polymerization reactions

POLYMERIZATION REACTIONS

Polymerization is the chemical process occurring when simple molecules react to form polymers

Polymerization occurs by the sequential reactions of monomers, which means that a successive series of reactions occurs as the repeating units are linked together.

This can proceed by the reaction of monomer to form a dimer, which in turn reacts with another monomer to form a trimer and so on.

Reaction may also be between dimers, trimers, or any molecule species within the reaction mixture to form progressively larger molecule.

In either case, a series of linkages is built between the repeating units, and the resulting polymer molecule is often called a polymer chain, a description that emphasizes its physical similarity to the links in a chain.

Low-molecular-weight polymerization products such as dimers, trimers, tetramers, etc., are referred to as oligomers. They generally possess undesirable thermal and mechanical properties. A high degree of polymerization is normally required for a material to develop useful properties and before it can be appropriately described a polymer.

Example: polystyrene DP = 7 - a viscous liquid (not of much use) DP > 1000 a solid - commercial grade

However, there is no demarcation has been established between the sizes of oligomers and polymers.

Polymerization mechanism The classification of polymers depending on the reaction involved in their formation : step-reaction (condensation) chain-reaction (addition) polymerization. These terms focus more on the manner in which the monomers are linked together during polymerization.

Step polymerization (condensation) Involves a series of reactions in which any two species (monomers, dimers, trimers, etc.) can react at any time leading to a larger molecule. Most step polymerization involves a classical condensation reaction such as esterification, ester interchange or amidization.

In step polymerization, the stepwise reaction occurs between pairs of chemically reactive or functional groups on the reacting molecule.

In the process, a small molecule, usually water or ammonia, is eliminated.

The formation of a polyesterR and R are the unreactive part of the molecules

Step polymerization generally involve either one or more types of monomers. In either case, each monomer has at least two reactive (functional) groups.

One type of monomer is involved : the functional groups on the monomer are different and capable of intramolecular reactions. A-B step polymerization

Example : the formation of an aliphatic polyester by the self-condensation of -hydroxycaproic acid

When more than one type of molecule is involved, the functional groups on each type of monomer are the same, but capable of intermolecular reaction with the other type of monomer. A-A/B-B step polymerization

Synthesis of Polyesters:

Other commercially important polyesters:

Polycarbonates: Complete the polymerization reactionIdentify the type of polymerization reaction

Polyamides:

Step polymerization can be divided into two main categories:a)Polycondensation in which a small molecule is eliminated at each stepA-R-A + B-R-B A-R-R-B + ABb)Polyaddition- monomers react without the elimination of a small molecule A-R-A + B-R-B A-R-AB-R-B

nHO-(CH2)4-OH + nO=C=N-(CH2)6-N=C=O

basic catalyst O H H O || | | || O-(CH2)4-OC-N-(CH2)6-N-C

Example of polyaddition polymerization : preparation of polyurethane

Chain polymerization (addition) Is an important industrial method of polymer preparationPolymers are produced by reactions in which monomers are added one after another to a rapidly growing chain.Monomer generally employed in addition polymerization are unsaturated (usually with C=C).

The most common unsaturated compounds that undergo chain reaction polymerization are olefins, as exemplified by the following reaction of a generalized vinyl monomer.

nCH2 = CH -CH2-CH- | | R R

PolystyrenePolyethylenePolyacrylonitrilepoly(methyl methacrylate) poly(vinyl chloride). Examples of addition polymers:

Chain reaction polymerization may be classified as (depending on the nature of the reactive centre):a)free-radicalb)cationicc)anionicd) coordination polymerization

Chain reactions, involved three fundamental steps: initiation , propagation and termination

Free-radical Polymerization The growing polymer in chain-reaction polymerization is a free radical, and polymerization proceeds via chain mechanism.Chain reaction polymerization is induced by the addition of free-radical-forming reagents.

InitiationInvolves the acquisition of an active site by the monomer. This may occur spontaneously by the absorption of heat, light (ultraviolet), or high energy irradiation.

