Introduction

The study of polymer chemistry starts through understanding the techniques in that such substances are synthesized. Polymer synthesis is a compound process and can happen in a diversity of ways. Polymers are organized via the procedure of joining small molecules (monomers) via covalent bonds. Therefore, polymerization is a procedure of reacting monomer molecules mutually in a chemical reaction to form three-dimensional networks or polymer chains.

In chemical compounds, polymerization happens via a variety of reaction mechanisms that fluctuate in complexity due to functional groups there in reacting compounds and their inherent steric results described via VSEPR Theory. There are many shapes of polymerization and different systems subsist to categorize them such include, the Addition polymerization as well said Chain-growth polymerization and Condensation polymerization sometimes said Step- growth polymerization.

Addition (Chain-growth) Polymerization

Addition polymerization explains the technique where monomers are added one via one to an active site on the growing chain. This is distinguished via self-addition of molecules to each other, extremely quickly through chain reaction. Monomer in this case keeps its structural identity when it obtains changed to polymer (product has similar elemental composition as the monomer). It can be symbolized through the chemical equation:

nM(monomer)  (-M-)n(polymer)

Where n is the degree of polymerization.

In addition polymerization, no by product is formed (for example, no net loss of atom). The bifunctionality is provided through the double bonds present in the monomer. The most general kind of addition polymerization includes Free radical chain reaction of molecule that have C=C bond. For olefins (CH₂=CHR), vinyl compounds (CH₂=CHX), allyl compounds (CH₂=CH-CH₂X) and dienes (CH₂=CR-CH=CH₂), addition reaction polymerization involves the opening of the bond through a free radical initiator. Under addition polymerization, another procedure recognized as Ionic polymerization (for instance cationic chain expansion and anionic chain growth mechanism) can happen. All such polymerization procedures follow 3 principal steps - Initiation (birth), Propagation (growth), and Termination (death). Typical addition polymers are polystyrene, poly(acrylic acid), polyethylene, poly(vinyl chloride), poly acrylonitrile, poly(vinyl fluoride) and so on.  

Free Radical Polymerization

The most general kind of addition polymerization is free radical polymerization. A free radical is merely a molecule through an unpaired electron. The tendency for this free radical to expand an additional electron in order to shape a pair makes it extremely reactive so that it splits the bond on another molecule via stealing an electron, leaving that molecule through an unpaired election (that is another free radical). 

Free radicals are frequently generated through the division of a molecule (known as an initiator) into to fragments along a single bond. Initiators for free radical polymerization comprise any organic compound by a labile bond, these as an azo (-N=N-), disulphide (-S-S-), or peroxide (-O-O-) compound. The labile bond can be broken down either through heat or irradiation these as uv or γ-radiation giving a radical. An instance of a free radical initiator 

                R-O-O-R¹   Where R and R¹ represent an alkyl/benzoyl group. 

The subsequent diagram illustrates the formation of a radical from its initiator, in this case benzoyl peroxide.

The stability of a radical refers to the molecule's tendency to react through other compounds. An unstable radical will eagerly join through many dissimilar molecules. Though a stable radical won't simply interact through other chemical substances. The stability of free radicals can fluctuate extensively depending on the properties of the molecule. The active center is the location of the unpaired electron on the radical since this is where the reaction happens. In free radical polymerization, the radical attacks 1 monomer, and the electron migrates to an additional part of the molecule. This recently shaped radical attacks another monomer and the procedure is repeated. Therefore the active center shifts down the chain as the polymerization happens until it is ended.     

Mechanism of Free radical Polymerization    

The formation of a polymer via this procedure is an instance of a chain reaction. Once a chain reaction obtains started, it is capable to stay itself going. The three steps of this reaction to focus on are Initiation, Propagation and Termination.

