Step growth reaction

Step growth reactions , also called step growth polymerizations , are polymerizations that lead to the formation of polymers in steps in independent reactions . During polymerization, monomers react to form dimers , dimers with monomers and dimers with one another. The intermediate products of any degree of polymerization continue to react gradually to form macromolecules . Thus, step growth reactions differ fundamentally from chain polymerizations , in which only monomers are continuously added to an active center of a growing chain.

Two types of step polymerization can be distinguished:

The most common stage polymerizations are polycondensations : each stage is a condensation reaction that takes place with the elimination of small molecules such as H ₂ O , HCl , NH ₃ or H ₃ C – OH .

Certain stage polymerizations are polyadditions : each stage is an addition reaction and therefore takes place without the splitting off of small molecules.

The IUPAC recommends the collective name Stufenwachstumspolymerisationen (engl .: Step-growth polymerization) or step-growth reaction is not defined and polycondensation and polyaddition as independent of the polymerization. The two polymerizations have in common that they only form macromolecules at very high conversions.

General course of the reaction

First, dimers (P ₂ ) and then trimers (P ₃ ) or tetramers (P ₄ ) are formed from the monomers (M ). As growth progresses, molecules with any degree of polymerization react with one another. In the reaction equation, i and j can therefore have all integer values ≥ 1. In the polycondensation, a molecule (L), such as. B. water, split off. The progress of growth is otherwise similar to polyaddition. The actual reaction mechanism of a given polymerization is not relevant. There are also exceptional cases in which radical chain reactions proceed as polyaddition and not as radical chain polymerization .

Scheme for the course of a step growth reaction with conversions 0, 41 and 67%. Long chains only form when sales are very high.

Step growth reactions	First step	Follow-up steps	General steps
Polyaddition	M + M → P ₂	P ₂ + M → P ₃ P ₂ + P ₂ → P ₄	P _i + P _j → P _{i + j}
Polycondensation	M + M → P ₂ + L	P ₂ + M → P ₃ + L P ₂ + P ₂ → P ₄ + L	P _i + P _j → P _{i + j} + L

The polymerization requires monomers that carry at least two functional groups . Two variants are possible: The conversion of monomers of only one type with two different functional groups (A – B monomers) and reactions between two different types of monomers (A – A monomers and B – B monomers), each with two identical functions wear. In order to achieve a high degree of polymerization, A – A monomers and B – B monomers must be used in a ratio of 1: 1.

The green segments of the circles represent two reactive functional groups that all monomers carry at the beginning of the reaction (0% conversion). At 25% conversion, essentially only dimers have formed; reactive groups have combined to form a chemical bond.

Type	functional groups	general chemical formula	Structure of the polymer
A-B monomer	Amino acid with an amino group and a carboxy group	HOOC-R-NH ₂
A – A monomer and B – B monomer	Dicarboxylic acid and diamine	HOOC-R ¹ -COOH and H ₂ N-R ² -NH ₂
Example: monomers for polyamides

Degree of polymerization and weight fraction distribution


Development of the mean degree of polymerization over the conversion P according to the Carothers equation . The stoichiometric factor r indicates the ratio of the A – A / B – B monomers. If the ratio is 1: 1 or if only A – B monomers are used, r = 1. ${\ displaystyle {\ overline {X}} _ {n}}$	Weight fraction distribution w _p over the degree of polymerization X _i with a linear step growth reaction of A – B monomers with different conversions U according to the Schulz-Flory distribution .

The plot of the conversion P against the logarithmic representation of the mean degree of polymerization shows that macromolecules are only formed at high conversions. In the case of polycondensations in particular, a very high conversion is only achieved by aftertreatment of the polymer under reduced pressure and at elevated temperature, the elimination product being removed from the system. If a monomer is volatile under these conditions and was used in a slight excess, a 1: 1 ratio of the monomers can be set subsequently and a higher degree of polymerization can be achieved. The weight fraction distribution shows the distribution of the chain lengths in a polymer at different conversions. Even at a conversion of 0.99 the distribution is very broad and the polymer still contains oligomers. Classic chain polymerizations, by the way, lead to macromolecules immediately after the start of the reaction. The weight fraction distributions are similar to the distributions in step growth reactions at high conversions. ${\ displaystyle {\ overline {X}} _ {n}}$

While bifunctional monomers lead to linear polymer chains, the use of (additional) tri- or polyfunctional monomers leads to crosslinked structures . The mean degree of polymerization increases significantly earlier and steeper than the conversion. Insoluble polymers form, which leads to gelling of the reaction mass.

