Chemistry · Ch 15 — Chemistry in Everyday Life
Preparation of Some Important Addition Polymers
Preparation of Some Important Addition Polymers
Three specific addition polymers are worked through as examples of everything covered in 15.4.2, each starting from a small, simple monomer.
Polythene comes in two variants from the same monomer, ethene. LDPE (Low Density Polyethylene) forms by heating ethene at 200-300 degrees Celsius under roughly 1000 atmospheres of pressure with oxygen present as catalyst; the reaction runs through a free-radical mechanism in which peroxides generated from the oxygen act as the initiator. LDPE is used as cable insulation and in making toys. HDPE (High Density Polyethylene) instead forms by polymerising ethylene at 373 K under 6-7 atmospheres, using a Ziegler-Natta catalyst (titanium tetrachloride combined with triethylaluminium). HDPE ends up denser and higher-melting than LDPE, which is why it is used for bottles and pipes rather than flexible insulation.
Teflon (PTFE) comes from the monomer tetrafluoroethylene. Heating this monomer with oxygen, or with ammonium persulphate, under high pressure gives Teflon -- the material used to coat non-stick cookware and other articles that need a chemically inert, low-friction surface. …
Worked out. An addition polymer of ethene, n CH2=CH2 -> -(CH2-CH2)n-. LDPE (Low Density Polyethylene) forms by heating ethene at 200-300 degC and ~1000 atm with oxygen as catalyst, via a free-radical mechanism in which oxygen-derived peroxides act as the free-radical initiator; it is used as cable insulation and for making toys. HDPE (High Density Polyethylene) instead forms by polymerising ethylene at 373 K and 6-7 atm using a Ziegler-Natta catalyst (TiCl4 + (C2H5)3Al); it has higher density and melting point tha …
Worked out. The monomer is tetrafluoroethylene, n CF2=CF2. Heating the monomer with oxygen or ammonium persulphate under high pressure gives Teflon, -(CF2-CF2)n-, used for coating articles and making non-stick coo …
Worked out. Prepared by addition polymerisation of vinyl cyanide (acrylonitrile, systematically prop-2-enenitrile), n CH2=CH-CN, using a peroxide initiator, to give -(CH2-CH(CN))n-. Used as a substitute for wool in blankets, sweaters and similar items. …
Condensation Polymers
Condensation polymers work by a fundamentally different logic than the addition polymers just covered: instead of monomers simply adding together, functional groups on adjacent monomers react with each other and, at each step, a small molecule -- water, ammonia or similar -- is eliminated. For the chain to keep growing at both ends, each monomer needs at least two reactive functional groups (it must be at least bi-functional).
Nylon-6,6 is built from two different bi-functional monomers mixed in equal proportion: hexan-1,6-dioic acid (adipic acid) and hexan-1,6-diamine (hexamethylene diamine). These first combine into an intermediate "nylon salt", which on heating eliminates water and forms amide bonds, ultimately building the repeating polyamide chain known as poly(hexamethylene adipamide) -- nylon-6,6 -- used in textiles and in manufacturing tyre cord.
Nylon-6, by contrast, comes from a single monomer, caprolactam, heated at 533 K in an inert atmosphere with a trace of water. The water first opens caprolactam's ring to give epsilon-aminocaproic acid, which then polymerises (again losing water at each step) into nylon-6. It is used to manufacture tyre-cord fabric.
Terylene (also sold as Dacron) is built from ethylene glycol and terephthalic acid (or its dimethyl ester), mixed and heated at 500 K with a zinc acetate plus antimony trioxide catalyst. The product is a polyester used blended with cotton or wool fibres, and as a glass-reinforcing material in safety helmets. …
Worked out. Made by mixing equimolar hexan-1,6-dioic acid (adipic acid) and hexan-1,6-diamine (hexamethylene diamine), which first form a salt ("nylon salt") that on heating eliminates water to form amide bonds, giving the repeating polyamide chain poly(hexamethylene adipamide) -- i.e. nylon-6,6. Used in textiles and t …
Worked out. Made by heating the monomer caprolactam at 533 K in an inert atmosphere with traces of water; this first opens the caprolactam ring to epsilon-aminocaproic acid, H2N-(CH2)5-COOH, which then polymerises (losing water) to nylon-6, -[NH-(CH2)5-CO]n-. Used to manufactu …
Worked out. The monomers are ethylene glycol (ethan-1,2-diol) and terephthalic acid (benzene-1,4-dicarboxylic acid, or its dimethyl ester). Mixing and heating them at 500 K with a zinc acetate + antimony trioxide catalyst gives terylene, a polyester with repeating -O-CH2-CH2-O-CO-C6H4-CO- units. Used blended with cotton or wool fibres and as glass-reinforcing mat …
Worked out. The monomers are phenol and formaldehyde (methanal), condensed under an acid or base catalyst. Phenol first reacts with methanal to give ortho- or para-hydroxymethylphenol, which reacts with more phenol to build the linear polymer novolac; further heating novolac with formaldehyde cross-links the chains into bakelite. Novolac itself is used in paints; soft bakelites make glue for laminated wooden planks and varnishes; ha …
Worked out. The monomers are melamine (a triazine ring bearing three -NH2 groups) and formaldehyde. Their condensation polymerisation gives melamine-formaldehyde resin, used to make unbreakable crockery. …
Worked out. Formed by condensation polymerisation of urea and formaldehyde, with water eliminated at each successive step as the chain of -NH-CO-NH-CH2- linkages builds up. …