Two typical examples are shown in Fig 4 below H H H H H C Page 59 of 73 C C C C

Two typical examples are shown in fig 4 below h h h h

This preview shows page 58 - 61 out of 73 pages.

Two typical examples are shown in Fig. 4 below. H H H H H C
Image of page 58
~ Page 59 of 73 ~ C C C C H H H H H Ethylene Propylene (derived from ethane) (derived from propane) Fig. 4 3) NAPHTHENES AND AROMATICS These both have ring-shaped molecules, as shown in Fig. 5 below. Materials made from a ring-shaped molecule have improved mechanical properties, for example the high tensile strength of nylon. Naphthenes have saturated molecules and names beginning with ‘cyclo’ (as in cyclohexane). Aromatics, on the other hand, are unsaturated and are chemically highly reactive, being used in solvents and explosives. Aromatics are rare in crude oils (except those found in California), but occur in coal. They form the basis of the styrene group of plastics. Fig. 5 Common naphthenes and aromatics POLYMERISATION Polymerization is the term for chemical reactions in which comparatively simple substances react together to give large, complex molecular forms with very high molecular weights. The reactants in polymerization reactions are known as monomers (single units) and the product of the reaction is a polymer (many units). Polymerization of monomers, the repeated linking of the monomers together to form the polymer which may contain thousands of monomer units, can be achieved in two ways: Addition polymerization (chain reaction); Condensation polymerization (step reaction). ADDITION POLYMERIZATION This polymerization process is of the simplest form. It involves the addition of monomers either of the same kind (homo-polymers) or of different kind (co- polymerization). This reaction generally occurs in unsaturated molecules by the breaking of double bonds between atoms in monomers and the forming of two single bonds in their place.
Image of page 59
~ Page 60 of 73 ~ The conditions required are generally pressure, temperature and a catalyst to break the multiple bonds which are unstable and weaker than single bonds. Double bond when opened has a free valence electron and can combine with additional molecules. Molecular weight of such chains can be up to 10,000. These are formed in case of thermoplastic polymers. This kind of polymerization takes place by three steps namely, (i) Initiation (ii) chain propagation (iii) termination a) HOMOPOLYMERS A homopolymer is made by the polymerization of a single monomer. Conversion of propylene to polypropylene is an example of addition polymerization of a homo-polymer. Once polymerization process is initiated, it does not continue indefinitely since it is impossible to link all the monomers in a polymer material into one long continuous chain. The polymerization is terminated by a collision between the active ends of two chains or by addition of a terminator, such as free radicals from the catalyst. b) COPOLYMERS A copolymer is made by the polymerization of two species of monomer.
Image of page 60
Image of page 61

You've reached the end of your free preview.

Want to read all 73 pages?

  • Fall '19

What students are saying

  • Left Quote Icon

    As a current student on this bumpy collegiate pathway, I stumbled upon Course Hero, where I can find study resources for nearly all my courses, get online help from tutors 24/7, and even share my old projects, papers, and lecture notes with other students.

    Student Picture

    Kiran Temple University Fox School of Business ‘17, Course Hero Intern

  • Left Quote Icon

    I cannot even describe how much Course Hero helped me this summer. It’s truly become something I can always rely on and help me. In the end, I was not only able to survive summer classes, but I was able to thrive thanks to Course Hero.

    Student Picture

    Dana University of Pennsylvania ‘17, Course Hero Intern

  • Left Quote Icon

    The ability to access any university’s resources through Course Hero proved invaluable in my case. I was behind on Tulane coursework and actually used UCLA’s materials to help me move forward and get everything together on time.

    Student Picture

    Jill Tulane University ‘16, Course Hero Intern

Stuck? We have tutors online 24/7 who can help you get unstuck.
A+ icon
Ask Expert Tutors You can ask You can ask You can ask (will expire )
Answers in as fast as 15 minutes
A+ icon
Ask Expert Tutors