Initiation Reaction

The 1^st step in producing polymers via free radical polymerization is initiation. This step starts when an initiator decomposes into free radicals in the presence of monomers. The instability of carbon-carbon double bonds in the monomer creates them susceptible to reaction through the unpaired electrons in the radical. In this reaction, the active center of the radical 'grabs' one of the electrons from the double bond of the monomer, leaving an unpaired electron to demonstrate as a new active center at the end of the chain. Calculation can happen at either end of the monomer. Instance: Consider the polymerization of ethylene (CH₂=CH₂) to polyethylene.

A peroxide molecule breaks up into two reactive free radicals. Light or heat can give the energy needed for this process- first part of the initiation step:

We can write an equation for this process: 

     ROOR + energy         2RO

   peroxide                          free radical initiator

The 2^nd part of initiation happens when the free radical initiator attacks and joins to a monomer molecule. This shapes a new free radical that is said the activated monomer. An equation for the reaction is as therefore:

                 RO?        +         CH₂=CH₂          RO-CH₂-CH₂

       Free radical initiator     ethylene                     activated monomer  

One of the two Π- electrons in the ethylene has been utilized to form a single bond through the R-O? radical. The other continues on the second carbon atom, leaving it as a seven-valence electron atom that reacts through another ethylene molecule.

Propagation Reaction 

After a synthesis reaction has been started, the propagation reaction takes over. In the propagation phase, the recently shaped activated monomer attacks and attaches to the double bond of one more monomer molecule. The procedure of electron transport and consequent motion of the active center down the chain proceeds. In this diagram, (chain) terms to a chain of connected monomers, and X refers to hydrogen atom. Therefore the monomer would be ethylene and the polymer polyethylene.     

In free radical polymerization, the whole propagation reaction generally happens inside a fraction of a second. Thousands of monomers are added to the chain inside this time. The complete procedure stops when the termination reaction happens.

Termination Reaction

This chain reaction can't go on forever. The reaction must terminate, but how. In theory, the propagation reaction could continue until the supply of monomers is exhausted. Though, this outcome is extremely unlikely. Most frequently the expansion of a polymer chain is halted via the termination reaction. Termination characteristically happens in 2 ways: amalgamation and disproportionation.

• Combination happens when the polymer's growth is stopped via radical end groups on two growing chains that connect and form a single chain. The subsequent diagram depicts combination, through the symbol (R) representing the rest of the chain.

Fig: Combination

• Disproportionation halts the propagation reaction when a free radical strips a hydrogen atom from an active chain. A carbon-carbon double bond takes the place of the missing hydrogen. Termination via disproportionation is revealed in the diagram.

Fig: Disproportionation

Disproportionation can as well take place when the radical reacts through an impurity. This is why it is so significant that polymerization be carried out under extremely clean situations.

As we know: Chemists can manage the way a polymer does each of such steps via varying the reactants, the reaction times and the reaction situation. The physical properties of a polymer chain based on the polymer's average duration, the amount of branching, and the constituent monomers.

Ionic Polymerization 

Ion initiated polymerization follow the similar basic steps as free radical polymerization (initiation, propagation and termination) though, there are several differences. Ionic mechanism of chain polymerization as well engages attack on the Π electron pair of the monomer.

As we know in free radical, the unpaired electron of the free radical does the attack but in ionic, the positive or negative ion does the attack.

In ionic polymerization, either a carbanion (c^-) or carbonium (c⁺) ionic site can be shaped in the initiation procedure. Radical-initiated polymerizations are usually non-specific but this isn't true for ionic initiators, because the formation and stabilization of a carbonium ion or carbanion depends mainly on the nature of the group R in the vinyl monomer,

