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CHEM 3090 (3)


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CHEM 3090
Gino Lavoie

354Sec R71PolymerizationR71PolymerizationPolymers are nding increasing use throughout our society Well over 100 billion pounds of polymer are produced each year and it is expected that this gure1110 lbyrwill double in the coming years as higherstrength plastics and composite materials replace metals in automobiles and other products Consequently the eldof polymerization reaction engineering will have an even more prominent placein the chemical engineering profession Because there are entire books on thiseld see Supplementary Reading it is the intention here to give only the mostrudimentary thumbnail sketch of some of the principles of polymerization A polymer is a molecule made up of repeating structural monomerunits For example polyethylene is used for such things as tubing and repeating units of ethylene are used to make electrical insulationnCHCHCHCH2222nwhere n may be 25000 or higherEveryday ExamplesPolymerization is the process in which monomer units are linked by chemical reaction to form long chains These long chains set polymers apart fromPolyethyleneSoftdrink cupsother chemical species and give them their unique characteristic properties TheSandwich bagspolymer chains can be linear branched or crosslinked Figure R711Poly vinyl chloridePipesShower curtainsTygon tubingPoly vinyl acetateChewing gumFigure R711Types of polymer chainsHomopolymers are polymers consisting of a single repeating unit such as Homopolymers can also be made from two differentCHCH22monomers whose structural units form the repeating unit such as the formationof a polyamide eg nylon from a diamine and a diacidPolymerization reactions are divided into two groups known as stepreactions also called condensation reactions and chain reactions alsoknown as addition reactions Step reactions require bifunctional or polyfunctional monomers while chain reactions require the presence of an initiatorUnit 1Unit 1Unit 2Unit 2HOROOCRCOOH2n1HOnHOROHnHOOCRCOOH21212nRepeating UnitnARAnB RBARRB2 n1AB 1212n355Chap Copolymers are polymers made up of two or more repeating units There areve basic categories of copolymers that have two different repeating units Qand S They areAlternatingQSQSQSQSQS12BlockQQQQQSSSSSCategories3RandomQQSQSSQSSSof Copolymers4GraftQQQQQQQQQQSSSSSS5Statistical follow certain addition laws1Examples of each can be found in Young and LovellR711Step PolymerizationStep polymerization requires that there is at least a reactive functional groupon each end of the monomer that will react with functional groups with othermonomers For example aminocaproic acidNHCHCOOH225has an amine group at one end and a carboxyl group at the other Some common functional groups areCOOHCOClNHOH2In step polymerization the molecular weight usually builds up slowly Structural UnitRepeating UnitDimer2HNHRCOOHHNHRCOOHHO 22For the preceding case the structural unit and the repeating unit are the sameLetting andWecanAHRN HRCOBO HABHO12write the preceding reaction asARBAB ARBDimerARB2ARBTrimerBAR ARBAB23ARBTetramerARBARBAB 34ARARBBARBAB 224PentamerARBARB ARBAB45ARARBB ARBAB235HexamerARBARBARBAB 56ARARBB ARBAB246ARARBB ARBAB3361R J Young and P A Lovell Introduction to Polymers 2nd ed New York Chapman Hall 1991
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