POLYMER CHEMISTRY Lecture/Lession Plan -2

Size: px

Start display at page:

Download "POLYMER CHEMISTRY Lecture/Lession Plan -2"

Angela Knight
5 years ago
Views:

1 Chapter 6 POLYMER CHEMISTRY Lecture/Lession Plan -2 POLYMER CHEMISTRY Classification on the basis of tactility On the basis of orientation of functional group or side groups throughout the long backbone chain of any polymer polymer can be divided into to three parts: i) Isotactic polymer ii) Syndiotactic polymer iii) Atactic polymer i) Isotactic polymer: Isotactic polymer Polymer in which orientation of functional group or side groups are at the same side throughout the main or long backbone chain are called isotactic polymer. Due to presence of functional group or side groups same side throughout the polymeric backbone chain these polymer cis polymer. Example: Cis polyisoprene (Natural Rubber). ii) Syndiotactic polymer: Syndiotactic polymer 11

2 12 CHAPTER 6. POLYMER CHEMISTRY LECTURE/LESSION PLAN -2 Polymer in which orientation of functional group or side groups are arrange in the alternate fashion throughout the main or long backbone chain are called syndiotactic polymer. Due to presence of functional group or side groups alternate fashion throughout the polymeric backbone chain these polymer also are trans polymer. Example: Trans polyisoprene (Guttaparcha) polypropylene. iii) Atactic polymer: Atactic polymer Polymer in which orientation of functional group or side groups are arrange in the Random or haphazard fashion throughout the main or long backbone chain are called atactic polymer. Example: polypropylene Classification on the basis of molecular force The quality of polymer can be characterized by their different mechanical properties like tensile strength modulus of rupture, modulus of elasticity, elongation at break, flexural strength, impact indentation strength, abrasion test, tearing strength etc. These mechanical properties are depending upon intermolecular forces like Vander Walls forces, dipole-dipole interaction etc. among the polymeric backbone chain and hydrogen bonding. These molecular forces are very much dependent on the functional group in the backbone chain and the size of the polymer molecule. On the basis of molecular forces polymer can be divided into to five parts: i) Elastomer ii) Fiber iii) Thermoplastic iv) Thermosetting plastic i) Elastomer: The polymers in which the molecular forces like Vander Walls forces dipole-dipole interaction among the polymeric backbone chain and hydrogen bondings are very weak resulting the weak mechanical properties like tensile strength modulus of ruptured modulus of elasticity elongation at break flexural strength impact indentation strength abrasion test tearing strength etc. These polymers are stretched over a long range during application of stress and when stress is removed they return back to their original shape. This is the main and important properties of elastomer. Elastomers are mainly having long chain in coiled form with some inter linking among the long chain. During application of stretching force these coils get straight towards the direction of applied force and when the force is withdrawal the interlinking helps to return back to its original form of coiling. Examples: Natural rubber. ii) Fiber: The polymers which have higher inter molecular forces like Vander Walls forces dipoledipole interaction among the polymeric backbone chain and hydrogen bondings than elastomers and fiber resulting comparatively strong mechanical properties like tensile strength modulus of ruptured modulus of elasticity elongation at break flexural strength impact indentation strength abrasion test tearing strength etc. These polymers are thin and thread type. Due to flexible nature these polymer can be woven. Examples: Nylon66 Terylene Dacron Polyester Orlon Acrilon. iii) Thermoplastic: The polymers which have intermediate inter molecular forces like Vander

3 6.1. ADDITION POLYMERIZATION 13 Walls forces dipole-dipole interaction among the polymeric backbone chain and hydrogen bondings in between elastomers and fiber resulting comparatively strong mechanical properties like tensile strength modulus of ruptured modulus of elasticity elongation at break flexural strength impact indentation strength abrasion test tearing strength etc. higher than elastomer but lower than fiber.it have normally linear polymeric chain sometimes cross linking between chains. The main property of thermoplastics is these are hard at room temperature and on application of heat the polymer become soft and fluid like but when heat source is removed again it become hard. Depending on this characteristic of thermoplastics they can moulded any desire shape after melting. Due to presence of no or few cross linking the polymer chains are slide over other on heating. So thermoplastics polymers are reversible and recyclable. Examples: polyethylene, polyvinylchloride, polypropylene etc. 6.1 Addition polymerization During the process of polymeric chain growth if there is only addition of monomers with out the loss of any small molecule and the polymer is an exact multiple of used monomer unit is called addition polymerization. Example of addition polymers are PE PVC, PP etc Mechanism of addition polymerization Addition polymerization proceed through two types of mechanism A) Free radical mechanism B) Ionic mechanism. A) Ionic mechanism are two typei) Cationic mechanism ii)) Anionic mechanism Every polymerization reaction proceeds through three steps- 1) Chain initiation 2) Chain propagation 3) Chain termination 1) Chain initiation: This is the first step of polymerization reaction where initiator activate the monomer unit resulting in the formation of active center in monomer unit. M(Monomer) + I(Initiator) = M (active center in monomer unit) I (Free radical, heat, pressure, catalyst etc.) 2) Chain propagation: This is the second step of polymerization reaction where second third forth... monomer units are added with activated monomer unit consecutively resulting in the formation of large chain unit.

