L17PolymerII145s09(2)

L17PolymerII145s09(2) - Lecture 17: Polymer Structure &...

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Lecture 17, spring 2009 ENGR 145, Chemistry of Materials Case Western Reserve University Reading assignment : C&R §14.11; OGC §23.1-23.2 Learning objectives: Understand the two basic types of polymerization reactions Addition Condensation Describe three types of addition polymerization reactions: Free radical polymerization Ionic polymerization Coordination polymerization Be able to explain how these reaction mechanisms affect the structure and properties of the resulting polymer Lecture 17: Polymer Structure & Synthesis II 1
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Lecture 17, spring 2009 ENGR 145, Chemistry of Materials Case Western Reserve University Free radical polymerization Ionic polymerization Coordination polymerization Condensation polymerization Types of Polymerization Reactions 2
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Lecture 17, spring 2009 ENGR 145, Chemistry of Materials Case Western Reserve University low density poly(ethylene) LDPE (CH 2 -CH 2 ) n • poly(vinyl chloride) PVC (CH 2 -CHCl) n poly(methyl methacrylate) CH 3 PMMA | (-CH 2 C-) n | COOCH 3 Common Polymers Formed via Free Radical Polymerization [OGC §23.1; C&R §14.11] 3
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Lecture 17, spring 2009 ENGR 145, Chemistry of Materials Case Western Reserve University What are free radicals? Two ways to break an A-B single bond into two fragments: ionization: A-B A + + B (we’ll use this later) free radicals: A-B A + B the dots denote unpaired electrons very reactive Typical initiator: benzoyl peroxide • C 6 H 5 COO-OOCC 6 H 5 2 C 6 H 5 COO benzoyl free radicals General behavior for peroxide initiators: Free Radical Polymerization 4
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Lecture 17, spring 2009 ENGR 145, Chemistry of Materials Case Western Reserve University • Ethylene (C 2 H 4 ) forms polyethylene (PE) in the presence of free radical R (catalyst or initiator) Free Radical Polymerization monomer initiation propagation electrons in π bond electrons in π bond 5
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Lecture 17, spring 2009 ENGR 145, Chemistry of Materials Case Western Reserve University Note: Propagation is a chain reaction Unpaired electrons are not consumed in propagation reaction Number of free radicals is unchanged during propagation But the reaction can stop via termination reactions: Combination reactions between free radicals Transfer of hydrogen between free radicals Free Radical Polymerization 6
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Lecture 17, spring 2009 ENGR 145, Chemistry of Materials Case Western Reserve University Possible termination reactions between two free radicals Two radicals combine to form one new (~unreactive) molecule: or a hydrogen atom transfers from one free radical to another: and there is yet another possibility … Free Radical Polymerization double bond on the other saturated C on one chain 7
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Lecture 17, spring 2009 ENGR 145, Chemistry of Materials Case Western Reserve University Some termination reactions transfer the radical to a new chain R-(CH 2 -CH 2 ) n can react with molecules besides the monomer: R-(CH 2 -CH 2 ) n + A-B R-(CH 2 -CH 2 ) n -A + B • Typically: A-B is another polymer molecule often A is a hydrogen Free Radical Polymerization 8
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L17PolymerII145s09(2) - Lecture 17: Polymer Structure &...

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