Consider the following statements regarding polymeric materials : 1. A plastic material that requires heat to make it formable (plastic) and upon cooling, retains its shape is known as thermosetting plastic. 2. The chemical reaction in which high molecular mass molecules are formed from two or more monomers is called chain polymerization. 3. A polymer chain consisting of two or more types of monomeric units is called a copolymer. Which of the above statements is/are correct? Correct Answer 2 and 3 only Explanation: Plastics can be divided into two types thermoplastics and thermosetting plastics. Thermoplastics: They are polymers that can be deformed on heating and can be reformed into different shapes. Examples: Polythene and PVC. Thermosetting plastics: They are polymers that cannot be deformed on heating or remoulded into a new shape. Thermosetting plastics are polymers that become irreversibly rigid on heating. Initially, it is in liquid form or soft solid, heat provides energy for covalent bond formation, crosslinking the polymer subunits. Thus the polymer becomes rigid. Examples: Bakelite, epoxy resin, melamine, etc. Copolymers: Copolymers are those polymers which are made by the copolymerisation of more than one monomeric species. They have different properties than the homopolymers. It contains multiple units of each monomer used in the same polymeric chain. They can be made either by chain growth polymerisation or by step growth polymerisation. Ex. butadiene-styrene copolymer, ethylene-vinyl acetate etc. Polymerization The process by which the monomers get linked up is called polymerization. Types of Polymerization Depending upon the mode of reaction, polymerization is classified as : Addition polymerization Condensation polymerization Addition Polymerization: This process involves the addition of monomer units to themselves to form a growing chain by a chain reaction mechanism. It is for this reason that the process is also known as chain growth polymerization. Addition polymerization is achieved by adding a catalyst (known as initiator), which provides some reactive species like free radicals. Condensation Polymerization: In this, the monomers combine with the elimination of a small molecule like H2O, ROH or NH3 , etc. The reaction is called (step growth) condensation polymerization and the product formed is called condensation polymer. The process involves the elimination of by product molecules, therefore, the molecular mass of the polymer is not the integral multiple of the monomer units