4 14 CHAPTER 6. POLYMER CHEMISTRY LECTURE/LESSION PLAN -2 M (active center in monomer) + M(Monomer) = M M M M + M(Monomer) = M M M M M M + M(Monomer) = M M M M = (M)n M 3) Chain termination: This is the third and final step of polymerization reaction where termination of active center is removed by the following possible way a) Recombination of two smaller activated monomers b) Recombination of two larger activated polymeric chain c) Recombination of one smaller activated monomer with a larger activated monomer d) Disproportion(Simultaneous oxidation and reduction of two activated monomers) A) Free radical mechanism: It is the way of polymerization where the initiator of the reaction is a free radical. This initiator free radical is produced by many way but one of the important way is decomposition of the compound by sun light, heat, catalyst etc. As example peroxide, hydroperoxide, peracid are very useful initiator in organic polymerization reaction. Benzoyl peroxide Decomposition of benzoyl peroxide in presence of sun light or heat form free radical as initiator- Now, the following three common steps of free radical polymerization reaction are 1) Chain initiation: This is the first step of polymerization reaction where initiator is added to the monomer unit resulting in the formation of free radical center in monomer unit. Free radical chain initiation of vinyl chloride 2) Chain propagation: This is the second rapid step of polymerization reaction where second third forth... monomer units are added with generated free radical monomer unit consecutively resulting in the formation of large chain radical.

5 6.1. ADDITION POLYMERIZATION 15 Free radical chain prpagation of vinyl chloride 3) Chain termination: This is the third and final step of polymerization reaction where termination of free radical after long propagation is takes place by the following possible way A) Recombination of two smaller free radicals: Free radical chain termination of vinyl chloride B)Recombination of two larger polymeric free radical chain: C) Recombination of one smaller free radical monomer with a larger free radical: D) Disproportion:(Simultaneous oxidation and reduction of activated long chain free radical) In this process any one free radical centered carbon atom abstracted hydrogen free radical(reduction) from other(oxidation) free radical and thereby chain termination takes place. One important think is one of the products is unsaturated as below. Other important way of chain termination is- 1) Inhibitor reaction: The process of termination is done by inhibitor, a compound which inhibit the growth of polymerization reaction by termination. This termination is done when inhibitor combine with generated free radical and give comparatively stable product. Oxygen, Nitrobenzene, Dinitrobenzene and phenolic materials are act as inhibitor in many organic polymerization reaction. Example of polymers formed by free radical mechanism are polyethylene, polypropylene, styrene, styrene butadiene etc. B) Ionic polymerization:it is the way of polymerization where the initiator of the reaction is an ion. This initiator ion is formed from specific Lewis acid or base produce cation(carbonium ion) or anion(carbanion)on the monomer unit. Depending up on this cation and or anion formation ionic polymerization is devided into two parts - i) Cationic mechanism: It is the way of polymerization where the initiator of the reaction is formed by the combination of Lewis acid(alcl 3, FeCl 3, BF 3 ) used as catalyst with a co catalyst like water. 1)Chain initiation: This is the first step of cationic polymerization reaction where initiator is added to the monomer unit resulting in the formation of cation center(carbinium ion) in monomer unit.

