Introduuction to polymer Chemistry

Now a days, polymers have completely invaded our life; and it seems that by the end of this century, our life would be practically impossible without polymers. Some of these polymers occur naturally. The natural polymers e.g. Carbohydrates which provide us food, cloths and shelter. Proteins which are the building blocks of life and nucleic acids - responsible for the storage and transfer of the genetic information. So, in this article let's know about the introduction to polymer Chemistry in easy ways.

Polymer Chemistry

Polymers are man made also- The synthetic polymers-These nit not only resemble the natural polymers but in some respects they are far more superior. We can have fire proof cloths and wear a bullet proof jacket, which is as light as cotton but as tough as steel. We can now have bullet proof glass -which is as light and transparent as ordinary glass. By using special type of plastics, the blinds can be made to see and cripples to walk; heart valves can be replaced and blood arteries can be repaired and many parts of our body can be completely or partly replaced. In near future, some of these polymers can be used as filters to convert sea water into drinking water!

The world of polymers is not only exciting and fascinating but equally challenging. In this chapter, we describe the different classes of synthetic polymers, the reactions by which they are made, and the properties and uses of some important polymers.

Historical Background:

Long long about at least few billion years - life evolved on earth and this can be considered as the beginning of the world of polymers. It is because, from simple molecules like, CO2, H2O and NH3 complex molecules such as proteins were made, which are probably the first kind of polymers that have been produced by nature. With evolution of life, more complex molecules were made by nature such as wood, cellulose and starch and nucleic acids.

Christopher Columbus on his visit to America observed that people played with round bouncing object, which was obtained from the juice of a tree, knows as 'Weeping Wood'. It was nothing but what we now call a rubber ball. This substance could rub off the markings of pencil on paper and hence the name rubber was given to it.

What are Polymers?

Polymers

Polymers are very large molecules (macromolecules) which are made up of many repeating molecular units, which are joined to each other in a regular fashion. Such substances are very common among natural organic substances. Most of the organic polymers are based on a frame work of C-C bonds with O and N atoms are the additional structural units.

The repeating unit of the polymer is termed as monomer and the process by which large numbers of monomers join to form the polymer is termed as polymerisation. The number of monomers present in a polymer is called as the Degree of Polymerisation.

Polymer structure and Nomenclature:

Resins and plastics are polymeric substances. They are derived from monomeric precursors. The monomeric units can be placed together in many ways. The chain can be linear, branched or cross-linked. A homopolymer is derived from a single monomer. These monomeric units can be arranged in various ways. It depends on monomers and other reactants used in synthesis. It also depends on polymerisation conditions. They can be arranged in a regular way or alternate randomly. They can also be arranged in blocks and in branches that arrange from the main chain.

A polymer is named by enclosing within parenthesis the name of the repeating unit, or name of the monomer(s) from which it is derived. It is preceded by the prefix poly. The systematic name of the polymer is derived from the IUPAC desiganation of the repeating units. Occasionally polymers with identical repeating units have different common names. This includes different modes of synthesis. For example, both polyethylene and polyethylene have the same backbone structure.The backbone structure can be shown either -(CH2CH2) - or - (CH2)-.

Molecular weight and Degree of Polymerisation:

The size of a polymer molecule depends on the number of repeat units it contains and we know that this number shows the degree of Polymerisation. If there are 1000 repeat units in a polymer molecule, the degree of Polymerisation is 1000. The molecular weight of a polymer can be expressed with the help of DP by another way as:
M=Dp.m
Where, M= Molecular weight of a polymer,
Dp= degree of Polymerisation
m= the molecular weight of a monomer or repeat units.
Both molecular weight and DP are related to the molecular size. Therefore, like the molecular weight, DP can also be averaged over the size of the sample.

Classification of Polymerisation reactions:

Polymerisation of monomers take place by two chemical reactions.

A) Chain-reaction Polymerisation or Addition Polymerisation reaction:

In this Polymerisation, the monomers add on to each other by a chain reaction. The monomer is converted into a reactive species which reacts with another monomer and a new reactive species is generated, this reacts with next monomers and in this way a large polymer is produced, e.g. free radical Polymerisation of ethylene.
R• + CH2 = CH2 = R-CH2-CH2•
R-CH2-CH2• = R-CH2-CH2-CH2-CH2•
Addition or chain Polymerisation

B) Condensation Polymerisation reaction:

In this type two monomer molecules combine with each other with loss of some small molecules such as water or alcohol. Here individual reaction does not depend on the previous one and proceeds in a stepwise manner, e.g., formation of a polyester from ethylene glycol and terephthalic acid.

