Polymers Chapter 30 Light weight Flexible Easily processable Transparent (sometimes) Strong Elastic Cheap

Polymers Polyisoprene (natural rubber) Macromolecules > 10,000 grams/mole (e.g. proteins, DNA) poly = many mer = units or pieces 1000 g/mole Polyisoprene (natural rubber)

Polymers in Common Products They are everywhere

Polymers have non-Newtonian Properties Long macromolecules: 100,000 x longer than diameter Entanglements are slow to disentangle Result: Flexible, tough, strong materials Sticky & viscous in solution or melted

Types of Polymers Elastomers Thermoplastics Thermosets Polyisoprene, Neoprene, Spandex or Lycra Silicones Polystyrene Polycarbonate Polyethylene Nylon Polyester Epoxies Some urethanes Cured polyesters Formaldehyde resins Strong Inflexible Insoluble and does not soften with heat Rubbery Elastic Tough Flexible Softens with heat

Polymer Structure and Properties The large size of polymer molecules gives them some unique physical properties compared with small organic molecules. Linear and branched polymers do not form crystalline solids because their long chains prevent efficient packing in a crystal lattice. Most polymers have crystalline regions and amorphous regions.

Crystallites Crystallites: These are ordered crystalline regions of the polymer that lie in close proximity and are held together by intermolecular interactions, such as van der Waals forces or hydrogen bonding. Crystalline regions impart toughness to a polymer. The greater the crystallinity (i.e., the larger the percentage of ordered regions), the harder the polymer.

Amorphous Regions Amorphous regions: These are segments of the polymer structure where the polymer chains are randomly arranged, resulting in weaker intermolecular interactions. Amorphous regions impart flexibility. Branched polymers are generally more amorphous, and since branching prevents chains from packing closely, they are also softer.

Polymer Transition Temperatures Two temperatures, Tg and Tm, often characterize a polymer’s behavior. Glass transition temperature (Tg): temperature at which a hard amorphous polymer becomes soft. Melt transition temperature (Tm): temperature at which crystalline regions of the polymer melt to become amorphous. More ordered polymers have higher Tm values.

Processing Thermoplastics Rule of Thumb Amorphous: Tg + 80 °C Crystalline: Tm + 30 °C

Chain-Growth and Step-Growth Polymers Synthetic polymers may be classified as either chain-growth (addition) or step-growth (condensation) polymers. Chain-growth polymers are prepared by chain reactions. Monomers are added to the growing end of a polymer chain. The conversion of vinyl chloride to poly(vinyl chloride) is an example.

Step-Growth Polymers Step-growth polymers are formed when monomers containing two functional groups are joined together and lose a small molecule such as H2O or HCl. In this method, any two reactive molecules can combine, so that monomer is not necessarily added to the end of a growing chain. Step-growth polymerization is used to prepare polyamides and polyesters.

Molecular Formulae of Polymers Polymers generally have high molecular weights ranging from 10,000 to 1,000,000 g/mol. Synthetic polymers are really mixtures of individual polymer chains of varying lengths, so the reported molecular weight is an average value based on the average size of the polymer chain. By convention, the written structure of a polymer is simplified by placing brackets around the repeating unit that forms the chain. Figure 30.2 Drawing a polymer in a shorthand representation

Chain-Growth (Addition) Polymers Chain-growth polymerization is a chain reaction that converts an organic starting material, usually an alkene, to a polymer via a reactive intermediate—a radical, cation, or anion.

Chain growth or Addition polymerizations: Monomers & polymers

Radical Polymerization Radical polymerization of CH2=CHZ is favored by Z substituents that stabilize a radical by electron delocalization. Each initiation step occurs to put the intermediate radical on the carbon bearing the Z substituent. With styrene as the starting material, the intermediate radical is benzylic and highly resonance stabilized.

Disproportionation Chain termination can occur by radical coupling, or by disproportionation, a process in which a hydrogen atom is transferred from one polymer radical to another, forming a new C–H bond on one polymer chain, and a double bond on the other.