                                        CH₂=CHR 

For this reason cationic initiation is generally limited to monomers with electron-donating groups (for example, alkoxy, alkyl, alkenyl, phenyl) which assist to stabilize the delocalization of the positive accuse in the Π- orbitals of the double bond. Anionic initiators need electron withdrawing substituents (for example cyano (-CN), carbonyl -aldehyde, ketone, acids and ester) to promote the configuration of a stable carbanion, and when there is an amalgamation of together mesomeric and inductive consequences the stability is greatly increased. As such ions are connected through a counter-ion the solvent has a profound influence. Thus chain propagation will based significantly on the separation of the two ions and this will also control the mode of entry of an adding monomer. While polar and highly solvating media are obvious choices for ionic polymerizations many can't be utilized since they react through and negate the ionic initiators. Ionic-initiated polymerizations are much more compound than radical reactions. When the chain carrier is ionic, the reaction rates are speedy, difficult to reproduce, and yield elevated molar mass material at low temperatures via mechanisms that are frequently hard to describe. Initiation of an ionic polymerization can occurs in one of 4 ways involving basically the loss or gain of an electron e- via the monomer to create an ion or radical ion.                                      

(a)   M + I⁺         MI⁺

   Cationic                                                    

(b)   M + I-                 MI^-

    Anionic 

(c)   M + e-          ?M^-

    Anionic

(d)   M - e-                 ?M⁺

    Cationic (charge transfer)         

Anionic Polymerization

Anionic polymerization is a form of addition polymerization that involves the polymerization of vinyl monomers with strong electronegative groups. This polymerization is carried out through a carbanion active species. Like all calculation polymerizations, it occurs in three steps: chain initiation, chain propagation, and chain termination. The initiator in an anionic polymerization might be any strong nucleophile that includes covalent or ionic metal amides, alkoxides, hydroxides, cyanides, phosphines, amines and organometallic compounds (alkyl lithium compounds and Grignard reagents). The most commercially utilized of such initiators is the alkyl lithium initiator - butyl lithium.

CH₃-CH₂-CH₂-CH₂-Li

Butyl lithium

This can split to form a positive lithium cation and a negative butyl anion. We call an anion as this where the negative charge is on a carbon atom a carbanion.                                                                                

CH₃-CH₂-CH₂-CH₂-Li      CH₃-CH₂-CH₂-C^-

 Example 1: Polymerization of acrylonitrile.

           nCH₂=CHCN             [CH₂CHCN]n

                                  polymerization

 Note: The butyl anion is (abbreviated Bu-)

Chain initiation 

Using butyl lithium as an anionic initiator, it reacts through the end carbon in a molecule of acrylonitrile to provide new anion.

    Bu^-Li⁺+CH₂=CHCN          Bu-CH₂-CH^-CNLi⁺                                                

                                           Activated monomer initiator     

During initiation procedure, the addition of the butyl anion to acrylonitrile creates a carbanion at the head end in connection through the positively charged Lithium counterion. The new anion then reacts by an additional molecule of acrylonitrile.

Chain Propagation

Propagation in anionic polymerization consequences in the complete consumption of monomer. It is extremely fast and happens at low temperatures. This is due to the anion not being extremely stable; the speed of the reaction in addition to that heat is liberated during the reaction.  

Bu-CH₂-CH-CN Li⁺ + CH₂=CHCN    Bu-CH₂-CHCN-CH₂-CH-CN   Li⁺   

                                                                      Propagation stage

Anionic polymerization is said Living polymerization since when additional monomer is added (even months later) they resume expansion and enhance in molar mass.

Chain Termination

When carried out under the appropriate situations, termination reactions don't occur in anionic polymerization. Though, termination can happen through unintentional quenching due to trac impurities. This comprises trace amounts of oxygen, carbon dioxide or water. Intentional termination can take place through the addition of water or alcohol.  

Bu-(CH₂-CHCN)_n-CH₂-CH^-CN  Li⁺ + H₂O     Bu-(CH₂-CHCN)_n-CH-CH₂CN  +  Li⁺OH^-

                                                                                       Termination stage

Termination is attained here via addition of water to replace the Li⁺ through a hydrogen ion.                                     

Example 2: Using Potassium amide as an initiator. In the initiation step, the base adds to a double bond to form a carbanion.

Fig: Form a carbanion

In the chain propagation, this carbanion adds to the double bond and the procedure repeats to form a polymeric carbanion.

