N O O 2 O R O + O P O + O R' OR'' CN CN R C C C C R' C O C O OR'' 2 O CN R C C C C O + CN O C R' C O OR'' OR''' OR'' OR'''
APPLICATIONS OF BIODEGRADABLE POLYMERS: Poly (β-hydroxy butyrate) or PB: PB is used in the manufacture of shampoo bottles. β-hydroxy butyrate-β-hydroxy valerate or B-V copolymers: The B-V copolymers are suitable as matrices for controlled release of drugs due to their favorable biocompatibility and biodegradation properties. Poly (lactic acid) or PLA: As PLA breaks down in the environment back to lactic acid, which can be metabolized; it has found commercial use in medical applications such as sutures, drug-delivery systems and wound clips. It is also used in some agricultural applications, such as timed-release coatings for fertilizers and pesticides.
LIMITATIONS OF BIODEGRADABLE POLYMERS: Biodegradable polymers are not suitable for recycling, especially in the case of commingled plastics. Generally, biodegradable polymers are very expensive. Biodegradable polymers are not easily available.