But, most frequently, initiation of free radical polymerization is brought about by the addition of small quantities of compounds called initiators such as:a)peroxides: [Dialkyl peroxides (ROOR), Diacylperoxides (RCO-O-O-CO-R), hydroperoxides (ROOH)]. Example : Benzoyl peroxide and t-butylperoxide are commonly used free-radical initiatorsb)azo compounds : [RN=NR] . Example: azo-bisisobutyronitrile Lewis acids (AlCl3,BF3, SnCl4, SbCl5, ZnCl2, TiCl4) and organometallic reagents (RAlCl2, R2AlCl, R3Cl).

Initiators are not exactly catalyst since they changed chemically in the course of polymerization.

An initiator usually a weak organic compound that can be decomposed thermally or by irradiation to produce free radicals (molecules containing atoms with unpaired electrons). Peroxides such as benzoyl peroxide can be decomposed thermally between 60 and 90C. On the other hand, azo compounds such as azo-bisisobutyronitrile is decomposed photochemically (short-wavelenght visible light or near-ultraviolet radiation).

The thermal decomposition of organic compound is appropriate only for polymerizations carried out at room temperature or higher.

In free-radical polymerization carried out in aqueous medium, the decomposition of peroxides and persulfate (K2S2O8) is generally accelerated by the presence of a reducing agent (bisulfite ion (from NaHSO3) or ferric ion). This method of free radical initiation is referred to as redox initiation. The enhance rate of free-radical formation in redox reactions permits polymerization at relatively low temperature. Example: emulsion polymerization

High energy irradiation (as X-rays, -rays and -rays ) of monomers can be carried out in bulk or in solution. It is not as selective as photolytic initiation.

When choosing an initiator for free-radical polymerization, the most important parameters that must be considered are the temperature range to be used for the polymerization and the reactivity of the radicals formed. The presence of certain promoter and accelerators and the nature of the monomer often affect the rate of decomposition of initiators.

Free-radical initiation processes do not require stringent exclusion of atmospheric moisture, but can be inhibited by substances such as oxygen. Free radicals are inactivated by reaction with oxygen to form peroxides or hydroperoxides.

a)the formation of radicals :I-I 2 IThe initiation of polymerization occurs in 2 successive steps :

O-centered radicals (oxyl radicals) are un-stable and transformed into relatively stable C-centered radicals

2 C centered radicals Nitrogen gas

H H | |I + CH2=C I-CH2-C | | R Rb)the addition of the initiator radical to a vinyl monomer : Initiator fragments have been shown by end-group analysis to become part of the growing chain.

PropagationDuring propagation, the initiated monomer described above adds other monomers - usually thousands of monomer molecules-in rapid succession.This involves the addition of a free radical to the double bond of a monomer, with generation of another radical.

H H | |I-CH2-C + CH2=CHR I-CH2-CH-CH2-C | | | R R RThe active centre is thus continuously relocated at the end of the growing polymer chain

Propagation continues until the growing chain radical is deactivated by chain termination or transfer

H H H | | |I-CH2-C + CH2=CHR I-CH2-C-CH2-C | | | R R R3 possible ways for the propagation step to occur:head to tailThe substituted carbon atom is regarded as the head and the unsubstituted carbon atom the tail of the vinyl monomer. HeadTail

A random distribution of these species along the molecular chain might be expected.It is found, however, that head-to-tail linkages in which the substituents occur on alternate carbon atoms predominate; only occasional interruptions of this arrangement by head-to-head and tail-to-tail linkages occur. In addition, exclusive head-to-head or tail-to tail arrangements of monomers in the chain are now known.

TerminationIn termination, the growth activity of a polymer chain radical is destroyed by reaction with another free radical in the system to produce polymer molecule(s).

H H H | | |I- -CH2-C + I I- - CH2-C-C-I | | | R R RTermination can occur by the reaction of the polymer radical with initiator radicals. This type of termination process is unproductive and can be controlled by maintaining a low rate of initiation.