Polystyrene Tensile Strength: 45 MPa, Modulus = 3.2 GPa Elongation 4% Amorphous Polystyrene Commercial poly(styrene), PS, is a substantially linear, atactic polymer. Chain stiffness induced by the phenyl substituent creates a high Tg (105°C), Tensile Strength: 45 MPa, Modulus = 3.2 GPa Elongation 4% Styrofoam, molded objects such as tableware (forks, knives and spoons), trays, videocassette cases. Styrofoam, molded objects such as tableware (forks, knives and spoons), trays, videocassette cases.

semicrystalline Teflon PTFE – Polytetrafluoroethylene – aka Teflon long name, simple structure: Exceptional resistance to solvents, great lubricant, nothing sticks to it! The fluorine-carbon bonds are very strong, fluorines protect carbon backbone. High melting point 330 C High electrical breakdown – artificial muscle. Technically a thermoplastic, but hard to process. Opaque due to crystallinity Tensile Strength: 30 MPa Modulus: 410 MPa 350% elongation

Polyvinyl Chloride PVC No Plasticizer: Rigid Polymer (pipe) amorphous Polyvinyl Chloride PVC No Plasticizer: Rigid Polymer (pipe) Tensile Strength: 65 MPa, Modulus = 3.5 GPa Elongation 10% Saran Wrap, floor tiles, bottles 40 wt% Plasticizer: soft pliable (Tygon tubing) Tensile Strength: 15 MPa Elongation 400% Synthetic leather, shower curtains

Chain Branching There are two common types of polyethylene—high-density polyethylene (HDPE) and low-density polyethylene (LDPE). HDPE consists of long chains of CH2 groups joined together in a linear fashion. It is strong and hard because the linear chains pack well, resulting in stronger van der Waals interactions. It is used in milk containers and water jugs. LDPE consists of long chains with many branches along the chain. The branching prohibits the chains from packing well, so LDPE has weaker intermolecular interactions, making it a much softer and pliable material. It is used in plastic bags and insulation.

Chain Branching High density polyethylene Low density polyethylene

Branching in LDPE

Chain Branching Mechanism Branching occurs when a radical on one growing polyethylene chain abstracts a hydrogen atom from a CH2 group in another polymer chain. Incorrect mechanism

Cationic Polymerization of C=C monomers Cationic polymerization is an example of electrophilic addition to an alkene involving carbocations. Cationic polymerization occurs with alkene monomers that have substituents capable of stabilizing intermediate carbocations, such as alkyl or other electron-donor groups. The initiator is an electrophile such as a proton source or Lewis acid. Since cationic polymerization involves carbocations, addition follows Markovnikov’s rule to form the more stable carbocation. Chain termination occurs by a variety of pathways, such as loss of a proton to form an alkene.

Polymers from Cationic Polymerization Figure 30.4a

Anionic Polymerization Alkenes readily react with electron-deficient radicals and electrophiles, but not (generally) with anions and other nucleophiles. Anionic polymerization takes place only with alkene monomers that contain electron-withdrawing groups such as COR, COOR, or CN, which can stabilize an intermediate negative charge. The initiator in anionic polymerization is a strong nucleophile, such as an organolithium reagent, RLi.

Anionic Polymerization There are no efficient methods of terminating anionic polymerizations. The reaction continues until all the initiator and monomer have been consumed so that the end of the polymer chain contains a carbanion. Anionic polymerization is called living polymerization because polymerization will begin again if more monomer is added at this stage. To terminate anionic polymerization an electrophile such as H2O or CO2 must be added. Diene polymerizations, polystyrene

Polymers from Anionic Polymerization Figure 30.4b NO!!!!! Water is the initiator

Copolymers Copolymers are polymers prepared by joining two or more monomers (X and Y) together.