In order to produce crosslinked polymers, however, only oligomeric precursors ( prepolymers ) are generally first produced and mixed with other monomers or prepolymers and auxiliaries to form synthetic resins . The crosslinking is carried out only when the synthetic resin (eg. As in the desired shape or location of the application of adhesives ) brought has been, since the reaction product no longer meltable or malleable a thermoset or at lower crosslinking a not fusible elastomer is .

Examples

As a rule, its starting materials or the mechanism of its polymerization cannot be derived from the structure of a polymer under consideration. Often there are several ways to get to the polymer.

The polycondensation

of dimethyl terephthalate and ethylene glycol leads to the polyester polyethylene terephthalate (PET)
of AH salt , a salt of adipic acid and hexamethylenediamine leads to the polyamide PA 6.6
of the A – B monomer ε-aminocaproic acid leads to the polyamide PA 6

There are more efficient ways of producing PA 6. All examples lead to macromolecules that are used directly as plastics. In polyaddition, often only prepolymers are produced that are mixed to form synthetic resins or used as components in 2-component resins (adhesives, paints).

The polyaddition

of 1,6-hexane diisocyanate and 1,4-butanediol leads to a polyurethane
of bisphenol A diglycidyl ether and bisphenol A results in an epoxy resin

In the case of the epoxy resin mentioned, a prepolymer with two epoxy groups is generated via the steering system with the aid of the stoichiometric factor r . The average degree of polymerization remains low even with very high conversion and a diglycid polyether is formed: ${\ displaystyle {\ overline {X}} _ {n}}$

This higher molecular weight bisphenol A diglycidyl ether is reacted, for example, with the trifunctional monomer diethylenetriamine as hardener , which leads to crosslinked macromolecules.

Typical functional groups

Typical reactions of functional groups in step growth

Individual evidence

↑ Entry on step growth polymerization. In: Römpp Online . Georg Thieme Verlag, accessed on November 17, 2019.
↑ Bernd Tieke: Makromolekulare Chemie , 3rd edition, Wiley-VCH, Weinheim, 2014, p. 15ff.
↑ Ulrich Jonas, Patrick Theato: Glossary of terms related to kinetics, thermodynamics and mechanisms of polymerizations (IUPAC recommendations) , In: Angew. Chem. , 2009, Vol. 121, No. 50, pp. 9725-9738.
↑ Entry on polycondensation . In: IUPAC Compendium of Chemical Terminology (the “Gold Book”) . doi : 10.1351 / goldbook.P04722 Version: 2.3.3.
↑ Entry on polyaddition . In: IUPAC Compendium of Chemical Terminology (the “Gold Book”) . doi : 10.1351 / goldbook.P04720 Version: 2.3.3.
↑ Entry on chain polymerization . In: IUPAC Compendium of Chemical Terminology (the “Gold Book”) . doi : 10.1351 / goldbook.C00958 Version: 2.3.3.
↑ Erich Fitzer, Werner Fritz: Technische Chemie: Introduction to chemical reaction engineering , 3rd edition, Springer, Berlin, Heidelberg, 1989, p. 413.

[roempp-1] Entry on step growth polymerization. In: Römpp Online . Georg Thieme Verlag, accessed on November 17, 2019.

[MakroChem-2] Bernd Tieke: Makromolekulare Chemie , 3rd edition, Wiley-VCH, Weinheim, 2014, p. 15ff.

[IUPAC-de-3] Ulrich Jonas, Patrick Theato: Glossary of terms related to kinetics, thermodynamics and mechanisms of polymerizations (IUPAC recommendations) , In: Angew. Chem. , 2009, Vol. 121, No. 50, pp. 9725-9738.

[Gold_Book_con-4] Entry on polycondensation . In: IUPAC Compendium of Chemical Terminology (the “Gold Book”) . doi : 10.1351 / goldbook.P04722 Version: 2.3.3.

[Gold_Book_add-5] Entry on polyaddition . In: IUPAC Compendium of Chemical Terminology (the “Gold Book”) . doi : 10.1351 / goldbook.P04720 Version: 2.3.3.

[Gold_Book_chain-6] Entry on chain polymerization . In: IUPAC Compendium of Chemical Terminology (the “Gold Book”) . doi : 10.1351 / goldbook.C00958 Version: 2.3.3.

[FF-TC-7] Erich Fitzer, Werner Fritz: Technische Chemie: Introduction to chemical reaction engineering , 3rd edition, Springer, Berlin, Heidelberg, 1989, p. 413.