Current Affairs JEE Main & Advanced

(1) Thermocol is polystyrene foamed with vapour of pentane.   (2) Cups used for hot drinks are made up of polystyrene. It does not become soft like other plastics at temperatures near boiling point of water.   (3) A major development of co-ordination polymerisation is stereochemical control. For example Propene could polymerise to any of the three different arrangements.   (i) Isotactic (Same order) : When groups are arranged on one side of the chain. All y group i.e. on one side and all Z groups on the opposite side of the chain.     (ii) Syndiotactic (Alternating order) : The Y and Z groups lie alternately on each side of the chain.     (iii) Atactic (Random order) : The Y and Z groups are arranged in a random fashion.     (4) Addition polymers, generally, have only carbon atoms in their main chain. On the other hand, condensation polymers, generally, have atoms other than carbon atoms, in their main chain.     (5) Polyurethanes : Polyurethanes are polymers obtained by the polymerisation of a urethane.   It is used for heat and sound insulation in the form of polyurethane foam. Mattresses, cushions and pillows made out of polyurethane foam are washable and long lasting.     \[\underset{A\,\text{urethane}}{\mathop{R-\overset{H}{\mathop{\overset{|}{\mathop{N}}\,}}\,-\underset{O}{\mathop{\underset{|\,|}{\mathop{C}}\,}}\,-O{R}'}}\,\]     (6) Epoxy resins : These are obtained by copolymerisation of epichlorohydrin and bisphenol-A. These resins have good adhesive strength. These are used for making adhesives (Araldite, M-seal etc.) for making glass reinforced plastic (fibre glass), for lamination, to impart crease resistance and shrinkage control to cotton, rayon and for making anti-skid surface for highways.     (7) Polycarbonates : These are obtained by copolymerisation of diphenyl carbonate and bisphenol-A. It has very high optical transparency, high impact strength over wide range of temperature. It is used for making bullet-proof glass, baby-feed bottles, fridge containers, mixi jars etc.     (8) Thermoplastics are also called cold setting polymers. They are moulded when hot but set into the required shape only on cooling. Thermosetting polymers are also called heat setting polymers. Such polymers are supplied in the partially polymerised form.     (9) On long exposure to air and sun-light thermo-plastics becomes brittle. It is due to the evaporation of plasticizer with time. The faint smell associated with various thermoplastics is due to slow evaporation of this plasticizer.     (10) High density polyethene is a linear polymer. Carry bags made out of it are not so soft and make a crackling sound when crushed in hands. You can easily tear them in one direction, but not at right angle to it. Plastic twine is made out of such a polymer. They have very high tensile strength in one direction (along the polymer chain) and a low tensile strength at right angle to it. Such carry bags are used to carry clothes, note-books etc. Carry bags made more...

A polymer sample contains chain of varying lengths and therefore its molecular mass is always expressed as an average on the other hand natural polymer such as proteins contain chain of identical length and therefore they have definite molecular mass.     The molecular mass of a polymer can be expressed in two ways.     (1) Number average molecular mass \[({{\bar{M}}_{N}})\]\[\]     (2) Weight average molecular mass \[({{\bar{M}}_{W}})\].     (1) Number average molecular mass \[({{\bar{M}}_{N}})\] : If \[{{N}_{1}},\,{{N}_{2}},\,{{N}_{3}}\]….. are the number of molecules with molecular masses \[{{M}_{1}},{{M}_{2}},\,{{M}_{3}}\]…… respectively, then the number average molecular mass is     \[{{\bar{M}}_{N}}=\frac{{{N}_{1}}{{M}_{1}}+{{N}_{2}}{{M}_{2}}+{{N}_{3}}{{M}_{3}}+...}{{{N}_{1}}+{{N}_{2}}+{{N}_{3}}...}\]     This may be expressed as : \[{{\bar{M}}_{N}}=\frac{\sum {{N}_{i}}{{M}_{i}}}{\sum {{N}_{i}}}\]     Where \[{{N}_{i}}\] is the number of molecules of the ith type with molecular mass \[{{M}_{i}}\].     (2) Weight average molecular mass \[({{\bar{M}}_{W}})\] : If \[{{m}_{1}},\,{{m}_{2}},\,{{m}_{3}}\]…. are the masses of species with molecular masses \[{{M}_{1}},\,{{M}_{2}},\,{{M}_{3}}\]….. respectively, then the weight average molecular mass is     \[{{\bar{M}}_{W}}=\frac{{{m}_{1}}{{M}_{1}}+{{m}_{2}}{{M}_{2}}+{{m}_{3}}{{M}_{3}}....}{{{m}_{1}}+{{m}_{2}}+{{m}_{3}}+...}\] or \[=\frac{\sum {{m}_{i}}{{M}_{i}}}{\sum {{m}_{i}}}\]     But     \[{{m}_{i}}={{N}_{i}}{{M}_{i}}\], so that \[{{\bar{M}}_{W}}=\frac{\sum {{N}_{i}}M_{i}^{2}}{\sum {{N}_{i}}{{M}_{i}}}\]     where \[{{N}_{i}}\] is the number of molecules of mass \[{{M}_{i}}\].  