6 16 CHAPTER 6. POLYMER CHEMISTRY LECTURE/LESSION PLAN -2 Cationic chain initiation of vinyl chloride 2) Chain propagation: This is the second rapid step of cationic polymerization reaction where second third forth...monomer units are added with generated cation center(carbinium ion) in monomer unit consecutively resulting in the formation of large chain cation. Cationic chain propagation of vinyl chloride 3) Chain termination: This is the third and final step of polymerization reaction where termination of cation after long propagation is takes place by a) Proton release b) or by combination with any part of initiator Cationic chain termination of vinyl chloride a) Proton release: It is the way of long chain termination of cationic polymerization by release of proton from carbonium ion chain to the counter ion attached consequently form unsaturated compound. b) combination with any part of initiator: It is the way of long chain termination of cationic polymerization by the formation of covalent bond by the combination of carbonium ion with negative part of initiator. Example of polymers formed by cationic mechanism are Vinyl ethers, Styrene Isobutylene etc. ii) Anionic mechanism: It is the way of polymerization where the initiator of the reaction is formed by the combination of any Lewis base (n-buli, Alkyl-Na) alkali meal amide etc. )used as catalyst Chain initiation: This is the first step of anionic polymerization reaction where initiator is added to the monomer unit resulting in the formation of anion center(carbanion) in monomer unit.

7 6.1. ADDITION POLYMERIZATION 17 Anionic chain initiation of vinyl chloride 2) Chain propagation: This is the second rapid step of anionic polymerization reaction where second third forth...monomer units are added with generated anion center(carbanion) in monomer unit consecutively resulting in the formation of large chain anion. Anionic chain propagation of vinyl chloride 3) Chain termination: This is the third and final step of polymerization reaction where termination of anion after long propagation is takes place by the combination with any foreign compound(such as CH 3 OH, H 2 O, CO 2 ) added from outside. With out the addition of foreign compound the termination of anionic polymerization is not possible Anionic chain termination of vinyl chloride Electron withdrawing group like -CN, -COOH, -CHO stabilized the carbanion and hence boost up the anionic polymerization reaction. Example of polymers formed by anionic mechanism are Styrene, Isoprene, butadiene etc. Condensation polymerization: During the process of polymeric chain growth if there is reaction takes place between different functional group of single monomer or different monomers with the loss or elimination of compound like HCl, H 2 0 or other compound and the polymer is not an exact multiple of used monomer unit is called condensation polymer polymerization. Example of condensation polymer are PF UF, MF, Nylon PET etc.

8 18 CHAPTER 6. POLYMER CHEMISTRY LECTURE/LESSION PLAN -2 Coordination polymerization:organometallic compound has a great role in many chemical reaction as well as polymerization reaction also. When Organometallic compound act as initiator in Polymerization reaction is called coordination polymerization reaction. Zieglar and Natta first prepared polymer by coordination polymerization halide of Ti V, Zr, Cr, Wand Mo.The coordination polymerization can be done by the combination of halide of different transition metal like TiCl 2, TiCl 3, TiCl 4 etc. and organometallic compound such as trialkyl(trimethyl, Triethyl) aluminium. The most common and useful catalyst is the combination of triethyl aluminium with titanium trichloride. These catalysts are heterogeneous catalyst. The monomer catalyst complex involves coordination between a carbon atom of a monomer and a metal atom of the catalyst. In this polymerization the triethyl aluminium act as electron donor and and meal halide acts as an electron acceptor. The different catalysts for coordination polymerization reaction. Mechanism of coordination polymerization: 1) Chain initiation: This is the first step of coordination polymerization reaction where complex catalyst is added to the monomer unit resulting in the formation of metal-carbon bond in monomer unit. 2) Chain propagation: This is the second rapid step of coordination polymerization reaction where second third forth... monomer units are added with generated metal-carbon bond in monomer unit, consecutively resulting in the formation of large chain metal-carbon bond. 3) Chain termination: This is the third and final step of coordination polymerization reaction where termination of large chain metal-carbon is terminated by the transfer of hydrogen atom from monomer or from active hydrogen containing compound. Importance of coordination polymerization: 1) Normal free radical polymerization involves atactic polymer whereas coordination polymerization isotactic polymer. That means stereospecific polymer can be prepared by this coordination polymerization. Using this process(zeiglar-natta catalyst) of polymerization isoprene produces only stereospecific Cis-1,4-polyisoprene 2) Coordination polymerization always involves mainly linear polymer hence highly crystalline in nature.

MATERIALS SCIENCE POLYMERS

MATERIALS SCIENCE POLYMERS POLYMERS 1) Types of Polymer (a) Plastic Possibly the largest number of different polymeric materials come under the plastic classification. Polyethylene, polypropylene, polyvinyl chloride, polystyrene,