Step-growth polymerization
Many naturally-occurring and some synthetic polymers are produced by step-growth polymerization, e.g. polyesters, polyamides, polyurethanes, etc.
There also is the possibility to have more than two reactive sites on a monomer: In this case branched polymers production take place.
Most natural polymers being employed at early stage of human society are of condensation type.
The pioneer of synthetic polymer science, Wallace Carothers, developed a new means of making polyesters through step-growth polymerization in 1930s as a research group leader at DuPont.
Collaborating with Paul Flory, a physical chemist, they developed theories that describe more mathematical aspects of step-growth polymerization including kinetics, stoichiometry, and molecular weight distribution etc.
The simple esterification is an acid-catalyzed process in which protonation of the acid is followed by interaction with the alcohol to produce an ester and water.
For a system with equivalent quantities of acid and glycol, the functional group concentration can be written simply as After integration and substitution from Carothers equation, the final form is the following For a self-catalyzed system, the number average degree of polymerization (Xn) grows proportionally with
Hence, and integration gives finally For an externally catalyzed system, the number average degree of polymerization grows proportionally with
For theoretical and practical reasons it is of interest to discuss the distribution of molecular weights in a polymerization.
The molecular weight distribution (MWD) had been derived by Flory by a statistical approach based on the concept of equal reactivity of functional groups.
[12][13] Step-growth polymerization is a random process so we can use statistics to calculate the probability of finding a chain with x-structural units ("x-mer") as a function of time or conversion.
[1] Notes: Substituting from the Carothers equation We can now obtain: The polydispersity index (PDI), is a measure of the distribution of molecular mass in a given polymer sample.
Impurities with A or B functional groups may drastically lower the polymer molecular weight unless their presence is quantitatively taken into account.
[13] More usefully, a precisely controlled stoichiometric imbalance of the reactants in the mixture can provide the desired result.
The advantages of lightweight polymers include: high strength, solvent and chemical resistance, contributing to a variety of potential uses, such as electrical and engine parts on automotive and aircraft components, coatings on cookware, coating and circuit boards for electronic and microelectronic devices, etc.
To obtain desired mechanical strength, sufficiently high molecular weights are necessary, however, decreased solubility is a problem.
One approach to solve this problem is to introduce of some flexibilizing linkages, such as isopropylidene, C=O, and SO2 into the rigid polymer chain by using an appropriate monomer or comonomer.
Polyethersulfones are partially crystalline, highly resistant to a wide range of aqueous and organic environment.
The polyketones are finding applications in areas like automotive, aerospace, electrical-electronic cable insulation.
The first stage forms a soluble and fusible high-molecular-weight poly(amic acid) in a polar aprotic solvent such as NMP or N,N-dimethylacetamide.
The poly(amic aicd) can then be processed into the desired physical form of the final polymer product (e.g., film, fiber, laminate, coating) which is insoluble and infusible.
Alkyne, nitrile, and cyanate end-capped oligomers can undergo cyclotrimerization yielding aromatic structures.