In condensation polymers, the empirical formula of the polymers are different from that of the monomer and the molecular weight of the polymer product is lesser by the weight of the simple molecule eliminated during condensation.

Transport properties of a polymer:

Transport properties like diffusivity refer to how fast molecules move through the polymer matrix. These transport properties are very significant in many applications of polymers for films and membranes.

Phase behavior

1)Melting point (Tm) :

The term melting point indicates not a solid-liquid phase transition from a crystalline or semi-crystalline phase to a solid Amphorus phase. It is generally shown as Tm. It means crystalline melting temperature. In case of synthetic polymers, it means decomposition temperature.

2) Glass transition temperature:

It is the temperature at which amorphous polymers go through a transition from rubbery viscous liquid to a brittle, glassy amorphous solid on cooling.

3) Mixing behavior:

This effect results from the reality that the driving force for mixing is usually entropy. In short, miscible materials generally form a solution.

The phase behavior of polymer solutions and mixtures is more complicated than that of small molecule mixtures. Small molecule solutions show only an upper critical solution phase transition where phase separation occurs with cooling. On the other hand polymer mixtures usually presents a lower critical solution temperature phase transition, at which phase separation occurs with heating.

4) Inclusion of Plasticizers:

It tends to low Tg and polymer flexibility. Plasticizers are small molecules. They are chemically similar to polymers. They initiate gaps between polymer chains for higher mobility and lowered interchain interactions. For example, an unplasticized PVC is used for making pipes. A pipe has no plasticizers in it, since it needs to remain strong and heat resistant.

5) Chemical Properties:

Various side groups on the polymer show ionic bonding or hydrogen bonding between its own chains. It results in higher tensile strength and higher crystalline melting points.

Thermodynamics of polymers :

Thermodynamics of Polymerization help to determine the position of the equilibrium between polymer and monomers. The Gibbs free energy of Polymerization is normally used to qualify the course of a polymeric reaction.

In general, Polymerization is an exothermic process. As the process is exothermic, addition of a monomer to the developing polymer chain includes the conversion of pi-bonds into sigma bonds. The Polymerization Will be Thermodynamically favoured by the resulting ∆Gp. Depolymerization is favoured at high temperatures whereas Polymerization is favoured at low temperatures. At definite temperature, the Polymerization touches equilibrium. It is called the ceiling temperature. At this stage ∆Gp=0.

Classification of polymers:

Polymer is genetic name given to a vast number of materials of high molecular weight. Depending upon different chemical structures for monomers, and polymer, their physical properties, thermal characteristics and the ultimate uses, polymers can be classified in different ways as follows:

A) Method 1 : Natural and Synthetic polymers

During on their origin, polymers are classified as natural and synthetic polymers.

Natural polymers:

Polymers found in nature or isolated from natural materials are called natural polymers e.g. starch, cellulose, silk, wool, rubber etc. Starch and cellulose molecules consist of large number of glucose molecules joined together. Protein is obtained as long chain or cross linked Polymerization of amino acids.

Synthetic polymers:

Polymers which are synthesized in the laboratory from low molecular weight compounds are known as synthetic polymers or man made polymers.

B) Method 2): Organic and Inorganic polymers-

Depending upon the nature of atoms present in the background chain of polymers, they are grouped as organic and inorganic polymers. The majority of synthetic polymers are organic and inorganic polymers.

a) Organic polymers:

A polymer in which the backbone chain is made up of carbon atoms is called as organic polymers. The majority of synthetic polymers are organic polymers.

b) Inorganic polymers:

A polymer in which the backbone chain is made up of atoms other than carbon.

C) Method 3: Based on synthetic modes of Polymerization-

There are two modes of Polymerization- addition Polymerization and condensation Polymerization. Based on these modes of ssynthesis the polymers are classified into two main groups- addition polymers and condensation polymers.

D) Method 4: Based on Inter-molercular forces-

Depending upon the Inter-molercular forces present in the polymer molecules, they have been classified into following four categories-

1) Fibres:

These are the polymers having long, thin and thresd like structure with high tensile strength and high modulus.

2) Elastomers:

These are the polymer chains held together by weakest Inter-molercular forces. These weak forces permit the polymer to be stretched.

3) Plastics:

A polymer is shaped into hard and tough articles by application of heat and pressure, it is known as plastics. The molecular structure of plastics is of two general types.