Structure of Copolymers The structure of a copolymer depends on the relative reactivity of X and Y, as well as the conditions used for polymerization. Several copolymers are commercially important: Saran food wrap is made from vinyl chloride and vinylidene chloride. Automobile tires are made from 1,3-butadiene and styrene.

ABS: High strength, dimensional stability, impact resistance Poor UV resistance Telephones, PC housing & keyboards, ... Grafted with polybutadiene

Anionic Polymerization of Epoxides Anionic polymerization of epoxides can be used to form polyethers. For example, the ring opening of ethylene oxide with OH as initiator affords an alkoxide nucleophile which propagates the chain by reacting with more ethylene oxide. Polymerization of ethylene oxide forms poly(ethylene glycol), PEG, a polymer used in lotions and creams.

Anionic Polymerization of Epoxides Under anionic conditions, the ring opening follows an SN2 mechanism. Thus, the ring opening of an unsymmetrical epoxide occurs at the more accessible, less substituted carbon.

Polymer Stereochemistry Polymers prepared from monosubstituted alkene monomers (CH2=CHZ) can exist in three different configurations: isotactic, syndiotactic, and atactic.

Ziegler-Natta Catalysts (Coordination) The more regular arrangement of Z substituents makes isotactic and syndiotactic polymers pack together better, making the polymer stronger and more rigid. Chains of atactic polymer tend to pack less closely together, resulting in a lower melting point and a softer polymer. Radical polymerizations often afford atactic polymers. Reaction conditions can greatly affect the stereochemistry of the polymer formed. The use of Ziegler-Natta catalysts permits easy control of polymer stereochemistry, with the formation of isotactic, syndiotactic, or atactic polymers dependent on the catalyst used. Most Ziegler-Natta catalysts consist of an organoaluminum compounds such as (CH3CH2)2AlCl or TiCl4.

Polypropylene Tensile Strength: 31-41 MPa, Modulus = 1.2-1.7 Gpa semicrystalline Tensile Strength: 31-41 MPa, Modulus = 1.2-1.7 Gpa Elongation 100-600% Living Hinge

Mechanistic details are not known with certainty.

Natural Rubbers Natural rubber is a terpene composed of repeating isoprene units, in which all the double bonds have the Z configuration. Since natural rubber is a hydrocarbon, it is water insoluble, making it useful for water proofing. The Z double bonds cause bends and kinks in the polymer chain, making it a soft material.

Gutta-Percha Rubber The polymerization of isoprene under radical conditions forms a stereoisomer of natural rubber called gutta-percha, in which all the double bonds have the E configuration. Gutta-percha is also naturally occurring, but is less common than its Z stereoisomer. Polymerization of isoprene with a Ziegler-Natta catalyst forms natural rubber with all the double bonds having the desired Z configuration.

Polymer Stereochemistry Natural rubber is too soft to be used in most applications. When natural rubber is stretched, the chains become elongated and slide past each other until the material pulls apart. In 1939, Charles Goodyear discovered that mixing hot rubber with sulfur produced a stronger more elastic material. This process is called vulcanization. Vulcanization results in cross-linking of the hydrocarbon chains by disulfide bonds. When the polymer is stretched, the chains no longer can slide past each other, and tearing does not occur. Vulcanized rubber is an elastomer, a polymer that stretches when stressed but then returns to its original shape when the stress is alleviated.

Elastomers n = 40 Block copolymer elastomers

Vulcanized Rubber Figure 30.5 No, common mistake!!!!! WRONG!!!!!

