Organic Chemistry 219 Module 8

Organic Chemistry 219 Module 8 Polymer - -large molecule made by repetitive linking of smaller units (monomers) Macromolecule - -very large molecule composed of thousands of covalently bonded atoms (ex: polymer) 1. Natural (in nature) 2. synthetic (in lab) - -two ways polymers are made Rubber, carbs: starch & cellulose, proteins, nucleic acids DNA, RNA - -natural polymers Nylon, teflon, styrofoam, polyethylene, Dacron, - -synthetic polymers 1. Method of formation 2. Final composition 3. stereo chemical orientation - -3 main ways to differentiate polymers 1. chain-growth 2. step-growth - -Two classifications of Synthetic polymers Addition polymers - -chain growth polymers are also know as: Made by addition of one monomer unit to another in a repetitive pattern - -How are chain growth polymers made? Chain growth polymerization - -A polymerization involving sequential addition to monomers that are unsaturated (C=C) or have some other reactive functional groups (particularly ethylene and derivatives) 1. intervention 2. consumption of all available monomers - -what 2 things stops chain growth polymerization? Teflon - -chain-growth Polymer: polytetrafluoroethylene Monomer: F2C=CF2 Nonstick coating, goretex, electrical insulator, chem-resistant coating Saran - -chain-growth Polymer: polyvinylidenedichloride Monomer: H2C=CCl2 Cling-wrap Organic Chemistry Polypropylene - -chain-growth Polymer: polypropylene Monomer: H2C=CHCH3 carpet fibers, car parts, toys, packaging, houseware Orlon, Acrilan, Creslan - -chain-growth Polymer: polyacrylonitrile monomer: H2C=CH(CN) Textiles/fibers, carpets, upholstery Polyvinyl Acetate - -chain-growth Polymer: polyvinyl acetate Monomer: H2C=CH(OCOCH3) elmers glue, silly putty, latex paints Polyvinyl alcohol - -chain-growth polymer: polyvinyl alcohol monomer: H2C=CH(OH) eye med- artificial tears Plexiglass (Lucite) - -chain-growth polymer: polymethylmethacrylate monomer: H2C=C9CH3)COOCH3 clear plastic sheets , blocks, and tubing the final polymer retains all of the atoms of the monomer - -The defining characteristic of chain-growth polymers step growth polymerization - -formed by the reaction between 2 different functional groups on different monomer molecules with the accompanying loss of some small molecule (typically water) The final polymer chain does NOT include all the atoms initially present in the monomer molecules - -The defining characteristic of step-growth polymers Condensation polymers - -step-growth polymers also known as: Typically di or polyfunctional - -functionality of step-growth monomers Alternating order in the final polymer chain - -appearance of monomers in step-growth polymerization By carbon-heteroatom bond formation - -how step-growth polymers generally grow Carbon-carbon bond formation - -how chain-growth polymers generally grow Organic Chemistry Organic Chemistry Organic Chemistry Polyamide (Nylon) - -formed by combining 1,6-diaminohexane (amine) with 1,6 hexandioic acid (carboxylic acid) Amine functionality reacts with carboxylic acid functionality to form an amide with a corresponding loss of a water molecule Cellulose, polypetide chains, beta-Hydroxybutyric acid - -naturally occurring step growth/ condensation polymer examples Lexan - -step-growth polymer: polycarbonate eye glasses, auto parts, drinking glasses Kevlar - -step-growth polymer: polyamide body armor, tires, helmets Dacron, Mylar - -step-growth polymer: polyester eletric/thermal insulation, helium balloons, artificial limbs 1. Free radical 2. Cationic 3. Anionic - -three main mechanisms for addition/ chain-growth polymerization begins by creating a reactive intermediate that starts the chain reaction - -For all three main addition mechanisms, how does polymerization begin? Free-radical addition polymerization - -a monomer reacts with a free-radical initiator to create a free-radical Free-radical - -an atom or a group of atoms that has one unpaired electron radical initiator - -a reagent that creates radicals, has relatively weak covalent bonds which can be homolyzed 1. electrically neutral 2. quite reactive and reacts quickly to pair up the lone electron 3. formed from radical initiators - -3 main characteristics of free-radicals Homolyzed - -broken so that each atom joined by the bond gets one of the two electrons in the bond benzoyl peroxide - -example of a radical initiator (o-o bond = weak and can undergo homolysis upon heat or UV exposure to produce 2 benzoyl radicals) Organic Chemistry Organic Chemistry only a small amount of radical initiator is needed compared to the concentration of monomers present -some remains in final product but it does not affect the product - -how much initiator is needed in polymerization and does any remain at the end of the reactions? 1. the monomer used 2. the molecular weight of the final polymer chain - -two main things that control polymer properties Radical can attack + homolyze C=C of monomer to create a reactive intermediate that starts polymerization - -what happens after the radical initiator has formed radicals? Adds to the LEAST substituted carbon of the C=C bond because this carbon is easier to approach and less hindered to produce a more stable radical intermediate - -which substituent does the radical add to in the monomer? Produces a new radical that can continue the polymerization chain reaction: Each attack of the radical with a new monomer unit extends the chain by one unit - What happens during propagation of free-radical polymerization? Some step as initiation in that monomers add in head to tail fashion with substituents present on alternating carbons in the chain - -what is chain propagation the same as? 1. temperature 2. pressure nt used er concentration - -What 4 main