Polymers and polymeric materials offer a wide range of properties. But in practice, different proper — Analytical Chemistry Chemistry Question
Synthesis of block copolymers followed by size-exclusion chromatography
Polymers and polymeric materials offer a wide range of properties. But in practice, different properties are simultaneously desirable, such as different chemical, thermal or mechanical properties. A way to achieve such combinations is to combine existing polymer segments into block copolymers. Here we study the synthesis of a block copolymer of styrene and dimethylsiloxane (PS-b-PDMS-b-PS) and characterize the resulting polymers using Size-Exclusion Chromatography (SEC). Living anionic polymerization is used to achieve narrow molecular weight distributions and precise block lengths.
Anionic polymerization of styrene is typically initiated by an organolithium reagent. Identify the initiator used in this synthesis.
Model Answer
n-Butyllithium (n-BuLi)
Draw the chemical equation or reaction mechanism for the initiation of styrene polymerization using sec-butyllithium (s-BuLi) as the initiator.
Model Answer
s-Bu- Li+ + CH2=CHPh → s-Bu-CH2-CH(Ph- Li+
To obtain a polymer with a narrow molecular weight distribution, what is the relationship between the initiation rate constant ka and the propagation rate constant kp?
Model Answer
ka >> kp
Write the rate of disappearance of the monomer, -d[M]/dt, during the propagation step as a function of the propagation rate constant kp, the monomer concentration [M], and the active species concentration [AM_i^-].
Model Answer
-d[M]/dt = kp * \sum_i [AM_i^-] [M]
Assuming the total active species concentration is constant and equal to C (\sum_i [AM_i^-] = C), show that the rate of monomer consumption can be written as a first-order rate law, and write the integrated rate equation.
Model Answer
-d[M]/dt = kp * C * [M]
Integrated rate equation: [M](t) = [M](t=0) * e^(-kp * C * t)
Deduce from this equation the half-life time denoted t½ of the polymerization reaction as a function of kp and C.
Model Answer
t½ = ln 2 / (kp * C)
We now consider different synthesis conditions for styrene polymerization. Fill in with molecules the mechanism presented below: [VISUAL]
Model Answer
Initiation: Ph-COO-O-CO-Ph → 2 Ph* + 2 CO2; Ph* + CH2=CHPh → Ph-CH2-CH*(Ph)
Propagation: Ph-(CH2-CH(Ph))_i-CH2-CH*(Ph) + CH2=CHPh → Ph-(CH2-CH(Ph))_{i+1}-CH2-CH*(Ph)
Termination: Combination of two propagating radical chains to yield a dead polymer chain Ph-(CH2-CHPh)_{i+j+2}-Ph
Is this polystyrene synthesis regioselective? (Yes/No)
Model Answer
Yes, because of the preferential formation of one product over another (the reaction selectively generates one constitutional isomer over the other). Radicals add to the terminal carbon atom of styrene to form a benzyl-type radical.
Draw the structure of the intermediate that explains the regioselectivity observed during the addition of the propagating radical to the styrene monomer.
Model Answer
The intermediate is a resonance-stabilized secondary benzylic radical: Ph-CH2-CH*(Ph), where the radical is delocalized over the aromatic ring.
During this reaction the synthesis of macrocycles is observed. Draw a mechanism or a pattern to explain the formation of such macrocycles.
Model Answer
The macrocycles are formed by an intramolecular backbiting mechanism, where the active nucleophilic silanolate chain end attacks a silicon atom within its own polymer backbone, extruding a cyclic siloxane oligomer (such as D3) and leaving a shorter linear active chain.
Transfer reactions are also observed. Draw a mechanism showing what a transfer reaction in such a reaction medium could be.
Model Answer
The transfer reaction involves the transfer of an active center from the growing chain end to another molecule (such as the monomer, solvent, or polymer backbone) through a proton or atom transfer, terminating the growth of the active chain and starting a new propagating chain.
Fill in the gaps in the following sentence with the word “low” or “high”:
“Mn is more sensitive to molecules of molecular mass while Mw is more sensitive to molecules of molecular mass. Therefore the more the polymer chains approach uniform chain length, the more Ip is close to 1.”
Model Answer
Mn is more sensitive to molecules of low molecular mass while Mw is more sensitive to molecules of high molecular mass. Therefore, the more the polymer chains approach uniform chain length, the more Ip is close to 1.
Associate which reaction conditions lead to the highest Mn. Which of the 3 curves on the SEC chromatogram is related to the highest Mn? [VISUAL]
Model Answer
Curve (b) (the shorter the polymer, the higher the elution volume). Therefore, the curve with the lowest elution volume peak (curve b) corresponds to the highest molecular weight polymer.
Match each curve with reaction conditions (I, II or III with (a), (b) or (c)).
Model Answer
The full width half maximum of the SEC signal is related to the Ip value: the closer to 1, the narrower the peak. Curve (a) and (b) represent highly monodisperse samples (low Ip) with different molecular weights, while curve (c) shows a broader peak, representing a more polydisperse sample (high Ip).
Suggest a structure for the polymer that is finally obtained.
Model Answer
The final product is a symmetric triblock copolymer of styrene and dimethylsiloxane: PS-PDMS-O-Si(CH3)2-CH2-CH2-Si(CH3)2-O-PDMS-PS, formed by coupling two active PS-PDMSOLi chains with a bifunctional dichlorosilane coupling agent.
The final product is then fractionated (a fraction of the polymer chains are separated from the sample, according to their length). Associate the SEC experimental curves (1, 2, 3 or 4) measured at different stages of the synthesis (figure below) with the corresponding molecules (PS, PS-PDMS precursor, unfractionated product, or fractionated product). [VISUAL]
Model Answer
PS → 3 (highest elution volume, smallest size)
PS-PDMS precursor → 2 (intermediate size, larger than PS but smaller than the coupled product)
unfractionated product → 1 (bimodal curve showing both the desired coupled final product and the unwanted uncoupled precursor/homopolymer impurities)
fractionated product → 4 (highest molecular weight, lowest elution volume, narrow unimodal peak showing only the purified triblock copolymer)