1. IntroductionThe technology enhancement of any population is connected to the materials available.Therefore scientists have developed new materials to cope with the development of humancivilisation. With the use of organic chemistry, new material is producing commonlysynthetic polymers. Due to the versatility of these polymers it allows creation and fabricationof materials that cannot be attain from materials such as wood, metals and ceramics. The rawmaterials of polymer derived from petroleum and with the advance technology the synthesisof raw material became cheap and useful.2. Step Growth or Condensation PolymerizationsStep Growth polymerization is a polymerization process in which chain growth occurs instepwise manner. It is formed by reaction of difunctional molecules, with each new bondcreated in separate steps. Monomers will react with other monomers to form dimmers,dimmers than react with dimer to form tetramers, tetratmer react with monomers to formpentamers and so on. The stepwise manner plays an important role for both their molecularweights and molecular weight distribution. At the end high molecular weight polymermolecules is produce. The step growth process can be done in two ways; (1) reaction betweenA-A monomers will react with B-B monomers to give (A-A-B-B) polymer and (2) the selfcondensation A-B monomer to give (A-B)n polymers. New covalent bond is formed at theend of polymerization by polar reaction such as nuclephilic acyl substituition. 2.1. Polyamides Wallace M. Carothers and his associates E.I. Dupont de Numours & Company in the 1930s begin their fundamental research into the reaction of aliphatic dicarboxylic acids and diols. From adipic acid and ethylene; a high molecular weight polymer is obtained that could be drawn into fibres which are polyesters. O O HO H3C CH3 O OH + HO OH O O O Hexanedioic acid 1,2-Ethanediol Poly(ethylene adipate) (Adipic acid) (Ethylene glycol)
The polyesters fibres had melt transitions (Tm) too low for use as textile fibres. Thenhe concentrates to reactions of dicarboxylic acids and diamines to form polyamidesand 1934 synthesized nylon 6, 6 is made which is a purely synthetic fibres. Nylon 6,6 is synthesized from two different monomers, each containing six carbon atoms. OHO H2N + OH NH2 O Hexanedioic acid 1,6-Hexanediamine (Adipic acid) (Hexamethylenediamine) O -O + O- NH 3 +NH 3 O Nylon salt O -O NH CH3 NH n O Nylon 6,6The conditions that is suitable to form nylon 6,6 is the Tm has to be between 250-260C and the molecular weight is in the range of 10,000-20,000 g/mol.The first step of fiber production, crude nylon is melted, spun into fibers and cooled.The melt-spun fibers are cold drawn (drawn at room temperature) to about four timestheir original length to increase their degree of crystallinity. Next, individual polymermolecules become orientated in the fiber axis, and hydrogen bond form betweencarbonyl oxygen of one chain and amide hydrogen of another chain. The physicalproperties of the fiber from polyamide are impressive because both tensile strengthand stiffness increase due to the effects of orientation of polyamide molecules.The existing raw material base for the production of nylon 6, 6 is benzene whicharises from catalytic cracking and reforming of petroleum. A mixture ofcyclohexanol and cyclohexanone is produce when benzene is catalyzed tocyclohexane by catalytic reduction and then followed by catalyzed air oxidation.Oxidation of this mixture by nitric acid will gives adipic acid.
O 3H 2 O2 OH O HNO 3 OH catalyst catalyst + OHbenzene cyclohexane cyclohexane cyclohexanone O hexanedioic acid (Adipic acid)Adipic acid is the starting material for synthesis of hexamethylenediamine. Reactionod adipic acida and ammonia give ammoniums salt, which when heated will giveadipiamide. If adipic acid is catalytic reduce it will give hexamethylenediamine.Hence the formation of nylon 6, 6 are completely from petroleum which is sadlyunrenewable. + ONH 4 O O + H2N O NH 4 NH2 O O Ammonium Hexanedioate Hexanediamide (Ammonium adipate) (Adipamide) H2N NH2 1,6-Hexanediamine (Hexamethylenediamine)Nylon can be categories as polymers, the members of which have tactfully differentproperties that suits them to one use or another. Nylon 6,6 and nylon 6 are the mostpopular polymers. Nylon 6 is so named because it is synthesized from caprolactam asix carbon monomer. In the synthesis of nylon 6, caprolactam is partially hydrolyzedto 6-aminohexanoic acid and then it is heated to 250C to start the polymerization.Nylon 6 is fabricated into fibers. Brush bristle, ropes, high impact molding and tirecords. O Hydrolysis O NH NH Heat H CH3 n Nylon 6Caprolactam