Vulcanization of dienes with sulfur Allylic sites react with sulfur by alder-ene chemistry

Elasticity of polymers High entropy Low entropy At temperatures above a polymers glass transition temperature it is a rubber Under stress, the polymer chains elongate, but are held in check by entanglements or crosslinks that prevent the bulk polymer from breaking. Entropy spring

Synthetic Rubber NO!! Free radical The degree of cross-linking affects the rubber’s properties. Harder rubber used for automobile tires has more cross-linking than the softer rubber used for rubber bands. Other synthetic rubbers can be prepared by the polymerization of different 1,3-dienes using Ziegler-Natta catalysts. For example, polymerization of 1,3-butadiene affords (Z)-poly(1,3-butadiene). Polymerization of 2-chloro-1,3-butadiene yields neoprene, a polymer used in wet suits and tires. NO!! Free radical

Step-Growth Polymers Step-growth polymers are formed when monomers containing two functional groups come together with loss of a small molecule such as H2O or HCl. Commercially important step-growth polymers include: Polyamides (can also be chain growth) Polyesters Polyurethanes Polycarbonates Epoxy resins

Polyamides Nylons are polyamides formed from step-growth polymerization. Nylon 6,6 can be prepared by the reaction of a diacid chloride with a diamine, or by heating adipic acid and 1,6-diaminohexane. A BrØnsted-Lowry acid–base reaction forms a diammonium salt which loses H2O at high temperature.

Nylon 6 Nylon 6 is another polyamide which is made by heating an aqueous solution of -caprolactam. The seven-membered ring of the lactam is ring opened to form 6-aminohexanoic acid, the monomer that reacts with more lactam to form the polyamide chain.

Nylon 6,6: Excellent wear resistance & slick surface semicrystalline Nylon 6,6: Excellent wear resistance & slick surface Poor dimensional stability & high cost Gear, engine fan Strengths: Good Toughness & Strength Good Chemical resistance Limitations: Strong acidic environments Areas where moisture absorption is of concern -20% strength with humid environment Areas experiencing high operating temperatures Nylon 6: Tensile yield 76 MPa; Tensile modulus 1.4 GPa, elongation 250% Nylon 6,6: Tensile yield 80 MPa; Tensile modulus 2 GPa, elongation 200% Interchangable for most applications

Kevlar Kevlar is a polyamide formed from terephthalic acid and 1,4-diaminobenzene. The aromatic rings of the polymer backbone make the chains less flexible, resulting in a very strong material. Kevlar is light in weight compared to other materials of similar strength. It is used for bulletproof vests, army helmets and protective clothing used by firefighters.

Polyesters Polyesters are formed using nucleophilic acyl substitution reactions. For example, the reaction of terephthalic acid and ethylene glycol forms polyethylene terephthalate (PET), a polymer commonly used in plastic soda bottles. It is also sold as Dacron, a lightweight and durable material used in textile manufacturing.

Polyester films: Mylar = PETE Film Dacron = PETE fiber semicrystalline Mylar = PETE Film Dacron = PETE fiber Tg = 80 °C Tm = 260 °C Tensile Strength: 48-72 MPa, Modulus = 2.7-4.1 Gpa 50-300% elongation Teonex = PEN Film Tg = 120 °C Tm = 262 °C

Biodegradible Plastic Although PET is a very stable material, some polyesters are more readily hydrolyzed to carboxylic acids and alcohols in aqueous medium, making them useful in applications where slow degradation is useful. Copolymerization of glycolic acid and lactic acid forms a copolymer used by surgeons in dissolving sutures.

Urethanes A urethane (also called a carbamate) is a compound that contains a carbonyl group bonded to both an OR group and an NHR or NR2 group. Urethanes are prepared by the nucleophilic addition of an alcohol to the carboxyl group of an isocyanate, RN=C=O.

Polyurethanes Polyurethanes are formed by the reaction of a diisocyanate and a diol. A well-known polyurethane that illustrates how the macroscopic properties of a polymer depend on its structure at the molecular level is Spandex. At the molecular level, it has rigid regions that are joined together by soft flexible segments. Spandex is routinely used in both men’s and women’s active wear.

Polycarbonates A polycarbonate is a compound that contains a carbonyl group bonded to two OR groups. Carbonates can be prepared by the reaction of phosgene (Cl2C=O) with two equivalents of an alcohol (ROH). Polycarbonates are formed from phosgene and a diol. The most widely used polycarbonate is Lexan, used in bike helmets, goggles, and bulletproof glass.

