Mechanisms in Addition Chain-growth Polymerization, Chemistry tutorial


There are 3 basic mechanisms via which addition chain-growth polymerization can take place. They could be radical or cationic or anionic. The structure of the monomer and the initiator determines which will happen. Each of them happens via 3 stages: 

(i) Initiation stage, which commences the polymerization via forming the reactive molecule that can be radical or cationic or anionic. This is where the 3 essential mechanisms take their names. 

(ii) Propagation stage: where the expansion of the chain continues. Finally is 

(iii) Termination stage: Here the chain stops producing and the reaction is ended.

Main content

Addition chain-growth polymerization happens via 3 essential mechanisms. They are radical, cationic and anionic mechanisms. Each involves 3 stages that are initiation (reasoned via an initiator, which generally will not alter the properties of the polymer), propagation (always by propagating site at the end of the most recent unit added) and the termination stage

Radical addition chain-growth polymerization:

Generally an initiator is utilized to start the addition chain-growth polymerization reaction. The initiator is a compound that can convert the monomer into a free radical (a substance that contains an unpaired electron). When free radicals react with a double bond, a chain reaction occurs, which could theoretically continue as long as there is monomer material present. This is because the radical molecules are extremely reactive. There is homolytic breaking of the initiator to form a radical that adds to the vinyl monomer, changing it as well into a radical [active molecule]. Equation below illustrates how the initiator forms radicals. [RO• is a generalized symbol for a free radical initiator, which generally doesn't alter the properties of the polymer formed because it isn't in significant amount]. 

The radical reacts by the monomer to provide another radical of it that adds to other monomers, process repeated over and over, so the chain remains growing for example propagates, by the radical at the end of the most recent unit added, recognized as the propagating site. 

426_Radical addition chain-growth polymerization.jpg

Fig: Radical addition chain-growth polymerization

Activity 2: Show the propagating site in the above dimmer. At last the chain-growth reaction stops once the propagating site is no more available, there is no formation of other free radicals. The stage is referred to as 'termination stage'. Such might be caused via any of the followings: two radicals combine at the propagating sites 

ii. Disproportionation of 2 radicals takes place as one radical oxidizes to alkene [looses an H to the second], and another gets decreased to the alkane through accepting the H. 

iii. Combines by an impurity at the propagating site. Another case is the 'chain transfer' where another molecule AB merges via a growing chain such that A. finishes it, and B. is left to initiate growth of another chain. Here growth of the chains is being controlled. The readiness of molecule AB to homolytically split is influential here. Steric controls and nature of other on it are as well significant here. Most alkenes and vinyl compounds undergo radical addition polymerizations. 

Cationic addition chain-growth polymerization:

It is an electrophilic addition reaction. An electrophile initiates this polymerization reaction. It could be a proton [H+], which add to the vinyl molecule to turn it to a cation. Lewis acid and base for example BF3 and H2O is generally involved in producing the proton that adds to the unsaturation therefore begins the formation of the reactive cation. This now adds to other monomers, procedure repeated over and over, so the chain keeps raising for example propagation stage, by the positive charge at the end of the most recent unit added, recognized as its propagating site. 

Activity 3: Identify the cationic propagating site in the above dimmer. Termination of the cationic addition chain-growth reaction can be caused via any of the subsequent: 

i. Loss of a proton to shape unsaturation [double bond] at the end of the chain.

ii. Nucleophile adds to the cationic propagating site to form saturated polymer chain. 

iii. When there is chain shift reaction by the solvent Structure of the monomer verifies whether rearrangement will take place or not, this is towards retaining more stability. Rearrangements might happen depending on the stability of the carbocation formed. [Recall that tertiary C+ is most stable than secondary C+, and primary C+ is least stable]. Monomers that undergo cationic addition reactions include styrenes, methl vinyl ethers, and 2-methyl propene. 

Anionic addition chain-growth polymerization:

Nucleophile [usually a strong base for example butyllithium and sodium amide] initiates this polymerization by adding to the alkene to form a reactive anion, a carbanion. Alkenes are generally electron rich, so reluctant to addition of electrons, but presence of electron withdrawing groups [e.g. carbonyls and amides] that decrease density of electrons on the alkene foster it for nucleophilic reactions. The reactive anion [carbanion] formed add to other monomers, passes the negative charge to the extreme newly added monomer which now carries the carbanion. The process is repeated over and over, so the chain keeps growing for example propagation stage, by the negative charge at the end of the most recent unit added, known as its propagating site. General monomers that undergo anionic addition chain-growth reactions comprise methyl methacrylate, styrene, acrylamide, and acrolein.

i. a chain transfer reaction by solvent that provides a proton at the end of chain.

ii. Impurities in the reaction mixture reacting by it. 

iii. Propagation of chain terminates when all the monomers have completely participated in the reaction to form the chain, leaving no monomer unreacted and the propagating site still available for reactions if more monomers are added. What is formed is termed a 'living polymer'. 

Termination never takes place via loss of protons as in cationic or through disproportionation and radical combination. A good instance is glue a polymer of methyl methacrylate that shows this result when poured in between fingers, and it tends to gum the fingers mutually. This implies which nucleophilic group from the skin triggers the living polymer to persist chain formation reaction. Capability to continue forming covalent bond by the surface of material it is poured on makes it to glue-up/ join. The kind of substituent on the vinyl group verifies that of such 3 mechanisms the addition chain-growth polymerization will happen. The subsequent polymers are via addition polymerization: polymethyl methacrylate (Lucite, or Plexiglass), Teflon, polyvinyl alcohol, polystyrene, polyvinyl chloride (PVC), polyacrylonitrile (Orlon), and natural rubber.

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