Polymers can be grouped based on their chemical properties. These we will be considered n details in Modules 3 to 5.
The two types of polymerizations lead to the formation of 'addition polymers' and 'condensation polymers'. Type of functional groups and number of functional groups in the monomer determines the chemical groupings polymers would have and be.
The polymerization is general amongst unsaturated monomers, and there is no loss of smaller units, as in the formation of condensation polymers. It is as well termed to as chain-growth polymerization. Ordinary monomers are the unsaturated olefin compounds. They shape vinyl polymers. Addition polymerization takes place in the subsequent patterns of reactions:
1. Radical and non-radical addition polymerization,
2. Cationic calculation polymerization;
3. Anionic calculation polymerization;
4. Catalyzed and uncatalyzed addition polymerization;
5. Ring opening polymerization.
Each is characterized via incidence of 3 steps in the mechanism of the reaction- the initiation, propagation and termination stages.
Formation of condensation polymers engages the linking of monomers through loss of smaller units as H2O, CO2, NH3 molecules. It is as well termed to as step-growth polymerization. The monomers involved are generally saturated. Polyamides and polyesters are good instances condensed polymers.
Depend on functional group(s) and number of kinds of monomers that form the polymer.
The number of functional groups present in the monomer (s) in addition to number of kinds of monomers that link to form the polymer is significant in classifying polymers. Linear polymers by simpler structures are shaped from a kind of monomer that has a single functional group. They are termed to as homopolymers. Instances include natural rubber [poly isoprene], polythene and polypropene.
Example: nCH2= CH2 [CH2-CH2] n [for example n=103]
Polymers are termed to as bifunctional when 2 functional groups are on each monomer that forms it for instance vinyl halides [with halogen alpha to double bond], acrylonitriles [with nitrile and double bond], 1,3-butadienes [with 2 double bonds], isoprene [2-methyl-1,3-butadiene], phenyl ethene [styrene].
Activity A: Classify each of the subsequent bifunctional monomers via matching by the appropriate names. As well identify functional groups on each.
Many times more than a kind of monomer is engaged in producing a polymer. Polymers shaped from only 2 kinds of monomers are termed Copolymers. They are diversified by different arrangements of the monomers. The subsequent copolymers are recognized depend on the pattern of arrangements that is linkages of the 2 kinds of monomers in the structure of each polymer [NB: X and Y represent each of the monomer types]:
(i) Linear alternating copolymer: There is frequent and regular alternation between the two monomers. For example X-Y-X-Y-X-Y- X-Y-X-Y-X-Y- X-Y-X-Y-
(ii) Statistical copolymers: The two monomers are randomly distributed depending on each monomer's reactivity. This also indicates its alternative name 'random copolymer'. For example Y-Y- X-Y-X-X-X-X-X-Y-Y-Y X-Y-X-Y-
(iii) Blocks: Sequence is observed in the copolymer with substantial blocks of each monomer type. i.e. -Y-Y-Y-X-X-X-X-X-Y-Y-Y-X-X-X-X-X-Y-Y-Y-X-X-X-X-X-Y-Y-Y
(iv) Grafted copolymer: Here blocks of the monomers are grafted on backbone of the other as branches.
Many useful and important products are results of different copolymers, like food wrappers, sporting equipments and wears, films etc. More complex polymers formed from not less than three different monomers include terpolymers. Complexity in the structure of polymers formed increases with increase in number of monomers involved, and also with increase in number of functional groups in each monomer.
Activity B: What are trifunctional polymers? Give examples.
Activity C: Identify the monomer(s) in each of the given polymers: polypropene, polytetrafluoroethene.
Allylic positions on polymers with double bonds are very reactive. Their reactions lead to formation of bridges between molecules. This is called cross-linkings. A good example is what we discussed on vulcanization of rubber, where sulphur is heated with natural rubber to form cross-linkings, leading to improvements, in the use of the polymer. Cross-linkings afford polymers to be crystallizable (since molecules lie close together and strong), of high melting point. The crystallite region is destroyed when polymer molecules are heated.
Based on the stereochemistry
Polymers can be grouped as syndiotactic, isotactic, and atactic polymers based on their stereochemistry. [p.1040M&B] Branchings on optically active polymers like methyl groups and their random arrangements called alignment determine its stereochemistry, and its nature and properties. For example in poly-2-methyl propene the following situations can be seen:
(a) When all the methyl groups alternate uniformly, it is the syndiotactic form.
(b) If all the methyl groups are on one side of the structure, it is the isotactic form. It is usually formed by the non-radical mechanism.
(c) For the non-uniform alternation of the methyl groups, it is the atactic form. It is obtained by the radical method.
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