Types of Polymerisation Preparation and Properties
→ There are mainly two types of polymerisation.
→ Addition Polymerisatlon or chain growth Polymerisation
→ This process involves the addition of monomer units to themselves to form growing chain by a addition chain mehanism. So this process is also known as chain growth polymerisation, the monomer units may be same or different.
→ Addition polymerisation is formed by adding a catalyst (known as initiator), which provides some reactive species like free radicals i.e., this process is initiated by inhibitors such as organic peroxides to produce free radicals to which the monomers are added and chain is formed.
→ The polymers ar formed by repeated addition of monomers to the polymer chain e.g. ethylene, propylene, butadiene, tetrafluoroethylene, vinyl chloride etc. The monomers are unsaturated compounds and commonly derivatives of ethene.
→ This mode of poymerisation leading to an increase in chain length or chain growth. This process can take place either through the formation of either free radicaLs or ionic species.
→ However, the free radical governed addition or chain growth polymerisation is the most common mode.
Free radical addition polymerisation: It involves the following steps:
→ Step 1 : A variety of alkenes or dienes and their derivatives are polymerised in the presence of a free radical generating initiator (catalyst) like benzoyl peroxide, acetyl peroxide, tert-butyl peroxide. etc. The process starts with the addition of free radical formed by the peroxide etc., to the monomer unit and thus generating a new and larger free radical. This step is called chain initiating step.
→ Step 2 : This step is Chian Propagation Step. In this, the radical reacts with another molecule to form another bigger sized radical. The repetition of this sequence with new and bigger radicals carries the reaction forward and the step is termed as chain propagating step.
→ Step 3 : This step is Chain Termination Step. Above two reactions continues till monomer units are present. When these monomer units are consumed, then these radical unites to form neutral molecules. Thus, this step is known as Chain Termination Step.
Step 3 : Chain Termination can take place by four methods.
→ Coupling : Two propagating chains containing free radicals combine to neutral molecule.
→ Disproportionation : Neutral molecule is formed by transfer of Hydrogen.
→ Chain Transfer : r1his takes place as:
→ This RH chain is chain transfer agent. In polymerisation, the concentration of monomers decreases and reactants are already taken in small proportions. Hence, when chain termination takes place by chain transfer, it should be done by solvent only.
→ Step 4 : Inhibitors : The propagating chains can be terminated by the use of inhibitors also. Inhibitors are chemical substances which are watched with active free radicals. Hydroquinone, Nit robenzene, Dinitrobenzene etc., are generally used as inhibitors. If P is propagating chain and Nitrobenzene is inhibitor, then chain termination can takes place as:
→ Ionic Addition Polymerisation: Polymers can be prepared by polyrnerisation of almost all the unsaturated monomer units by free radical addition mechanism but In sorne cases polymerisation is also done by ions. For example, polymerisation of vinylderivatives can also be done by this ionic mechanism. Polymerisation of monomers of vinyl derivative containing electorn donor group is done by electrophiles and polymerisation of electron attracting monomers is
done by nucleophiles. On the above basis, ionic addition polymerisation can be of two types:
- Cationic polymerisation
- Anionic polymérisation
→ Cationic Polymerisation : These reactions are catalysed by protons or Lewis acids. Hence these reactions are known as acid catalysed polymerisation. This process takes place in three steps:
→ Step 1 : This is chain initiation step. Lewis acid reacts with co-catalyst and forms proton. This proton react with monomer unit to form carbocation and starts polymerisation.
X is electron donor group.
→ Step 2 : This is chain propagating step. In this step, Carbocation formed in step 1 adds to another monomer unit and from a new carbocation chain. This reaction goes on continuously.
→ Step 3 : Chain Termination Step : In cationic polymerisation, chain termination is done either spontaneously or in the presence of any nucleophile.
→ In this, catalyst and co-catalyst is again generated.
→ Anionic Polymerisation : An anionic initiator will generate a carbanionic intermediate and thus the polymerization is of anionic addition type. Here the active centre of the propagating species is negatively charge. Hence it occurs easily with monomers containing electron withdrawing group such as Phenyl, Nitrile etc. which are able to stabilize the propagating species. It also occurs in three steps:
→ Step 1 : This is chain initiation step. Here, base acts as nucleophile and monomer form carbanion and process of polymerisation starts.
→ Step 2 : This is chain propagating step. In this step, carbanion formed in step 1 adds to another monomer unit and form a new carbanion chain. This reaction oes on continuously.
→ Step 3 : This is Chain Termination step. In anionic polymerisation, chain termination is not spontaneous but can be done in the presence of any electrophile.
Important Points :
→ All Vinyl polymers containing electron attracting groups (—CN, —COOR, Cl etc.) are polymerised by anionic polymerisation.
Example: Vinyl Chloride
Condensation Polymensation :
→ This type of polymerisation generally involves two repetitive condensation reaction between two bi-functional monomers. These polycondensation reactions may result in the loss of some simple molecules as water (H2O), alcohol ROH) ammonia (NH3), etc., and lead to the formation of ‘high molecular mass condensation polymers.
→ In these reactions, the product of each step is again a bi-functional species and the sequence of condensation goes on, Since, each step produces a distinct functionalised species and is independent of each other, this process is also called as step growth polymerisation.
→ In each step, one small molecules is released and product is also a bifunctional species.
→ Formation of Polyester by dialcohol and diamides.
→ Formation of Polyamides (Nylon) by diamines and diacids.
→ Formation of polyether by dialcohols
→ Formation of Polymides by amino acids
→ The formation of terylene or dacron by the interaction of ethy1ene glycol and terephthalic acid.
→ In this way, this reaction completes in many steps and product of each step is a bifunctional species.
Dacron or Terylene (Polyester) :