Fig: Polymeric carbanion

The chain reaction can be finished via addition of an acid.

Fig: Examples of polymers

Instances of polymers that can undergo anionic polymerization are polystyrene, poly acrylonitrile, poly (ethylene oxide), poly (methyl methacrylate).

Cationic polymerization

This is a kind of chain expansion polymerization in that a cationic initiator transfers charge to a monomer that becomes reactive. This reactive monomer goes on to react similarly through other monomers to form a polymer. The kinds of monomers essential for cationic polymerization are bounded to olefins through electron-donating substituents and heterocycles. Analogous to anionic polymerization reactions, cationic polymerization reactions are extremely sensitive to the type of solvent utilized. Specifically, the ability of a solvent to shape free ions will dictate the reactivity of the propagating cationic chain. Monomers for cationic polymerization are nucleophilic and shape a stable cation upon polymerization. 

Cationic polymerization of olefin monomers happens through olefins that enclose electron-donating substituents. Such electron-donating collections build the olefin nucleophilic sufficient to attack electrophilic initiators or growing polymer chains. At the similar time, such electron-donating groups joined to the monomer must be talented to stabilize the consequential cationic charge for additional polymerization. 

Initiator for Cationic Polymerization  

There are a variety of initiators obtainable for cationic polymerization, and several of them require a co-initiator to create the required cationic species.

• Classical protonic acids

Strong protic acids can be used to form a cationic initiating species. High concentrations of the acid are needed in order to produce sufficient quantities of the cationic species.

The counterion (A^-) produced must be weakly nucleophilic so as to prevent early termination due to combination with the protonated olefin. Common acids used are phosphoric, sulfuric, fluro-, and triflic acids. Only low molecular weight polymers are formed with these initiators. 

Fig: Initiation by protic acid

• Lewis acids/Friedel-Crafts catalysts

Lewis acids are the most general compounds utilized for initiation of cationic polymerization. The more popular Lewis acids are SnCl₄, AlCl₃, BF₃, and TiCl₄. Even though such Lewis acids alone are able to induce polymerization, the reaction happens much faster through a suitable cation source. The cation source can be water, alcohols, or even a carbocation donor these as an ester or an anhydride. In such systems the Lewis acid is termed to as a coinitiator while the cation source is the initiator. Upon reaction of the initiator through the coinitiator, an intermediate compound is shaped that then goes on to react through the monomer unit. 

The counterion generated via the initiator-coinitiator complex is less nucleophilic than that of the Bronsted acid A^- counterion. Halogens, these as chlorine and bromine, can as well initiate cationic polymerization upon addition of the more active Lewis acids.

  Fig: Initiation with boron trifluoride (co-initiator) and water (initiator)

• Carbonium ion salts

Stable carbonium ions are utilized to initiate chain expansion of only the most reactive olefins and are recognized to provide well described structures. Such initiators are most frequently utilized in kinetic studies due to the ease of being able to calculate the disappearance of the carbonium ion absorbance. General carbonium ions are trityl and tropylium cations. 

                       Initiation with trityl carbonium ion

Propagation by Ionic chain carriers

Chain growth occurs through the repeated addition of a monomer in a head-to-tail manner to the carbonium ion, through retention of the ionic character throughout.  The propagation mechanism depends on the counter-ion, the solvent, the temperature, and the kind of monomer. The solvent and the counterion (the gegen ion) have a important consequence on the rate of propagation. The counterion and the carbonium ion can have dissimilar connections, ranging from a covalent bond, tight ion pair (unseparated), solvent-separated ion pair (partially separated), and free ions (totally disconnected). 

