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1. UNIT-7 KINETICS AND MECHANISM OF POLYMERIZATION

2. Mechanism and kinetics of copolymerization Let two monomers, M1 and M2 be mixed together and polymerized using a free-radical initiator. Once the initiator decomposes, the free-radical formed can attack M1 or M2 as follows: R+M1 → RM1 R+M2 → RM2 Two polymer chains can thus be initiated, one carrying a free radical site on monomer unit M1 and the other on monomer unit M2. After initiation, comes the process of propagation. Here, the chain carrying a free-radical site on M1 can either add another M1 or M2. Similarly, the chain carrying a free-radical site on M2 can either add another M2 or an M1, i.e.:

3. 1. RM1+M1 → RM1M1 2. RM1+M2 RM1M2 3. RM2+M1 RM2M 1 4. RM2+M2 RM2M 2 Assuming that all the four types of addition take place, we still have the growing chains ending with M1 or M2 and the possibility of all the four types of propagation. Now, the rates of these four propagation reactions are as follows:

4. The basic assumption is that the reactivity of any growing chain depends only on the end monomer unit carrying the free-radical site and not on the number or type of monomer units already added to the chain.

5. The rate at which the monomers M1 and M2 are consumed during the course of propagation can be expressed as follows:

6. Now, assuming a steady state, wherein the rate of a particular chain end (say, M1) disappearing is equal to the rate of formation of the same chain end, we can write A combination of all the foregoing equations gives the ‘copolymer equation’:

7. The terms k11/k12 (denoted by r1) and k22/k21 (denoted by r2) appearing in this equation are two important terms, known as the reactivity ratios for any given pair of monomers M1 and M2. These ratios (r1 and r2) indicate whether a growing chain carrying a free radical on a particular monomer unit would prefer to add its own monomer species or the comonomer species.

8. In other words, the composition of the copolymer formed at any given instant is dependent not only on the concentration of the monomer species present in the system at that instant but also on their reactivity ratios. The point to be noted is that r1 and r2 for any given pair of monomers are dependent purely on the nature of the two monomers and temperature and independent of other parameters such as the solvent, initiator and chain transfer agent. The latter, however, will have a pronounced influence on the molecular weight and the molecular weight distribution of the polymer formed.

9. Addition polymerization. Addition polymerization occurs in monomer units having double or triple bonds. It involves the combination of a large number of monomer units by addition reaction. In general, compounds containing –C=C-Z (Z is substituent) undergo addition polymerization which is popularly known as vinyl polymerization. This polymerization is also known as chain growth polymerization. The monomer units may be joined in a head–tail arrangement, or a head–head and tail–tail arrangement, or it may be a random arrangement. The most favourable arrangement is head–tail arrangement.

10. Mechanism of Addition Polymerization In the formation of addition polymers (chain growth polymers), the reaction is initiated by a catalyst, which results in the formation of a reactive intermediate. This intermediate then adds on to the monomer unit to generate a new intermediate, which adds on to another monomer unit and the process goes on. Depending upon the reactive intermediate formed, the polymerization reaction may follow: Free radical mechanism Cationic mechanism Anionic mechanism (d) Ziegler–Natta polymerization

11. (a) Free radical mechanism. The addition polymerization reactions, which are carried out in presence of peroxides or potassium perborate, follow a free radical mechanism. The addition occurs through a free radical intermediate and is also known as radical polymerization. The radical polymerization occurs in head to tail manner. The mechanism involves following steps:

12. Step 1. Chain initiation

13. Step 2. Chain propagation

14. Step 3. Chain termination

15. (b) Cationic mechanism. The addition polymerization reactions, which are carried out in the presence of strong acids like sulfuric acid, halogen acids or in the presence of Lewis acids (BF3, AlCl3) follow a cationic mechanism. The protonation of alkene results in the formation of carbocation intermediates. Thus, the growing chains in polymerization are cations and the process is known as cationic polymerization. For example, the cationic mechanism for polymerization of 2-methylpropene (isobutene) involves following steps:

16. (c) Anionic mechanism. The addition polymerization reactions, which are carried out in presence of strong bases like sodamide or metal alkyls follow anionic mechanism. The alkenes containing an electron withdrawing group generally undergo anionic polymerization. The electron withdrawing groups facilitate the attack of base on the olefinic carbon to which they are attached. The mechanism involves following steps:

17. (d) Ziegler–Natta polymerization (Coordination polymerization). The free radical, cationic, or anionic polymerizations generally result in the formation of addition polymers, which are atactic. In 1953, two scientists Karl Ziegler (Germany) and Giulio Natta (Italy) independently synthesized “stereoregular” (isotactic and syndiotactic) addition polymers. The stereoregular polymers are high melting, crystalline solids. These have much better properties and wider range of applications than atactic polymers.

18. The Ziegler–Natta polymerization is carried out in the presence of a mixture of titanium tetrachloride (TiCl4 ) and triethylaluminium as catalyst, popularly called Ziegler–Natta catalyst. The polymerization is also known as coordination polymerization. The reaction is carried out at a low temperature (<100 degree Celsius) and low pressure (~10 atm) in a non polar solvent.

19. The reaction takes place as follows:

20. Mechanism:- The catalyst has a complex structure. The alkyl titanium bond undergoes insertion of monomer units. The π-electrons of monomer are coordinated to a vacant site on metal (This being the reason that it is known as coordination polymerization). The coordinated monomer then inserts into titanium–carbon bond and the process goes on to form a polymer. Since titanium is attached to different ligands, the coordination of monomer units follow a stereoregular pattern. The various steps of reaction mechanism are given below:

21. Ziegler–Natta polymerization: schematic and simplified representation of (a) formation of polyethylene (b) polypropylene formation (further simplified); (c) mechanism for the formation of polyethylene

22. Characteristics of Ziegler–Natta polymers. The polymers manufactured by using Ziegler–Natta catalyst have the following important characteristics: (1) They are high melting, highly crystalline, isotactic polymers having high molecular mass. (2) They are tough and flexible, so can be moulded easily and are resistant to acids and alkalis.

