Experiment 3
Radical Polymerisation Synthesis and UV-Vis, SEC and PNIPAM
Characterisation
Faculty of Science
School of Chemistry
SID: 510514495
Introduction
Radical polymerisation is a chain-growth polymerisation method used in industry to produce
synthetic materials such as polyvinyl and other polyethylene (PET) products. A method for
determining molecular weight distribution in a radical polymerised substance includes size
exclusion chromatography which is a process separating polymers by size instead of weight.
Similarly, to radical polymerisation, RAFT polymerisation is used in industry for such
purposes but involves a chain transfer agent that enables the chain of the monomer to grow,
creating uniformity in the polymer. Stemming from RAFT polymerisation the PET-RAFT
process encompasses a new element, light, to propel radical polymerisations, thus, rendering
initiators unnecessary (no heat required). The chain transfer agent is excited by a photredox
catalyst, in this case, ZnTPP which initiates the chain-growth. The chain transfer agent in this
experiment is HEAA with the role of controlling the polymerisation process. The aim of this
experiment is to investigate methods to polymerise and characterise acrylamides, hence, the
monomer used in this experiment is DoPAT as it provides suitable control over the
polymerisation process during the RAFT equilibrium. When characterising polymers, UVVis spectrophotometer can be used to determine the cloud point temperature of an aqueous
NIPAM solution [1].
Experimental Section
Stage 1:
The 4mL glass vial was weighed including the screw cap. Approximately 5 mg of DoPAT
monomer was added to the vial and the molar mass of DoPAT was calculated. 500 molar
equivalents of HEAA chain transfer agent was added to the vial using a pipette. The exact
amount of monomer added was recorded and 800 μL of DMSO was added to the vial. A
clean stir bar mixed the mixture until all contents were dissolved. Finally, 200 μL of ZnTPP
photocatalyst was added to the stock solution to give a concentration in DMSO of 5.0
mg/mL. The vial lid was screwed on tightly and placed into the photoreactor with the start
time noted. The polymerisation then proceeded for 105 minutes. While the polymerisation
underwent, the following steps were carried out:
1) 1.5 mL of SEC solvent was added to a glass vial a 1 mL syringe, a 0.45 um syringe
filter, and a SEC vial including lid with septum was set aside.
2) 100 mL of acetone and a medium-sized stir bar into a 250 mL beaker was added
and set aside.
After removing the polymerisation from the photoreactor, 2mL of DMSO was added and
shaken vigorously until the polymer solution was homogenous. Homogeneity was ensured
via using a vortex mixer. The polymer was then precipitated by adding dilute polymerisation
mixture by dropwise under stirring to an acetone flask. A yellow/white polymer immediately
precipitated and was separated using a Buchner funnel the distinguish the solvent from the
polymer. 2mg of the precipitated polymer was added to a glass vial containing the SEC
solvent which was then extruded into a 1 mL syringe and added into another SEC vial for the
SEC measurement. The remaining polymer was added to a clean glass vial.
Stage 2:
In this stage, the ZnTPP (photoredox catalyst) was prepared for its analysis. 0.100 g of
ZnTPP was weighed out into a 100 mL flask with a stirrer bar. 50 mL of chloroform and 5
mL of methanol was added, and the solution was stirred for 5 minutes (not to point of
dissolution). Zinc acetate (0.100 g) was then added to the reaction mixture and a reflux
condenser was fitted to the reaction vessel. The reaction mixture was then heated under reflux
for 15 minutes on a steam bath. The solvent was then removed onto a rotary evaporator and
50 mL of water was added. The solution was then sonicated for 5 minutes before the purple
coloured solid crystals were collected via suction filtration. The crystals were then washed
with ethanol and dried on the pump. Crude yield was recorded, and the purity of the product
was measured via thin layer chromatography using chloroform:hexane, 1:1 as an eluting
solvent. The ZnTPP was then characterised using UV-Vis spectroscopy by preparing a
ZnTPP solution of approximately 2 mg in 10 mL dichloromethane. The visible spectra (400700 nm) of ZnTPP was taken, a glass cuvette was used for this analysis.