• Polydispersity index : The ratio of weight average molecular mass to the number average molecular mass is called polydispersity index, PDI.

    \[PDI=\frac{{{{\bar{M}}}_{W}}}{{{{\bar{M}}}_{n}}}\]     This gives an idea about the homogeneity of a polymer.     (i) The polymers whose molecules have nearly same molecular masses are called monodisperse polymers. For these molecules, \[{{\bar{M}}_{W}}={{\bar{M}}_{N}}\] and therefore, PDI is one.       (ii) The polymers whose molecules have wide range of molecular masses are called polydisperse polymers. For these polymers, \[{{\bar{M}}_{W}}>{{\bar{M}}_{N}}\] and therefore, their PDI is greater than one.       Thus, it may be concluded that in general, natural polymers are more homogeneous than synthetic polymers.     For natural polymers, PDI is usually unity and therefore, natural polymers are monodisperse.     For synthetic polymers, the PDI is greater than one and therefore \[{{\bar{M}}_{W}}\] is always greater than \[{{\bar{M}}_{N}}\]. \[{{\bar{M}}_{N}}\] is always determined by employing methods which depend upon the number of molecules present in the polymer sample. For example, colligative property such as osmotic pressure is used. On the other hand, weight average molecular mass is measured by using the methods such as light scattering and ultracentrifugation, sedimentation, etc. which depend upon the mass of individual molecules.     (3) Polymer in increasing order of their intermolecular forces are polythene < Buna S < Nylon-66.     (4) We always use purest monomer in free radical polymerisation reaction because the impurities can act as chain transfer agent and may combine with the free radical to slow down the reaction or even stop the reaction.     (5) Benzoquinone inhibit the free radical polymerisation of vinyl derivative because it combine with free radical intermediate to form a non reactive radical which is highly stabilized by resonance because of the lack of reactivity of the new radical formed, it inhibit the further progress of the more...

