The simplest repeat unit is that for polyethylene, and consists of two carbon atoms linked to four hydrogen atoms. The difference between the monomer and the repeat unit is the loss of the double bond in the former to give the chain-linked repeating group. Thus the molecular masses of both monomer and unit are identical at The molecular mass of the repeat unit is usually designated M R and is simply the sum of the atomic masses A i of the component atoms i of the repeat unit:.
The number of repeat units in a chain is specified by the degree of polymerization, n , but a much more commonly used measure is the chain molecular mass M. Various types of polymer can be generated from similar monomer units, and Table 3 shows three families of closely related polymers.
Replacing a single hydrogen atom in ethylene by a methyl group —CH 3 yields propylene, which when polymerized forms polypropylene PP. When a chlorine atom Cl is substituted, the polymer is PVC and when a benzene ring —C 6 H 5 is substituted, the polymer is polystyrene. When just one hydrogen atom is substituted in the ethylene molecule, so-called vinyl polymers are created.
When both hydrogen atoms on one of the two carbon atoms in the repeat unit are substituted, vinylidene polymers are formed. While most vinyl polymers are rigid thermoplastics at room temperature, introduction of a double bond into the repeat unit creates some of the common rubbers already described see Table 1.
The precursor monomer units are dienes, that is, they possess two double bonds as shown by BR and NR in Table 2. A quite different way of constructing polymer chains is shown by the repeat units of polyamide 6 and 6,6 nylon 6 and nylon 6,6.
Here the monomer molecules are linked together by acid and amine groups at their ends. They are known as functional groups and play a significant role in controlling the physical properties of the final polymer. An important way of studying such functional groups, and other ways in which atoms are linked together in polymer chains, is spectroscopy Box 3. As you work through this course you will need various resources to help you complete some of the activities. Making the decision to study can be a big step, which is why you'll want a trusted University.
Take a look at all Open University courses. IUPAC system: identify repeat unit, starting with highest priority atoms in chain and putting substituents on lowest number positions.
Linear polymers that include rings cyclolinear polymers. Orientation and configuration in polymer chains 1. Head-to-tail orientation of monomer units predominates. Orientation and stereochemistry at double bonds. Note - natural rubber is head-to-tail: "The Isoprene Rule". Tacticity - configuration at chiral centers. Chain Growth Polymerization Step Growth Polymerization The only growth reaction is addition of monomer to a growing chain with a reactive terminus Reaction can occur independently between any pair of molecular species The reaction mixture consists of high polymer and unreacted monomers, with very few actively growing chains The reaction mixture consists of oligomers of many sizes, in a statistically calculable distribution Monomer concentration decreases steadily as reaction time increases Monomers disappear early, in favor of low oligomers High polymer appears immediately, average molecular weight doesn't change much as reaction proceeds Oligomers steadily increase in size, polymer average molecular weight increases as reaction proceeds Increased reaction time increases overall product yield, but doesn't affect polymer average molecular weight Long reaction times are essential to produce polymer with high average molecular weight.
Staudinger established the following major principles: 1 atoms in polymers maintain their normal valences; 2 ends of polymers are normal functional groups; 3 end group analysis can lead to estimate of chain length. These are called thermosetting polymers. Examples include Bakelite and vulcanized rubber. The structural difference between these polymers is that the thermosetting polymers have crosslinks between the chains and the thermoplastic polymers do not.
When a thermoplastic polymer is heated the chains are free to move past each other making the sample less rigid and eventually melting it. This cannot happen with a thermosetting polymer, since its chains are locked together by the cross links. The energy from the heat must eventually go into breaking bonds which leads to decomposition of the polymer. We noticed crosslinking earlier when we saw how the disulfide crosslink formed by oxidation of the SH group in cysteine was important in maintaining protein structure.
Since polymers are made by linking together many identical small molecules, there are repeating units in polymers. In poly vinyl chloride the repeating unit comes directly from the end-to-end linking of many vinyl chloride molecules. A molecule from which a polymer is made is called a monomer.
Each vinyl chloride monomer molecule contributes a CH 2 group joined to a CHCl unit by a single bond. This single bond is a remnant of the double bond which joined those groups in the vinyl chloride molecule.
This is just what happens in an addition reaction of an alkene. We'll see how an addition reaction leads to such polymers in a few paragraphs. We notice that the repeating unit is linked to the rest of the chain by amide functional groups, and that the repeating unit contains an amide group.
We can deduce the structrure of the monomers by imagining the compounds which might be used to make the amide group. In the industrial process these are a dicarboxylic acid adipic acid and a diamine 1,6-hexanediamine. These form a salt when dissoved in alcohol. The salt is heated at o under pressure to form the amide bonds.
Nylon 66 is the result.
0コメント