Based on extensive research of the relationship between molecular structure and bulk physical properties, scientist at Dupont reasoned that polyamide containing aromatic ring will become stiffer and stronger than either nylon 6 or nylon 6, 6. From there Kevlar where produce, a polyaromatic amide fiber synthesized from tetraphthalic acid and p-phenylenediamine. The amazing features of Kevlar are that it is light weighted and strong even if it is compared with other material that have the same strength. Kevlar is used as anchor cables for offshore drilling rigs and reinforcement fibres for automobiles tires. Because of its remarkable feature it is also woven into fabric such as bulletproof vest, jackets and raincoats.2.2. Polyesters Carothers and his associates deduce that polyesters fibers are produce from aliphatic dicarboxylic acids and ethylene glycol were not suitable for textile use because of their low melting point. In 1930s Winfield and Dickson studied polyesters and found that the a greater resistance to rotation of polymer backbones will stiffen the polymer, this is done by increase its melting point which will then result of an acceptable polyester fibres. Terephthalic acid is used to create the stiffness of polymer chain. Polymerization of this aromatic dicarboxylic acid with ethylene glycol will give poly(ethyleneterephthalate) or PET. O O O O OH O + HO H3C O CH3 HO OH n 1,2-Ethanediol Poly(ethylene terephthalate) 1,4-Benzenedicarboxylic acid (ethylene glycol) (Dacron,Mylar) (terephthalic acid) The crude polyester can be melte, extruded and then cold drawn to form tetxtile fiber Dacron polyester. Which are four times stronger than nylon 6,6. However Dacron polyester fibres are harsh to touch due to their stiffness hence they are mixed with cotton or wool to make acceptable textile fibres. PET is fabricated into Mylar films and recycle plastic beverages containers. For the synthesis of PET; ethylene glycol is obtained by air oxidation of ethylene to ethylene oxide followed by hydrolysis of glycol. Ethylene id obtained from cracking either from petroleum or the ethane from natural gas. Whereas by oxidation of p-
xylene will produce terepthalic acid; an aromatic hydrocarbon obtained from catalytic cracking or reforming of naptha or petroleum. O + H , H 2O H2C CH2 HO OH Phenol Oxirane 1,2-Ethanediol (Ethylene Oxide) CH3 O2 O O HO OH H3C catalyst p-xylene Terephthalic acid2.3. Polyurethane Urethane is and ester of carbamic acid H2NCOOH. Urethane is the name derived from the functional group formed from the reaction of an isocyanate group with a hydroxyl group. R N O R OH R NH O R O isocyanate carbamate Polyurethanes consist of a flexible polyester or polyether unit alternating with rigid urethane units block. The rigid urethane block is derived from diisocyanate, generally a mixture of 2,4 and 2,6 toulene diisocyanate. The flexibility of this polymer depend on the low molecular weight (MW 1000-4000) polyesters or polyether with –OH group at the ends of the polymer chain. Fibers forms from polyurethane are fairly soft and elastic. It is usually used as Spandex and Lycra. By adding small amount of water during polymerization polyurethane foams for upholstery and insulating materials are made. This is because water react with isocyanate groups to produce carbon dioxide gas which act as a foaming agents
O O N N HO polymer OH + 2,6- Toulene diisocyanate Low Molecular Weight polyester or polyether O O CH3 O CH3 NH NH n H3C O polymer polyurethane H H O O O O R N O + R NH OH R NH2 + isocyanate carbamic acid (unstable)2.4. Phenol-formaldehyde polymers Phenol-formaldehyde polymers are thermosetting polymers composed of long chains that are cross linked by covalent bonds. Leo Baekeland 1907 was the first to produce thermosetting polymer. Thermosetting polymer is produce by reacting phenol with formaldehyde to form Bakelite which is good electrical insulator. One of the monomer must be trifunctional in their preparation of thermoset. For Bakelite, the trifunctional monomer is phenol. Manufacture of thermosets begins with a fluid mixture of two monomers. It is first shaped and then polymerized wither by heating or by being mixed with an initiator. The product of polymerization is a network of covalently bonded atoms that is solid, even at room temperature. The thermoset polymers char and decompose at high temperature but they do not melt.
CH 2 CH 2 OH CH4 H4C + H2C O OH OH Formaldehyde CH 2 Phenol CH4 OH CH 2 CH 2 H4C o OH OH CH 2 CH4 Bakelite CH 2 H4C3. Summary Step growth polymerization is also known as condensation step For step growth polymerisation, monomers should have at least two functional group. Both functional group may be the same or different Monomers having two functional group always give linear polymer Step growth polymer do not contain all the atoms initially present in monomers The polymerization takes place between monomeric units with the elimination of small molecules such as water, ammonia, alcohol etc. Monomers having three functional groups alway gives cross-linked polymer. Example: urea-formaldehyde, phenol-formaldehyde resin, alkyd resin All resins are cross-linked polymer and all cross-linked polymers are condensation polymers
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