III. Synthesis A. Bulk polymerization 1. Initiator dissolved in monomer starts polymerization.

Transcription

1 1 III. Synthesis A. Bulk polymerization 1. Initiator dissolved in monomer starts polymerization. 2. If polymer is soluble, then reaction mix increases viscosity. 3. If polymer is insoluble, viscosity remains constant. 4. Increased viscosity leads to occluded radicals and a rapid autoacceleration. a. popcorn polymerization leads to nodules of highly crosslinked polymer. 5. Encapsuled monomer necessitates use of purification. B. Solution polymerization 1. Solvent must have boiling point above decomposition temperature of initiator and low chain transfer characteristics. 2. Solvent helps keep heat low. 3. Supercritical 2 is a good solvent (no chain transfer). 4. rganic solvent used for polystyrene, PMMA, PV, polybutadiene. 5. Water used for polyacrylic acid, polyacrylamide, poly(nvinylpyrollidone)[pvp].. Suspension polymerization 1. Monomer is dispersed as small droplets in nonsolvent such as water through constant mixing. 2. olloidal stabilizers, such as polyvinyl alcohol or methyl cellulose, are added to prevent droplets to coalesce. 3. Product is granular that is easily filtered. (Separation with spray drying as well.) 4. Temperature controlled with amount of nonsolvent. 5. Poor choice for elastomers, good for hard polymers such as polystyrene, PV, PMMA, poly(styrene-co-acrylonitrile), SAN, poly(vinylchloride-covinylidene chloride).

2 2 D. Emulsion polymerization 1. Monomer droplets created in a nonsolvent using and emulsifying agent. a. Emulsifying agent is a surfactant (to lower surface tension). i. soap salt of fatty acid ii. detergent synthesized anionic surfactant (sodium dodecyl sulfate) iii. nonionic surfactant dodecyl mercaptan b. Emulsified organic monomer (or oligomer) in nonsolvent (such as water) is a latex. 2. Emulsified droplets are called micelles. 3. Formation and size of micelles sensitive to surfactant concentration. a. critical micelle concentration - M 4. Latexes may have good long-term stability. 5. Addition of initiator migrates to micelle to start polymerization. 6. Synthesis may be complicated by the need to remove emulsifier. 7. Water-based paints and adhesives are latexes. (xygen is often initiator.) E. Miscellaneous synthesis issues 1. Synthesis of conjugated dienes a. A diene like 1,3-butadiene, isoprene or chloroprene (2-chlorobutadiene) can polymerize via 1,2 addition or 1,4 addition so that only one double bond used in the polymerization. i. 1,2 addition ii. 1,4 addition (trans) iii. 1,4 addition (cis)

3 3 b. Isoprene and chloroprene can also polymerize via 3,4 addition c. The 1,4-trans isomer is preferred with the 1,2 and cis-1,4 less preferred and the 3, 4 isomer least preferred. d. The type of addition can be controlled with temperature. Monomer Polymerization % cis-1,4 % trans-1,4 % 1,2 % 3,4 temperature ( ) Butadiene Isoprene hloroprene e. The percent of 1,2 isomer appears to somewhat temperature independent. f. The percent of cis isomer increases significantly with temperature. g. The percent of 3,4 isomer increases slightly with temperature.

4 4 2. Synthesis of isolated (nonconjugated) dienes a. In many cases, isolated dienes participate in the crosslinking of a polymer. b. Sometimes cyclic polymers can result if conformations are available to form five- and six-membered rings. + +

5 5 4 IV. Properties and Uses of Free-radical Polymers A. Polyethylene (low-density) LDPE a. Small branching due to back-biting chain transfer b. Branching reduces crystallinity i. 40 to 150 branches per 1000 ethylene units. c. Mechanical properties i. Tensile strength 8 30 MPa ii. Modulus MPa iii. Elongation % d. Melting point 100 e. Density g/cm 3 f. hemically resistant to acids, alcohols, bases and esters. g. Susceptible to attack from hydrocarbons and halohydrocarbons. a. Packaging for frozen foods b. Plastic trash bags c. Garbage cans d. Wash bottles e. Plastic trays f. Laminate on paperboard for juice and milk cartons g. Playground Slides