These are the polymers which are degraded by micro-organisms within a suitable period so that biodegradable polymers and their degraded products do not cause any serious affects on the environment.   In biological systems, biopolymers degrade mainly by enzymatic hydrolysis and to some extent by oxidation. Therefore, in view of the disposal problems of polymer waste and for developing polymers for other safe uses in human systems, attempts have been made to develop biodegradable synthetic polymers. These synthetic polymers mostly have functional groups which are normally present in biopolymers and lipids.   Among these aliphatic polyesters are one important class of biodegradable polymers which are commercially potential biomaterials. The common examples of biodegradable polymers are polyhydroxy butyrate (PHB), polyhydroxy butyrate –co-\[\beta \]-hydroxy valerate (PHBV), polyglycolic acid (PGA), polylactic acid (PLA), poly (Î-caprolactone) (PCL), etc.   Uses : Biodegradable polymers are used mainly for medical goods such as surgical sutures, tissue in growth materials or for controlled drug release devices, plasma substitutes etc. The decomposition reactions usually involve hydrolysis (either enzymatically induced or by non-enzymatic mechanisms) to non-toxic small molecules which can be metabolized by or excreted from the body. These are also finding use in agriculture materials (such as films, seed coatings), fast food wrappers, personal hygiene products, etc.   (i) Polyhydroxy butyrate (PHB)     Polyhydroxy butyrate (PHB) is obtained from hydroxy butyric acid (3-hydroxy butanoic acid)       (ii) Poly-Hydroxybutyrate-co-\[\beta \]-Hydroxy valerate (PHBV) : It is copolymer of 3-hydroxy butanoic acid and 3-hydroxy pentanoic acid, in which the monomer units are joined by ester linkages.     \[\underset{\text{3-Hydroxy butanoic acid}}{\mathop{nC{{H}_{3}}-\underset{OH}{\mathop{\underset{|}{\mathop{C}}\,H}}\,-C{{H}_{2}}COOH}}\,\,+\,n\underset{\text{3-Hydroxy pentanoic acid}}{\mathop{C{{H}_{3}}-C{{H}_{2}}-\underset{OH}{\mathop{\underset{|}{\mathop{C}}\,H}}\,-C{{H}_{2}}-COOH}}\,\]\[\to \underset{\text{PHBV}}{\mathop{{{\left( -O-\underset{R}{\mathop{\underset{|}{\mathop{C}}\,}}\,H-C{{H}_{2}}-\underset{O}{\mathop{\underset{|\,|}{\mathop{C}}\,}}\,O- \right)}_{n}}}}\,\], \[R=C{{H}_{3}}\], \[{{C}_{2}}{{H}_{5}}\]     The properties of PHBV vary according to the ratio of both the acids. 3-Hydroxy butanoic acid provides stiffness while 3-Hydroxypentanoic acid gives flexibility to the copolymer.   (iii) Polyglycolic acid (PGA) : Polyglycolic acid (PGA) is obtained by the chain polymerisation of cyclic dimer of glycolic acid, \[HO-C{{H}_{2}}-COOH\].     \[\underset{\text{Glycolic acid}}{\mathop{nHO-C{{H}_{2}}COOH}}\,\xrightarrow{\text{Heat}}\underset{\text{Polyglycolic acid (PGA)}}{\mathop{{{\left( -OC{{H}_{2}}\overset{O}{\mathop{\overset{|\,|}{\mathop{C}}\,}}\,- \right)}_{n}}}}\,\]     (iv) Polylactic acid (PLA) : Polylactic acid (PLA) is obtained by polymerisation of the cyclic dimer of lactic acid \[(HO-CH(C{{H}_{3}})COOH)\] or by microbiological synthesis of lactic acid followed by the polycondensation  and removal of water by evaporation.     \[\underset{\text{Lactic acid}}{\mathop{nHO\underset{C{{H}_{3}}\,}{\mathop{\underset{|}{\mathop{C}}\,H\overset{O}{\mathop{\overset{|\,|}{\mathop{C}}\,}}\,}}\,-OH}}\,\xrightarrow{\text{Condensati}\text{on}}\underset{\text{Polylactic acid (PLA)}}{\mathop{{{\left( -O\underset{C{{H}_{3}}\,}{\mathop{\underset{|}{\mathop{C}}\,H-}}\,\overset{O}{\mathop{\overset{|\,|}{\mathop{C}}\,}}\,- \right)}_{n}}}}\,\]     (v) Poly (\[\in \]-caprolactone) (PCL) : It is obtained by chain polymerisation of the lactone of 6-hydroxy hexanoic acid.     \[\underset{\text{PCL}}{\mathop{{{\left( -O-{{(C{{H}_{2}})}_{2}}-\overset{O}{\mathop{\overset{|\,|}{\mathop{C}}\,}}\,- \right)}_{n}}}}\,\]     Uses : PGA and PLA (90 : 10) is used to make absorbable structure to close an internal of external wound and has replaced cat gut these are completely degraded and absorbed by the body within 15 days to one month of the surgery.     Polyhydroxybutyrate (PHB) and (PHBV) have been used for making films for packaging and into moulded items.

  Rubber