Polycarbonates Excellent clarity Excellent toughness Good heat resistance Excellent electrical properties Intrinsic flame-retardancy Excellent strength Hot water = gradual embrittlement Crazed surface with exposure to organic solvents

Epoxy Resins Epoxy resins are the material of which “epoxy glue” is comprised. Epoxy resins consist of two components: A fluid prepolymer composed of short polymer chains with reactive epoxides on each end, and a hardener, usually a diamine or triamine that ring opens the epoxides and cross-links the chains together. The prepolymer is formed by reacting two different functional monomers, bisphenol A and epichlorohydrin.

Formation of the Fluid Prepolymer Nucleophilic attack by the phenolic OH groups on the strained epoxide ring affords an alkoxide that displaces Cl by an intramolecular SN2 reaction, forming a new epoxide. Ring opening with a second nucleophile gives a 2° alcohol. When bisphenol A is treated with excess epichlorohydrin, this step-wise process continues until all the phenolic OH groups have been used in ring-opening reactions, leaving epoxy groups on both ends of the polymer chains. This constitutes the fluid prepolymer.

Formation of an Epoxy Resin Epoxy pre-polymer Epoxy Linear Cured Epoxy catalyst

“Infinite” network One macromolecule Epoxy coats inside of steel cans to prevent heavy metals from contaminating food

Synthesis of Bakelite Figure 30.7

Plasticizers Not new car smell If a polymer is too stiff and brittle to be used in practical applications, low molecular weight compounds called plasticizers can be added to soften the polymer and give it flexibility. The plasticizer interacts with the polymer chains, replacing some of the intermolecular interactions between the polymer chains. This lowers the crystallinity of the polymer, making it more amorphous and softer. Not new car smell

New car smell

Plasticizers—Dibutyl Phthalate Dibutyl phthalate is a plasticizer added to poly(vinyl chloride) used in vinyl upholstery and garden hoses. Since plasticizers are more volatile than the high molecular weight polymers, they slowly evaporate eventually making the polymer brittle and easily cracked. Plasticizers like dibutyl phthalate that contain hydrolyzable functional groups are also slowly degraded by chemical reactions.

Environmental Impact of Polymers Polymer synthesis and disposal have a tremendous impact on the environment, and have created two central issues: Where do polymers come from? What raw materials are used for polymer synthesis and what environmental consequences result from their manufacture? What happens to polymers once they are used? How does polymer disposal affect the environment, and what can be done to minimize its negative impact?

Where do Polymers Come From? Until recently, the feedstock for all polymer synthesis has been petroleum. The monomers of virtually all polymer syntheses are made from crude oil, a nonrenewable raw material. For example, nylon 6,6 is prepared industrially from adipic acid and 1,6-diaminohexane, both of which originate from benzene, a product of petroleum refining. Figure 30.8 Synthesis of adipic acid and 1,6-diaminohexane for nylon 6,6 synthesis

Problems with Polymer Synthesis The adipic acid synthesis of nylon 6,6 has other problems. The use of benzene (a carcinogen and liver toxin) is undesirable, particularly in the large quantities demanded by large scale industrial reactions. The required oxidation with HNO3 in step 3 produces N2O as a by-product. N2O depletes ozone in the stratosphere. It also absorbs thermal energy from the earth surface like CO2, and may thus contribute to global warming.

Green Polymer Synthesis The negative environmental impact of polymer synthesis has prompted the development of Green Polymer Syntheses—the use of more environmentally benign methods to synthesize polymers. To date, green polymer synthesis has been approached in a variety of ways: Using starting materials that are derived from renewable sources, rather than petroleum. Using safer less toxic reagents that form fewer by-products. Carrying out reactions in the absence of solvent or in aqueous solution (instead of an organic solvent).

