Polymers

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

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

3. Polymers in Common Products They are everywhere

4. 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

5. 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

6. 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.

7. 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.

8. 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.

9. 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.

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

11. 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.

12. 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.

13. 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

14. 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.

15. Chain growth or Addition polymerizations: Monomers & polymers

17. 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.

18. 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.

19. 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.

20. semicrystalline Teflon • PTFE – Polytetrafluoroethylene – aka Teflonlong 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

21. 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

22. 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.

23. Chain Branching High density polyethylene Low density polyethylene

24. Branching in LDPE

25. 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

26. 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.

28. Polymers from Cationic Polymerization Figure 30.4a

29. 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.

31. 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

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

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

34. 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.

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

36. 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.

37. 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.

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

39. 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.

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

41. Mechanistic details are not known with certainty.

42. 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.

43. 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.

44. 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.

45. Elastomers n = 40 Block copolymer elastomers

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

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

48. 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

49. Synthetic Rubber • 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

50. 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