H H H H | | | | I- -CH2-C + C-CH2- -I I -CH2-C-C-CH2 I | | | | R R R RCoupling reactions produce a single polymercombination of growing polymer radicals predominates at low temperatureThe termination reaction that are more important in polymer production are: a) Combination (or coupling) Two growing polymer chains react with the mutual destruction of growth activity

H H H H | | | |

I- -CH2-C + C-CH2- -I I- -CH2-C-H + C=CH I | | | | R R R R

b) Disproportionation. A labile atom (usually hydrogen) is transferred from one polymer radical to another.Produce two polymers from the two reacting polymer chain radicals

The predominant termination reaction depends on the nature of the reacting monomer and the temperature.

Since disproportionation requires energy for breaking of chemical bonds, it should become more pronounced at high reaction temperatures

Chain transfer a fourth step called chain transfer is usually involved. In chain-transfer reactions, a growing polymer chain is deactivated or terminated by transferring its growth activity to previously inactive species (TA) (monomer / polymer / solvent molecule or other molecules deliberately or inadvertently introduced into the reaction mixture)

H H | |

I- -CH2-C + TA I- -CH2-C-T + A | | R R

Depending on its reactivity, the new radical, A may or may not initiate the growth of another polymer chain:If the reactivity of A is comparable to that of the propagating chain radical, then a new chain may be initiated.If its reactivity toward a monomer is less than that of the propagating radical, then the overall reaction rate is retarded.If A is un-reactive toward a monomer, the entire reaction could be inhibited.

Transfer reactions do not result in the creation or destruction of radicals; at any instant the overall number of growing radicals remains unchanged.

However, the occurrence of transfer reactions results in the reduction of the average polymer chain length, and in the case of transfer to a polymer it may result in branching.

Exercise 1:Prepare a styrene polymer via radical polymerization using benzoyl peroxide as the initiator, head to tail propagation and termination via a)coupling b)chain transfer to another polymerc)dispropotionation

Exercise 2:Prepare a vinyl chloride polymer via radical polymerization using benzoyl peroxide as the initiator (4 marks), tail to tail propagation (3 marks). In the termination step describe on how the following type of PVC can be obtained:A high molecular weight PVC (3 marks)An un-saturated PVC (4 marks)A branched PVC (6 marks)

Exercise 3:Show the initiation steps in a polymerization of ethylene using the following technique:Gamma raysCl2

Ionic Polymerization The growing polymer molecule is associated with counterions in ionic (cationic and anionic) polymerizationInvolves chain carriers or reactive centers that are organic ions or charged organic groups.The mechanism of ionic polymerization is more complex and is not as clearly understood as those of free radical polymerization.Initiation of ionic polymerization usually involves the transfer of an ion or an electron TO or FROM the monomer

Cationic Polymerization In cationic polymerization, the growing chain end carries a positive charge or carbonium (carbenium) ion. Monomers with electron donating groups like isobutylene form stable positive charges and are readily converted to polymers by cationic catalyst.Any strong lewis acid (AlCl3,BF3, SnCl4, SbCl5, ZnCl2, TiCl4) or Friedal-Crafts catalyst such as AlCl3 can readily initiate cationic polymerization in the presence of cocatalyst like water which serve as a Lewis Base or source of protons.

During initiation a proton adds to the monomer to form a carbonium ion, which forms an association with the counterion. Example : isobutylene and boron trifluoride

First, the catalyst and cocatalyst (e.g. water) form a complex: BF3 + H2O H+(BF3OH)-

The complex then donates a proton to an isobutylene molecule to form a carbonium ion:

Propagation involves the consecutive additions of monomer molecules to the carbonium ion at the growing chain end :

Since cationic polymerization is generally carried out in hydrocarbon solvents that have low dielectric constant, separation of the ions would require a large amount of energy. Consequently, the anion and cation remain in close proximity as an ion pair. Therefore, the growth rate and subsequent reaction (termination) are affected by the nature of the ion pair. If the intimate association of the ion pair is too strong, however, monomer insertion during propagation will be prevented

Termination a) Rearrangement to produce a polymer with an unsaturated terminal unit and the original complex

2.Chain Transfer (to a monomer).