6 6 2 B. Polyethylene (high-density) DPE a. Low degree of branching ( 6 per 1000 ethylene units) b. Synthesis with low branching is done via Ziegler-Natta polymerization. c. Mechanical properties i. Tensile strength MPa ii. Modulus MPa iii. Elongation % d. Melting point 100 e. Density 0.94 g/cm 3 f. More chemically resistant than LDPE a. Milk jugs b. utdoor furniture c. Soda bottles d. oaxial cable insulation e. Industrial drums f. Fuel tanks g. Natural gas piping h. Water pipes i. Tyvek i. DuPont ii. Fiber and non-woven fabric iii. Difficult to tear, yet easy to cut. iv. Uses a) Mailing envelopes b) ouse wrap c) Disposable laboratory clothing

7 7 5. Polypropylene a. Most industrial PP is isotactic, some atactic, very little syndiotactic b. Tacticity controlled with Ziegler-Natta catalyst. c. Mechanical properties i. Tensile strength MPa ii. Modulus MPa iii. Elongation % d. Melting point e. Density g/cm 3 f. Fatigue resistant g. Autoclavable a. Electrical insulation b. lear bags (atactic) c. oversheets d. Medical equipment e. Food containers (Tupperware, ubbermaid) f. Moisture-wicking clothing

8 8 D. Polytetrafluoroethylene (PTFE) a. Trade name Teflon (DuPont) b. Very low electrical conductivity over wide frequency range c. Very resistant to strong alkali and acid d. Very resistant to solvent, except perfluoroalkanes at high temp. e. Mechanical properties i. Tensile strength MPa ii. Modulus MPa iii. Elongation % f. Melting point 327 g. Density 2.30 g/cm 3 h. oefficient of friction i. Good impact resistance j. Poor abrasion resistance k. Difficult to machine l. Difficult to adhere on surface m. Decomposition temperature above 250 n. Maintains mechanical properties over a wide temperature range o. Made with emulsion polymerization with perfluorooctanoic acid (PFA) as emulsifier. (PFA is toxic and carcinogenic.)

9 9 a. Gaskets b. Low friction machine parts (bearings and bushings) c. ookware i. oating is created by high-pressure sintering of powder on metal surface. ii. PTFE film maintains shape over large temperature range d. Electrical insulation e. Plumber s pipe dope f. Porous membrane for Gore-Tex fabric. 3 E. Polyvinyl chloride (PV) a. Mostly syndiotactic polymer is highly crystalline. b. Most PV is made with suspension polymerization for heat control. c. Mechanical properties i. Tensile strength MPa ii. Modulus MPa iii. Elongation 40 80% d. Melting point e. Density 1.38 g/cm 3 f. Easily extruded above glass transition temperature T g = 82 3 g. Plasticizer must added to reduce crystallinity. i. Dibutyl phthalate ii. Dibutyl sebacate iii. Tributyl phosphate iv. tri-2-ethylhexyl trimellitate (TTM)

10 N h. elatively unstable to heat and light. i. Light stabilizers must be added. ii. eat stabilizers must be added to retard elimination of l during heating. i. Low electrical conductivity at low frequency (power lines) a. Piping b. Gutters c. Siding d. Window frames e. Fabric i. rlon copolymerized with polyacrylonitrile f. Power line insulation g. Food wrap (Polyvinylidene chloride) F. Polyvinylpyrrolidone (PVP) a. Monomer is N-vinyl-2-pyrrolidone. b. Water soluble, dry polymer readily absorbs atmospheric water vapor. 2 2 c. Density 1.2 g/cm 3 d. Glass transition temperature T g = 110 e. Non-toxic f. Forms solutions with Newtonian viscosity a. Blood plasma substitute b. Pharmaceutical binder c. Blended with other polymers (polysulfone) to form wettable membranes d. Binder for ink-jet printer ink. e. Thickener for cosmetic products (shampoo, toothpaste, hair spray, etc )