This is a comprehensive summary describing the synthetic part of the most common polymers, taught by Prof. Niko Van Den Brande. Prof. Ulrich Hennecke's part that considers the synthesis and applications of nucleic acids is also described here. Ultimately, the peptide section of Prof. S. Ballet is a...
Partim 1: Polymer synthesis and molecular characterisation
Chapter 1: Introduction
1. Basic concepts
Most polymers are composed of a carbon backbone. Because polymers have a large number
of atoms, their molecular mass (MM) is high => hampers their solubility. Moreover, their MM
is often referred to as a distribution. The MM of polymers depends on the reaction kinetics
and it defines the macromolecular characteristics of the polymer.
Polymers can be:
- Amorphous (no ordered state)
- Semi-crystalline and amorphous
Brittle polymers easily break when being stretched ductile polymers are deformable.
Recrystallization is a way of purifying the compounds, but not applicable for polymers due to
their restricted viscosity. The glass transition temperature (Tg) is the temperature range after
which the polymer coils possess sufficient mobility to go from brittle to ductile.
1.1. Macromolecular architecture
- Thermoplastic
o Linear and branched macromolecules
o Can be reprocessed by ∆T
o Commodity thermoplastics (PVC, PE, PP, PS) and engineering thermoplastics
(PA, PET)
- Elastomers (rubber)
o Low cross link density networks
o Infinity MM
o E.g. Rubber (very stretchable)
- Thermoset
o High cross link density networks
o Infinite MM
o Can’t be reprocessed, instead degradation will occur
o E.g. Bakelite, epoxy resin
, 1.1.1. MM, distribution, and averages
The degree of polymerization X(N) = Mpoly/Mmono
The MM is not perfectly determined as it is a continuous distribution.
- Number average MM Mn (average)
- Weight average MM MW (mostly for scattering techniques)
- Z-average MM Mz (z of zentrifugation)
General average of a property distribution:
Mn and Mw are mostly used, Mz emphasis the high MM tail => more for industry purposes.
∑ 𝑛𝑖 𝑀𝑖 ∑𝑛 ∑ 𝑛𝑖 𝑀𝑖
Mn = ∑ 𝑛𝑖
with ∑ 𝑛𝑖 the molfraction ∑ xi => Mn = ∑ 𝑛𝑖
= ∑ xi Mi
𝑖
The polydispersity D (or PDI) is a measure that describes how wide the mass distribution is.
1.2. Polymerization: general considerations
nX monomers → n polymer chains (with X the degree of polymerization)
The functionality f is the amount of chain bonds the monomer unit can form. The synthesis
of polymers requires monomers with f ≥ 2 + high purity of the monomers because the reaction
has to be repeated many times.
f = 2 => linear or branched architecture
f ≥ 2 => networks
In order for the polymerization to occur spontaneously ∆G has to be negative:
∆G = ∆H – T∆S with ∆S that will be negative due to a decrease of the # of molecules
Therefore, the reaction has to be exothermic => ∆H < 0 ~heat transfer is important
2 polymerization mechanisms can be distinguished:
- Step growth polymerization
o Coupling of polymer chains
o Herein everything reacts with each other => quickly no monomers anymore,
but only oligomers that react together
o Should go to 100% conversion in order to have long polymer chains
- Chain growth polymerization
o Coupling of monomers
o Radical, anionic, cationic, ROP …
o The monomers are constantly added to the growing polymer chain
o The polymerization should be stopped before a full conversion because
otherwise too much side reactions could take place
,Chapter 2: Step growth polymerization
2. Step growth polymerization
2.1. What is step growth polymerization?
The rapid use of monomers, after which the polymer chains are coupled
Examples are Kevlar (bulletproof vest), polyurethane, PET, polycarbonates …
2.2. Linear step polymerizations
2.2.1. Introduction
A linear step growth polymerization is a succession of reactions between pairs of functional
groups (FG) with f ≥ 2.
, For a linear polymerization
(f =2) 2 types of reactions can be distinguished:
- Polycondensation
o The formation of small molecules besides the polymer
- Polyaddition
o The addition of 2 oligomers without the formation of a byproduct
2.2.2. Polycondensation reactions
A step growth linear polymerization of monomer units with f = 2 during which a byproduct
will be formed, that has to be eliminated in order to drive the reaction to full conversion.
2.2.2.1. Types of polycondensations
With A and B as FG.
The difficulty of polycondensations is that an equivalent amount of each FG has to be present
in order for the reaction to continue, hence for the polymer chain to grow. This exact
stoichiometry requirement is complicated for industrial purposes. A solution could be that 1
molecule has both functional groups (A and B) and can therefore react with each self. Self-
reacting FG can also be an option (A and A).
2.2.2.2. Polycondensations – Polyesters
Polyesters are formed via the esterification of a carboxylic acid with an alcohol at elevated
temperatures. The reaction is catalysed with a Lewis acid.
The reaction between terephthalic acid and etheleneglycol => PET
In the case of lactic acid (LA) an ARB polycondensation can take place => PLA
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