Examples of Green Polymer Synthesis Chemists at Michigan State University have devised a two-step synthesis of adipic acid (used to make nylon) from glucose. The synthesis uses a genetically altered E. coli strain (called a biocatalyst) to convert D-glucose to (2Z,4Z)-2,4-hexadienoic acid, which is then hydrogenated to adipic acid.

Green Polyester Synthesis Sorona, DuPont’s trade name for polypropylene terephthalate, can now be made at least in part from glucose derived from a plant source such as corn. A biocatalyst converts D-glucose to 1,3-propanediol, which forms polypropylene terephthalate on reaction with terephthalic acid. Figure 30.9 A swimsuit made (in part) from corn—The synthesis of Poly(trimethylene terephthalate) from 1,3-propanediol derived from corn

Avoiding Solvent Use Other approaches have concentrated on using less hazardous reagents and avoiding solvents. Lexan can now be prepared by using bisphenol A with diphenyl carbonate in the absence of solvent. This avoids the use of phosgene, an acutely toxic reagent.

Problems with Polymer Disposal The same desirable characteristics that make polymers popular materials for consumer products—durability, strength, and lack of reactivity—also contribute to environmental problems. Because polymers do not degrade readily, billions of pounds of them end up in landfills every year. Two solutions to address the waste problem are: Recycling existing polymer types to make new materials Using biodegradable polymers that will decompose in a finite and limited time span.

Polymer Recycling Currently, ~23% of all plastics are recycled in the United States. Although thousands of different synthetic polymers have now been prepared, six compounds called the “Big Six,” account for 76% of the synthetic polymers produced in the U.S. each year. Each polymer is assigned a recycling code (1–6) that indicates its ease of recycling; the lower the number, the easier it is to recycle. Recycling begins with sorting plastics by type, shredding the plastics into small chips, and washing the chips to remove adhesives and labels. After the chips are dried and any metal caps or rings are removed, the polymer chips are melted and molded for reuse.

Chemical Polymer Recycling An alternative recycling process is to re-convert polymers back to the monomers from which they were made, a process that has been successful with acyl compounds that contain C–O or C–N bonds in the polymer backbone. For example, heating PET with CH3OH cleaves the esters of the polymer chain to give ethylene glycol and dimethyl terephthalate. These monomers can serve as starting materials for more PET. Similar treatment of discarded nylon 6 polymer with NH3 cleaves the polyamide backbone, forming -caprolactam, which can be purified and re-converted to nylon 6.

Examples of Chemical Polymer Recycling

Biodegradable Polymers Another solution to the accumulation of waste polymers in landfills is to design biodegradable polymers. A biodegradable polymer is a polymer that can be degraded by microorganisms—bacteria, fungi, or algae—naturally present in the environment. Several biodegradable polyesters have now been developed [e.g., polyhydroxyalkanoates (PHAs), which are polymers of 3-hydroxybutyric acid or 3-hydroxyvaleric acid].

Biodegradable Polymers—PHAs The two most common PHAs are polyhydroxybutyrate (PHB) and a copolymer of polyhydroxybutyrate and polyhydroxyvalerate (PHBV). PHAs can be used as films, fibers, and coatings for hot beverage cups made of paper. Bacteria in the soil readily degrade PHAs, and in the presence of oxygen, the final degradation products are CO2 and H2O.

PHAs An additional advantage of the PHAs is the polymers can be produced by fermentation. Certain bacteria produce PHAs for energy storage when they are grown in glucose solution in the absence of certain nutrients. The polymer forms as discrete granules within the bacterial cell. These are removed by extraction to give a white powder that can be melted and modified into a variety of different products.

Biodegradable Polymers Biodegradable polyamides have also been prepared from amino acids (e.g., aspartic acid can be converted to polyaspartate, abbreviated TPA). It is a commonly used alternative to poly(acrylic acid), which is used to line pumps and boilers of wastewater treatment facilities.