23. Synthesis and uses of vinyl polymers The polymerization of vinyl monomers generally occurs in the presence of peroxides and follows a free radical mechanism. However in case of polyethylene and polypropylene an ionic or Ziegler–Natta mechanism are also followed. The preparation of polypropylene is carried out exclusively through Ziegler–Natta polymerization to obtain a stereoregular polymer of high quality. The general reaction for vinyl polymerization is represented as follows:

24. SOME OF THE VINYL POLYMERS, ALONG WITH THEIR MONOMER UNITS AND COMMON USES ARE GIVEN BELOW:-

25. Condensation polymerization. It involves the combination of a large number of monomer units with the elimination of simple molecules like water, ammonia, and so on. This polymerization is also known as step growth polymerization. In condensation polymers the monomer units contain two or more functional groups and generally the condensation occurs between two different monomer units. For example, polyesters, polyamides, polyurethane, and bakelite .

26. POLYAMIDES Nylon 6,6. Nylon 6,6 is a copolymer and is prepared by condensation of hexamethylenediamine and adipic acid. During condensation, loss of water molecule takes place to form a number of amide linkages (polyamide) to yield nylon 6,6. Each monomer unit (diamine and diacid) has six carbons each and therefore the name, nylon 6,6. Adipic acid is obtained by oxidation of cyclohexanone .

27. Polyesters Dacron [terylene]. Dacron, a copolymer, is prepared by the condensation of methyl ester of terphthalic acid with ethylene glycol. The condensation involves loss of a methanol molecule, which results in the formation of a number of ester linkages (polyester).

28. This polyester can be fabricated into a strong film, which is known by the name Mylar. It is a light weight, high strength, transparent film, which is used for protection of artwork and important documents.

29. (ii) Polycarbonates. These are polyesters formed by condensation of derivatives of phenol and carbonic acid. For example, condensation of bisphenol A with phosgene results in the formation of a polycarbonate, known by the name of lexan . The reaction takes place as follows:

30. Bakelite (Phenol-formaldehyde resin). Bakelite is a copolymer of phenol and formaldehyde. It is a thermosetting polymer and has highly cross-linked structure. The methylene groups at ortho and para positions participate in cross linkages. The condensation of phenol and formaldehyde can occur in acidic or alkaline medium. • In acidic medium, condensation results in the formation of a linear polymer known as Novolak . This polymer further polymerizes in basic medium to yield bakelite .

31. Here, polymerization is shown with o- methylolphenol (other type of linear polymers are also formed). • In alkaline medium, condensation results in the formation of a linear polymer known as Resol . Heating resol polymer results in the formation of bakelite .

32. Both novolak and resol are thermoplastic polymers but they result in the formation of a thermosetting polymer, namely Bakelite.

33. Urea-formaldehyde resin . Urea on condensation with formaldehyde results in the formation of urea-formaldehyde resin. Initially, the reaction results in the formation of monomethylol , which further condenses with urea and formaldehyde to give the resin. The polymerization may occur in a linear, three-dimensional, or cyclic structure.

34. Epoxy Polymers The epoxy polymers are basically polyethers. One type of epoxy polymer (or epoxy resins as they are generally called) is prepared from epichlorohydrin and bisphenol-A. The reaction is carried out with excess of epichlorohydrin. The scheme is as follows:

35. Instead of bisphenol-A, many other compounds with hydroxyl groups (such as glycols, glycerols and resorcinols ) can also be used. The epoxy resins obtained through these reactions will be either highly viscous liquids or solids with high melting points. The epoxy resins can be further cured with substances such as amines, poly sulfides and polyamides. Epoxy resins find a large number of uses because of their remarkable chemical resistance and good adhesion. Epoxy resins are excellent structural adhesives. When properly cured, epoxy resins can yield very tough materials. They are used in industrial floorings, foams, potting materials for electrical insulations, etc. One of the principal constituents in many of the fibre -reinforced plastics (FRP) is an epoxy polymer.

36. POLYURETHANES The reaction of an isocyanate and alcohol results in the formation of a carbamate, which is known as urethane .

37. Polyurethanes are copolymers and are prepared by the reaction of alkyl or aryl diisocyanates with diols. In general, the diol used in the preparation is a copolymer of ethylene glycol and adipic acid, which has free hydroxy (– OH) end groups. The commonly used diisocyanate is toluene-2,4- diisocyanate

38. The polymerization in the presence of water results in the formation of polyurethane foam. Water reacts with the isocyanate group to produce carbamic acid, which spontaneously looses carbon dioxide. The carbon dioxide thus generates bubbles (giving porosity to polymer) to yield polyurethane foam. The density of foam is dependent on the amount of carbon dioxide evolved during the process.

39. Some condensation polymers along with their monomer units and common uses are given below:-

40. QUESTIONS: 1. What is Zeigler-Natta catalyst? Explain the mechanism for the polymerization of propene in presence of Zeigler- Natta catalyst. 2. Give the free radical addition mechanism involved in the polymerization of chloroprene. 3. Name some vinyl polymers along with their monomer’s units and common uses. 4. Give the mechanism of condensation polymerization with the help of suitable examples. 5. What are the characteristics and advantages of Zeiger-Natta polymerization? 6. Give the synthesis of polyurethanes.

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