factors determine the extent of the polymerization? Extremely rapid- can grow by thousands of monomers in less than a second - -speed of free radical chain growth Stops the chain growth, involves 2 radical species- pairing of 2 unpaired electrons and formation of new covalent bond - -Termination of free-radical polymerization 1. Radical Coupling 2. Radical Disproportionation - -2 pathways of termination Radical Coupling - -termination of free-radical polymerization where 2 radicals combine using the unpaired electrons on each to make a new covalent bond Head-to-head arrangement, substituents are attached to adjacent carbons - -monomer arrangement in radical coupling Organic Chemistry Organic Chemistry radical disproportionation - -One radical abstracts a hydrogen atom from another radical species which forms a new covalent bond and makes and alkane- then an alkene is formed by the combination of unpaired electrons on adjacent carbons Both form non-radical species from radical species which stops the growth of the polymer chain - -Radical Coupling AND radical disproportionation form what type of species from radical species? 1. Initiation 2. Propagation 3. Termination - -3 steps in a Chain Reaction of free-radical polymerization Initiation of Free Radicals - -production of a radical species from a non-radical species using heat or UV light Propagation of Free Radicals - -reaction of a radical with a non-radical producing a new radical species and continuing the chain reaction Termination of Free Radicals - -reaction of 2 radicals with each other producing a non radical- no intermediate reactive = reaction stops chain transfer reaction - -type of propagation step where a radical in one polymer chain abstracts a hydrogen atom from a position in a different polymer chain which causes branching in a growing polymer chain Rates of the straight chain propagation steps versus the chain transfer steps -Rates are controlled by stability of the intermediate radicals that form as a product in each - -what is chain transfer dependent on? Cation Chain-Growth Polymerization - -best for substrates that can form stable carbocation intermediates (typically unsubstituted substrates like bulky alkenes or alkenes with electron-donating substituents) How is cation addition polymerization initiated? - -By adding a strong acid to an alkene to form a carbocation intermediate Cation addition polymerization propagation - -carbocation intermediate + new alkene monomer molecule adds one unit to the polymer chain each time Cation addition polymerization termination - -terminates by the removal of a hydrogen atom from a carbon atom adjacent to the positively charged carbon to form an alkene (like in an elimination reaction) Anionic addition polymerization - -a form of chain-growth polymerization or addition polymerization that involves the polymerization of monomers initiated with anions. The type of reaction has many manifestations, but traditionally vinyl monomers are used. Organic Chemistry Organic Chemistry Why use anionic addition polymerization - -useful for alkenes with electron-withdrawing substituents such as cyano groups, phenyls, esters Grignard Reagents (R: -[MgBr]+) Alkyllithium (R:-Li+) - -main catalysts used for anionic addition polymerization Anionic addition polymerization propagation - -chain grows by 1 anionic intermediate adding to the C=C bond of a monomer molecule Monomer anion adds in place of a Grignard Reagent or Alkyllithium reagent -each addition grows by one unit Anionic addition polymerization termination - -accomplished by quenching reaction with a proton source such as water or alcohol a new chiral center is formed at every position where the substituent branches from the back bone of the chain - -what is formed when a monosubstituted alkene monomer is polymerized? Tactility - -describes the location in the polymer chain of chiral centers rather than using R or S designation 1. Atactic 2. Isotactic 3.Syndiotactic - -3 main classes of polymer tacticity atactic - -stereocenters have random configurations Isotactic - -all stereocenters have the same configuration syndiotactic - -Stereocenters alternate in configuration Stereorandom - -atactic configuration is labeled as Stereoregular - -isotactic and syndiotactic are labeled as The same monomers with different tacticity will have different physical properties EX: atactic polypropylene (soft matrix adhesive) vs isotactic polypropylene (high-melting solid that can be molded or machined) - -How does having different tacticity with the same monomer effect the monomer? 1. chain-end control 2. Site control - -2 ways to control polymer stereochemistry Chain end control - -at an existing stereocenter and the end of the chain, the next monomer will be influenced by how the chiral center projects into space Organic Chemistry Organic Chemistry Preferential formation of chain-end control - --1st chiral center that forms will determine all subsequent stereochemistry in the polymer -an existing chiral center will force an incoming group to one side of the molecule Site Control - -The specific shape of the reagent or catalyst that facilitates the reaction also determines the stereochemistry. Ziegler-Natta catalyst - -most famous site control of stereoregular polymers that uses various transition metal catalysts to control sites where monomers are added to the chain Ligands - -A molecule that binds specifically to a receptor site of another molecule. -coordinated to the metal ion that bind monomers prior to insertion into the growing chain -specifically utilized in Ziegler-Natta catalysts Shape and size hold monomers into position in ONE orientation - -what 2 factors of ligands orient monomers into position Isotactic and Syndiotactic (only ste