Polymerization and polycondensation reactions

Main characteristics and stages

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Poly­mer­iza­tion and poly­con­den­sa­tion re­ac­tions are wide­ly used in or­gan­ic chem­istry for the syn­the­sis of new sub­stances. Poly­mers ob­tained in the poly­mer­iza­tion re­ac­tion have enor­mous sig­nif­i­cance for the in­dus­tri­al sphere, so the top­ics of poly­mer­iza­tion and poly­con­den­sa­tion are es­pe­cial­ly im­por­tant.

Poly­mer­iza­tion re­ac­tion

Poly­mer­iza­tion is the process of form­ing a high-molec­u­lar com­pound (poly­mer). In the course of the process, low-molec­u­lar com­pounds grad­u­al­ly at­tach to each oth­er. Ini­tial­ly they start to at­tach to the ac­tive cen­ter, which is lo­cat­ed at the very start of the grow­ing chain.

Poly­mer­iza­tion is of­ten clas­si­fied ac­cord­ing to the fol­low­ing char­ac­ter­is­tics:

by the num­ber of monomers par­tic­i­pat­ing in the process:

  • ho­mopoly­mer­iza­tion (only 1 monomer par­tic­i­pates);
  • copoly­mer­iza­tion (sev­er­al monomers par­tic­i­pate);

by the na­ture of the ac­tive cen­ter:

  • rad­i­cal (ac­tive cen­ter formed by a free rad­i­cal);
  • ion­ic (ac­tive cen­ters formed by ions).

The poly­mer­iza­tion process also has the fol­low­ing main char­ac­ter­is­tics:

  • usu­al­ly only sub­stances with mul­ti­ple bonds en­ter into poly­mer­iza­tion;
  • the process of­ten takes place with the re­lease of heat in the course of re­ac­tions;
  • poly­mer­iza­tion can take place with sub­stances which may open cyclic groups;
  • the poly­mer­iza­tion process may be car­ried out by var­i­ous meth­ods de­pend­ing on the state of the sub­stance (in a so­lu­tion, cast­ing poly­mer­iza­tion, gaseous or sol­id poly­mer, in a liq­uid medi­um).
An example of alkene polymerization, in which each styrene monomer's double bond reforms as a single bond plus a bond to another styrene monomer. The product is polystyrene. [Wikimedia]

Usu­al­ly poly­mer­iza­tion takes place in sev­er­al suc­ces­sive stages:

  1. Ini­ti­a­tion. At this stage, cer­tain parts of the mol­e­cule (monomers) turn into ac­tive cen­ters. This is pos­si­ble un­der the im­pact of spe­cial­ly in­tro­duced sub­stances (cat­a­lysts). At this mo­ment, we may ob­serve the re­lease of light, heat, elec­tric cur­rent and en­er­gy in the process of poly­mer­iza­tion.
  2. Growth of the chain. The grad­u­al suc­ces­sive at­tach­ment of monomers to the ac­tive cen­ter of the chain takes place.
  3. Break of the chain. Takes place from the de­struc­tion of the ac­tive cen­ter of the macro­molecule. This is pos­si­ble be­cause of the col­li­sion of two ac­tive cen­ters.

It is also pos­si­ble for the ac­tive cen­ter to shift to an­oth­er par­ti­cle of the mol­e­cule (monomer or sol­vent). In this case, the growth of a new chain be­gins (split­ting).

If the first two stages are al­ways present in the course of any poly­mer­iza­tion process, a break in the chain or the shift of its ac­tive cen­ter may not even take place at all in some cas­es.

In­ter­est­ing demon­stra­tion of poly­mer­iza­tion:

Poly­con­den­sa­tion re­ac­tion

Poly­con­den­sa­tion is the process of form­ing poly­mers by the com­bi­na­tion of dif­fer­ent monomers. The process is fre­quent­ly ac­com­pa­nied by the re­lease of var­i­ous sub­sidiary low-molec­u­lar prod­ucts (wa­ter, al­co­hol, salt).

