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What is step growth polymerization?
A stairway to success Polymerization is a gradual procedure in which high-molecular-weight polymers are generated after a significant number of steps between bifunctional or multifunctional monomers. All monomers are responsive in contrast to chain growth.
As a result, most monomers are utilized quickly in the polymerization process to generate short chain oligomers that later interact to make long polymer chains. A condensation reaction occurs when two polyfunctional molecules combine to form a more enormous polyfunctional compound with the loss of a minor molecule like water. The reaction will proceed until one of the reagents has been exhausted.
The reactivity of dicarboxylic acids with diamines to generate polyamides and the reactivity of organic acids with alcohols to form polyesters such as polyethylene terephthalate are well-known step polymerization examples.
Historical aspects
The majority of natural polymers used in early human civilization are condensation polymers. Leo Baekeland revealed the production of bakelite, the first genuinely synthetic polymeric material, in 1907 using phenol and formaldehyde in a classic step-growth polymerization pattern.
As head of a research team at DuPont in the 1930s, Wallace Carothers, a founder of synthetic polymer science, invented a new method of producing polyesters using step-growth polymerization. It was the first polymerization reaction that outcomes were anticipated by scientific theory.
It was also the first reaction devised and explicitly performed to produce a high molecular weight polymer network. Carothers invented a set of mathematical equations to capture the dynamics of step-growth polymerization processes, which are still in use today and are referred to as the Carothers calculations.
Condensation polymer
A vast number of significant and valuable polymer composites are generated via ordinary functional group conversions of polyfunctional reactants rather than chain-growth processes that require reactive species such as radicals. These polymerizations usually result in the loss of a minor byproduct, such as water, and join two main components in an alternate structure.
Synthetic condensation polymers, often known as step-growth polymers, include the polyester Dacron and the polyamide Nylon 66. Contrary to chain-growth polymers, which develop by forming carbon-carbon bonds, step-growth polymers grow by forming carbon-heteroatom bonds. Even though polymers are alternate copolymers, the recurring monomeric unit is commonly referred to as a coupled moiety.
Mechanism of step growth polymerization
Flory’s equal reactivity concept can be used to characterize the kinetics of step-growth polymerization. Flory hypothesized that all steps, such as dimerization, trimerization, and so on, have the same rate constants. This hypothesis considerably simplifies the ordinarily complex dynamics of condensation polymerization. Flory evaluated the following two scenarios:
Polymerization without catalyst
The esterification process between alcohol and carboxylic acid is a notable case of step-growth polymerization. Titration of the unreacted acids in materials obtained from the mixture at different periods can be used to track the development of the polyester-forming process. Acids are considered to accelerate simple esterification processes. A secondary acid molecule acts as a catalyst without a strong acid.
Polymerization with catalyst
The polymerization is performed in concentrated sulphuric acid, and the catalyst concentrations are maintained steady during the operation; the synthesis will follow second-order reaction kinetics. The average degree of polymerization increments with the rate of reaction, which is a much better condition than just the weak-acid accelerated third-order reactions for generating high mean molecular weight polymers.
Classes of step growth polymer
High strength, strong flexibility, resistance to abrasion, good hardness, and excellent solvent resistance characterize polyamide nylon. The metal alternative in bushings, rope, belting, fiber cloths, thread, and jackets on electric cable are all examples of polyamide uses.
Polysiloxane comes in various physical bodies, greases, rubbers and waxes, and resins. Antifoam and release agents, wire insulation and gaskets, seals, cable, hot liquids, gas conduit, and other applications are all possible with this material.
Polycarbonates are self-extinguishing, translucent materials. They have crystalline thermoplasticity, strong impact resistance, and good thermal and oxidative stability, among other qualities. They have implications for manufacturing, the car industry, and medicine.
Step growth polymerization v/s chain growth polymerization
| Step polymerization | Chain polymerization |
| Any molecule like monomer, oligomer, or polymer existing can react with other components. | Monomers interact with the active center at the ending of the expanding chain during propagation. |
| Monomers are present all across the process. However, they are absorbed in high numbers early on. | Monomers are present throughout the process, although their concentration declines over time. |
| The oligomers and polymers’ end groups are responsive all across the polymerization process, hence no need for a termination step. | Propagation and initiation are two separate processes that occur during polymerization. In most circumstances, there is also a step to end the process. |
| The process continues quickly at first, but the molecular weight gradually increases. The high molecular weight is only achieved by lengthy oligomers interacting with each other at the conclusion of the process. | The reaction rate is influenced by the concentration of the initiation and co-initiator, and high-molecular-weight polymers are formed all across the reaction. |
| The creation of long high molecular weight polymers necessitates extensive reaction speed. | Long reaction periods result in high conversion rates but have little effect on the average molecular weight. |
| Molecular entities of any length of oligomers are present across the reaction, with the length dispersion widening and switching to higher molecular weight as response time progresses. | Only minor amounts of developing polymer chains are present in the mixture, consisting primarily of monomers and polymers. |
Conclusion
Not only is it vital to have a fundamental grasp of polymerization processes since they influence the structure and thus characteristics, but some processing pathways can transform monomers straight to a completed shape. They provide significant cost savings to the manufacturing sector both directly and indirectly.