Cationic polymerization are usually conducted in solutions and frequently at temperatures as low as -80 to - 100C .Polymerization of isobutylene (IB) was studied using the 2-chloro-2,4,4-trimethylpentane (TMPCl)/TiCl4/2,6 -di-tert-butylpyridine (DTBP) system in hexane (Hex)/methyl chloride (MeCl) at -25 to -80C Therefore, the choice of solvent has to be made carefully; a linear increase in polymer chain length an exponential increase in the reaction rate usually occur as the dielectric strength of the solvent increases. Polymerization rates at these low temperatures conditions are usually fast

Exercise:Prepare poly(methyl methacrylate) via cationic polymerization using AlBr3 as the initiator and BrC(CH3)3 as the co initiator. Show the termination step by: a)chain transfer to a monomer b)rearrangement

Exercise 2:Explain the following:It is difficult to separate the ion pair of the propagating species in cationic polymerization.The choice of solvent to conduct cationic polymerization is crucialThe rate of reaction of a cationic polymerization usually very fast

Anionic Polymerization In anionic polymerization, the growing chain end carries a negative charge or carbanions. Monomers that are suitable for ionic polymerization generally contain electron-withdrawing substituent groups such as CN, -COOR, -C6H5, -CH=CH2. The electronegative group pulls electrons from the double bond and consequently renders the monomer susceptible to attack by an electron donor.

Initiations involves the addition of the initiator to the double bond of the monomer. The initiator may be any compound including Grignard reagents (R-MgBr), organosodium compounds (NaOCH3), alkali metal amides (LiN(CH3H7)2 , alkoxide and hydroxides (OH). Initiation may occur in two ways:

a) direct attack of a base on the monomer to form a carbanion

b) transfer of electron from a donor molecule to the monomer to form an anion radical:

The reaction produces a carbanion at the head end to which is associated the positively charged counterion. M+B- may be a metal amide, alkoxide, alkyl, aryl and hydroxide depending on the nature of the monomer.The effectiveness of the catalyst in the initiation process depends on its basicity and the acidity of the monomer. For example, in the anionic polymerization of styrene, the ability to initiate reaction decreases in the order -CH2- - NH2- > NH2- >> OH-. Indeed OH- will not initiate anionic polymerization of styrene.

TWO EWGs are so effective in stabilizing anions. Therefore, weak base such as water can initiate this monomer

EGW = Electron Withdrawing Group

Propagation occurs by the successive insertion of monomer molecules by anionic attack of the carbanion. No chain transfer or branching occurs in anionic polymerization, particularly if reactions are carried out at low temperature:

Terminationof the growth activity of the polymer chain takes place either by the deliberate or accidental introduction into the system of oxygen, carbon dioxide, methanol, water or other molecules that are capable of reacting with the active chain ends.

In general, termination by transfer to the solvent predominates in anionic polymerization.In ionic as well as free-radical polymerization, the initiator or part of it becomes part of the resulting polymer molecule, attached to the non-growing chain end. Unlike cationic polymerization, the catalyst is regenerated at the termination step.

Living polymerizationIn some anionic polymerization system, termination can be avoided if the starting reagents are pure and the polymerization reactor is purged of all oxygen and traces of water.This produces polymer molecules that can remain active even after all the monomer molecules are consumed. When fresh monomer is added, polymerization resumes. Such polymeric molecules are referred to as living polymers because of the absence of termination.

MW in living polymerization :Since the chain ends grow at the same rate , the molecular weight of living polymer is determined simply by the ratio of monomer concentration to that of the initiator:DP = [monomer] / [initiator]The MW is predictable Polymers produced by living polymerization are characterized by very narrow molecular weight distribution. The polydispersity, D (approaches 1, typically 1.05-1.2)

Block Copolymerization The absence of termination in living polymerization permits the synthesis of unusual and unique block polymers.If a living polymer with one active end from monomer A can initiate the polymerization of monomer B, then an A-A-B-B type copolymer can be obtained (e.g. styrene-isoprene copolymer)

If both ends of polymer A are active, a copolymer of the type B-BA-AB-B results. (e.g. polystyrene-polyisoprene-polystyrene)

Living polymerization can also be employed to introduce a variety of desired functional groups at one or both ends of polymeric chains both in homo- and block polymers.