For poly­con­den­sa­tion, the fol­low­ing monomers are char­ac­ter­is­tic: com­pounds with mol­e­cules of at least 2 func­tion­al groups. They are usu­al­ly di­vid­ed for con­ve­nience into three groups:

  • iden­ti­cal func­tion­al groups which do not re­act among each oth­er (di­amines)
  • dif­fer­ent func­tion­al groups which may re­act among each oth­er and thus form poly­mers (amino acids);
  • iden­ti­cal func­tion­al groups which may re­act among each oth­er, form­ing sim­ple polyethers;

In this process, re­ac­tions of func­tion­al groups of monomers are some­times pos­si­ble not only with oth­er groups, but among each oth­er. This ex­plains why so many poly­mers can be formed.

General chemical structure of one type of condensation polymer [Wikimedia]

Poly­con­den­sa­tion is a process of sev­er­al suc­ces­sive stages. Monomers are used up rel­a­tive­ly quick­ly (at the ear­ly stage of re­ac­tion). Af­ter this, a high-molec­u­lar poly­mer is formed from oth­er oligomers which were pre­vi­ous­ly formed by func­tion­al groups. In this process, dif­fer­ent ex­change re­ac­tions are pos­si­ble. In the poly­con­den­sa­tion process, nu­mer­ous poly­mers are formed which take part in me­tab­o­lism and var­i­ous bi­o­log­i­cal pro­cess­es in the hu­man body.

Main dif­fer­ences

It may seem that poly­mer­iza­tion and poly­con­den­sa­tion are very sim­i­lar re­ac­tions. In fact, their only sim­i­lar­i­ty lies in the syn­the­sis of poly­mers. In poly­con­den­sa­tion, be­sides the main groups (poly­mers), sec­ondary sub­stances are also formed – low-molec­u­lar com­pounds (wa­ter, al­co­hol, am­mo­nia).

If in the poly­mer­iza­tion process, the com­po­si­tion of the monomer and poly­mer is iden­ti­cal, in the poly­con­den­sa­tion process the com­po­si­tion of the fi­nal macro­molecule may sig­nif­i­cant­ly dif­fer from the orig­i­nal com­po­si­tion of the monomer.

Im­por­tance of in­di­ca­tors

Poly­mer­iza­tion and poly­con­den­sa­tion have a di­rect in­flu­ence on the qual­i­ty of the ma­te­ri­als that are man­u­fac­tured in these pro­cess­es.

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The dura­bil­i­ty of the ma­te­ri­al de­pends di­rect­ly on the av­er­age de­gree of poly­mer­iza­tion. The high­er the de­gree of poly­mer­iza­tion, the more vis­cous, durable and long-last­ing the ma­te­ri­al can be ob­tained. The flex­i­bil­i­ty of the raw ma­te­ri­al is re­duced. But if tru­ly dense poly­mers are re­quired, the tem­per­a­ture of the process must be in­creased, in or­der to evap­o­rate the sol­vent (dur­ing poly­mer­iza­tion in a so­lu­tion) and raise the con­cen­tra­tion of monomers. This in­di­ca­tor is de­ci­sive in choos­ing raw ma­te­ri­als for the man­u­fac­ture of fi­nal prod­ucts. For ex­am­ple, if the poly­mer­iza­tion de­gree of cel­lu­lose is less than 100, this ma­te­ri­al is not con­sid­ered suit­able for fur­ther use.

Here you’ll find more ar­ti­cles about poly­mers.

Poly­con­den­sa­tion also has great im­por­tance for the syn­the­sis of poly­mers. Be­sides the syn­the­sis of the poly­mers them­selves, sub­sidiary prod­ucts may also be ob­tained, or in oth­er words, monomers may be pu­ri­fied of un­nec­es­sary ad­di­tives to ob­tain a more con­cen­trat­ed poly­mer.

Poly­mer­iza­tion in in­dus­try is used much more fre­quent­ly. This is the method used to ob­tain 75% of syn­thet­ic poly­mers man­u­fac­tured world­wide (poly­styrene, polyvinyl chlo­ride).