Fig: Range of associations between the carbonium ion (R⁺) and gegen ion (X^-)

The connection is strongest as a covalent bond and weakest when the pair exists as free ions. In cationic polymerization, the ions have a tendency to be in symmetry between an ion pair (either tight or solvent-separated) and free ions. The more polar the solvent utilized in the reaction, the better the solvation and division of the ions. Because free ions are more reactive than ion pairs, the rate of broadcast is faster in more polar solvents. The size of the counterion is as well a feature. A smaller counterion, by a higher charge density, will contain stronger electrostatic interactions by the carbonium ion than will a larger counterion which has a lower charge density.  Additional, a smaller counterion is more simply solvated via a polar solvent than a counterion through low charge density. The consequence is enhanced propagation rate by increased solvating capability of the solvent. Chain length is as well influenced via temperature. Low reaction temperatures, in the range of 170-190 K, are favored for producing longer chains. 

Termination

Termination usually happens via unimolecular rearrangement through the counterion. In this procedure, an anionic fragment of the counterion joins through the propagating chain end. This not only inactivates the producing chain, but it as well ends the kinetic chain through reducing the concentration of the initiator-coinitiator complex. 

 Fig: Termination by combination with an anionic fragment from the counterion

Chain transfer

Chain transfer can occur in 2 ways. One technique of chain move is hydrogen abstraction from the active chain end to the counterion. In this procedure, the growing chain is ended, but the initiator-coinitiator compound is redeveloped to initiate more chains. 

Fig: Chain transfer by hydrogen abstraction to the counterion

The 2^nd process engages hydrogen abstraction from the active chain end to the monomer. This ends the growing chain and as well shapes a new active carbonium ion-counterion complex that can maintain to propagate, therefore keeping the kinetic chain intact.  

 Fig: Chain transfer by hydrogen abstraction to the monomer 

Example:  Polymerization of Styrene                                                            

• Chain Initiation

Initiation utilizes a strong Lewis acid which requires or proceeds faster in the occurrence of a proton donor. The proton donor is termed to as the initiator whilst the Lewis acid is the co-initiator, because the proton donor eventually supplies the proton that adds to monomer that initiates polymerization. Here a strong lewis acid boron trifluoride is utilized as the co-initiator, while water is the proton source and it is recognized as the initiator.  

                   BF₃ +  H₂O              H+ (BF₃OH)^-

                                            Initiator co-initiator complex

The initiator,-co-initiator compound contributes a proton to the styrene molecule to provide carbonium ion. 

       H+ (BF₃OH)^- + (C₆H₅)-CH=CH₂      (C₆H₅)-CH₃-C+ (BF₃OH)^-

                             Styrene                                  activated monomer 

Proton addition yields a styrene carbonium ion that forms connection by a BF₃·OH counterion.

• Chain Propagation

Carbonium ion adds to double bond of another styrene molecule.      

 Fig: Propagation stage

• Chain Termination

Termination via amalgamation of 2 cationic polymer chains can't occur instead proton transfer to a monomer, polymer, solvent or counterion will usually happen           

Fig: Chain termination                                          

As annihilation step demonstrates, the original initiating specie is reinstated at the end of polymerization that can then reinitiate polymerization as an active catalyst. The termination process above is said ion-pair precipitation (there is a proton donation and reformation of BF₃ hydrate at the end of polymerization). Instances of polymers shaped via cationic polymerization are poly isobutylene, polystyrene, poly (oxymethylene) and so on.

Summary of Characteristics of Addition Polymerization: 

i. It happens in 3 distinct steps - chain initiation, chain propagation, and chain termination.

ii Several side reactions might occur, these as: chain transfer to monomer, chain transfer to solvent, and chain transfer to polymer.  

iii. High molecular weight polymer is shaped at low conversion.

iv. No small molecules, such as H₂O, are eliminated in this procedure. 

v. New monomer adds on the growing polymer chain via the reactive active centre that can be a free radical in radical polymerization, carbocation in cationic polymerization and carbanion in anionic polymerization.

 Table: Several significant Addition Polymers, their monomer precursors, properties and utilizes.