Functionalization of the Chain Ends

Carboxylation of end groups: Alcohol end groups via ethylene oxide:

In particular, living polymerization techniques provide a vital and versatile tool to control the architecture of a polymer; complicated macromolecules can be synthesized to meet the rigid specification imposed by a scientific or technological demand.However, the reactive end groups in living polymerization may be annihilated by a choice of suitable reactants by the experimenter at a desired stage of the polymerization process.

EXERCISE 1:Explain the following:CH3ONa is un able to initiate MMAIs it possible to initiate polycyanoacrylate using water? Explain? Write the polymerization mechanism of polyacrylonitrile using butyl lithium:i) with termination in the presence of CO2ii) termination by proton transferiii) without terminationExplain the polymer that is obtained in c(iii)Differentiate between anionic, cationic and radical polymerization

Example 2:Prepare a polyacrylonitrile polymer via anionic polymerization using n-butyllithium as an initiator. Describe the initiation of the polymerization by transfer of electron and termination by the addition of water.If the above polymerization takes place in an inert condition whereby oxygen and traces of water are purged out from the reactor; state:i) the type of polymer that is producedii) the advantages of preparing this type of polymerization processiii) the carboxylation of end group

Exercise 3: Synthesis the following polymer:

CH3(CH2)3-(CH2CH(C6H5))n-(CH2)5OH

Exercise 4:Prove that a branched polymer of PVC can be obtained from the free radical polymerization of PVC

Coordination Polymerization the mechanism of coordination polymerization is also complex and are not clearly understood Coordination polymerization may be anionic or cationic.Relatively fewer examples of cationic coordination polymerization leading to the formation of stereo regular polymers currently exists. These are limited almost exclusively to the polymerization of monomers containing heteroatoms with lone pair electrons such as oxygen or nitrogen.

In any case, the growth reaction in coordination polymerization is considered to be controlled by the counterion of the catalysts, which first involves the formation of a coordination compound between the catalyst, monomer and growing chain. The polarized double bond of the monomer is inserted in the polarized bond between the counter ion and the end of the growing chain.

Coordination involves the overlap of the electrons of the monomer with a vacant sp orbital in the case of groups I-III metals or a vacant d orbital in the case of transition metals

A typical catalyst complex is formed by trialkyl aluminum and titanium trichloride

The catalysts function by forming transient -complexes between the monomers and the transition metal species.

initiating The initiating species is a metal-alkyl complex

Propagationinvolves the consecutive insertion of monomer molecules into a polarized titanium-carbon bond. The proposed propagation mechanisms for both the monometallic and bimetallic catalyst are shown below:

Termination by introducing poisons such as water, hydrogen, aromatic alcohols, or metals like zinc into the reacting system.

Monomers with side groups asymmetrically disposed with respect to the double bond are capable of producing polymers in which the side groups have a specific stereochemical or spatial arrangement (isotactic or syndiotactic).Unbranced and stereospecific polymers are produced by the use of Ziegler-Natta catalyst. These are complex catalyst systems derived from a transition metal compound from groups IVB to VIIIB of the periodic table and an organometallic compound usually from a group IA or IIIA metal.Monoolefins such as propylene and dienes such as butadiene and isoprene can be polymerized using Ziegler-Natta coordination catalysts.

Example:Show the polymerization of PVC by coordination polymerization using Z-N [Al(CH2CH3)3 and TiCl4] catalyst by a) monocatallic using the d orbital as the overlapping orbital. b) bimetallicii)Show the termination step by :a) chain transfer b) addition of water iii)Name the stereoisomerism of PVC obtained from this type of polymerization.

Step polymerizationChain polymerizationAny two molecular species can react. Growth occurs only by addition of monomer to active chain endMonomer disappears early. Monomer is present throughout, but its concentration decreases.Polymer MW rises throughout.Polymer begins to form immediately. Growth of chains is usually slow (minutes to days).Chain growth is usually very rapid (second to microseconds).Long reaction times increase MW, but yield of polymer hardly changes. MW and yield depend on mechanism details.All molecular species are present throughout. Only monomer and polymer are present during reaction. Usually (but not always) polymer repeat unit has fewer atoms than had the monomer.Usually (but not always) polymer repeat unit has the same atoms as had the monomer

polymerization reactions

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