Condensation (step) polymerization

This takes place between 2 bifunctional molecules to create one larger polyfunctional molecule. The bifunctional molecules (monomers) usually condense through one another, and in so doing repeatedly eliminate a small molecule such as H₂O, NH₃, HCl, and so on. As the reaction proceeds. The reaction continues until approximately or all of the reactants are utilized up. This polymerization takes place through series of steps. The kind of end product consequential from a condensation polymerization is dependent on the number of functional end groups of the monomer that can react. Monomers by only one reactive group terminate a growing chain, and therefore provide end products by a lower molecular weight. Linear polymers are generated using monomers through 2 reactive end groups and monomers by more than two end groups provide 3 dimensional polymers that are cross linked. The monomers, which are engaged in condensation polymerization aren't the similar as those in addition polymerization. They have 2 major traits:

Instead of double bonds, such monomers have functional groups (as alcohol, amine, or carboxylic acid groups).  Each monomer has at least 2 reactive sites, which generally means two functional groups.                               

Several monomers contain more than 2 reactive sites, permitting for branching between chains, in addition to raising the molecular mass of the polymer. During the polymerization procedure, the monomers tend to form dimers (two linked monomers) and trimers (three linked monomers) first. Then, such very short chains react through each other and by monomers. It is only near the end of polymerization that very long chains are shaped. Instance of condensation polymers are polyamides, polyacetals, polyesters, polycarbonates and polyurethanes.

Mechanism of Condensation Polymerization 

Recall that monomers that are linked via condensation polymerization have 2 functional groups; therefore they have two reactive sites and can form long-chain polymers. Specified below are instances of condensation polymerization reactions.

A carboxylic acid and an amine forms an amide linkage  

         diacid                      diamine                                   polyamide     

A carboxylic acid and an alcohol forms an ester linkage - 

 nHO-R-OH + nHOOC-R-HOOH     H [OR-OOC-R₁-CO ]nOH + (2n + 1)

        diol                       diacid                                      polyester                                                                                    

Example 1:

A carboxylic acid monomer and an amine monomer can connect in an amide linkage.

Fig: Carboxylic acid monomer

As seen above, a water molecule is eliminated, and an amide linkage is formed. An acid group continues on one end of the chain, which can react through another amine monomer. Likewise, an amine group continues on the other end of the chain that can react by another acid monomer. Therefore, monomers can continue to connect via amide linkages to form a long chain. Since of the kind of bond that links the monomers, this polymer is said a polyamide. The polymer made from such two six-carbon monomers is recognized as nylon-6, 6. 

Example 2:

A carboxylic acid monomer and an alcohol monomer can connect in an ester linkage.

Fig: Alcohol monomer

A water molecule is eliminated as the ester linkage is shaped. Notice the acid and the alcohol groups that are still accessible for bonding. Because the monomers above are all joined via ester linkages, the polymer chain is polyester.  

Self-condensation

In condensation polymerization, it is possible sometimes to have 2 reactive functional groups on the similar monomer, in that case, it can lead to a self-polycondensation reaction. An instance is the polycondensation of amino enanthic acid to form polyenanthoamide:   

       nH₂N (CH₂)₆ COOH    H [NH(CH₂)₆CO]n OH  +  (n-1) H₂O   

Summary of the Characteristics of Condensation Polymerization

i. The molecules should have one or two functional groups and the reaction can occur between the similar or dissimilar functional group.

ii. They proceed through step-wise reaction between reactive functional groups. 

iii. Hydrogen bonding provides them a crystalline structure through a tensile strength.

iv. They have an extremely elevated melting point.

v. They are thermosets as once molded, they can't be remolded.

vi. They have high T_g and T_m values that build them rigid as glass and transparent.

vii. The reaction can happen between 2 monomers, one monomer and one dimer, or a dimer through an oligomer, or a chain by another chain of polymers.

viii. The polymers shaped still encloses both the reactive functional groups at its chain ends (as end groups) and therefore, is 'active' and not 'dead', as in chain polymerization. 

ix. They possess blended properties of both the molecules and functional group. 

Table: Several condensation polymer examples with their components and uses.

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