Stage 3:
In this stage, phase transitions were measured by monitoring the transmission of visible light
of a polymer solution with changing temperature. A 3 mL syringe and a 0.45 um Nylon filter
were used to add 2.5 mL of the poly(n-isopropylacrylamide (PNIPAM) solution into a UVVis cuvette. The spectrometer temperature cell was set to 30 ˚C and the visible spectrometer
was blanked with a clean MilliQ water sample and the transmission for the polymer solution
at 650 nm was collected.
In 1-degree increments, the transmissions for the polymer solution were collected with 5minute wait periods between each reading. These readings were then used to estimate the
temperature at which the phase transition occurred. A narrower temperature was chosen, and
the solution was cooled down and heated in smaller 0.2 ˚C increments, with the same timed
wait periods in between reading. This enable a more accurate cloud point temperature to be
obtained.
Results and Discussion
Figure 1: Reaction scheme for ZnTPP [1].
Results and Discussion
UV-Vis Spectrum ZnTPP
1,8
1,6
1,4
Absorbance (Au)
1,2
1
0,8
0,6
0,4
0,2
0
350
400
450
-0,2
500
550
600
650
700
Wavelength (nm)
Figure 2: UV-Vis Spectrum of ZnTPP
Figure 2 reveals a major peak at 424 nm at which ZnTPP absorbs photons, nearly within the
UV range of the visible spectrum. Absorption of these photons excites the electrons in its
HOMO (Highest Occupied Molecular Orbital) which allows it to act as a photocatalyst.
ZnTPP was activated with visible light which in turn, activated the DoPAT (chain transfer
agent) which is required for polymerisation. It is also important to note the minor peak at 550
nm which is within the visible light range, specifically the yellow region. The significance of
this peak is that it gives numerical reasoning for the purple appearance of the crystals
procured in Stage 2 of the experimental procedure.
Figure 3: UV-Vis absorption spectra of ZnTPP in neutral, first oxidation and first reduction
states in black, red and blue lines respectively [2].
Figure 3 depicts a literature example from the Journal of Chemical Research which depicts a
neutral state UV-Vis spectrum of ZnTPP with the same peaks as shown in Figure 2. Since
ZnTPP is highly conjugated, it is able to absorb visible light also due to the p-p transitions
occurring with the ligands which explains the major peaks at 424 nm and minor peaks at 550
nm in both Figure 2 and Figure 3, hence making ZnTPP an efficient photocatalyst. The
agreement between the peaks displayed in Figure 2 and the literature values shown in Figure
3 suggest the high level of purity of the compound obtained in Stage 2 of the experiment.
Calculating Average Degree of Polymerisation
Determining molar ratio between HEAA and DoPAT:
Moles of CTA, DoPAT:
0.0005 𝑔
350.60 𝑔 𝑚𝑜𝑙 −1
= 0.00001426 𝑚𝑜𝑙
𝑛(𝐷𝑜𝑃𝐴𝑇) =
Moles of monomer unit, HEAA:
1.111 × 0.00075
115.13
= 0.00000724 𝑚𝑜𝑙
𝑛(𝐻𝐸𝐴𝐴) =
Molar ratio:
0.00000724
0.00001426
= 506
𝐻𝐸𝐴𝐴: 𝐷𝑜𝑃𝐴𝑇 =
The degree of polymerisation can be calculated by a simplified equation:
𝐷𝑃 = 𝑀𝑜𝑙𝑎𝑟 𝑟𝑎𝑡𝑖𝑜 (𝐻𝐸𝐴𝐴: 𝐷𝑜𝑃𝐴𝑇) × 𝑐𝑜𝑛𝑣𝑒𝑟𝑠𝑖𝑜𝑛 %
Based on the fact that there was an 80% conversion where 80% of the DoPAT monomer was
consumed in the polymerisation and the molar ratio between HEAAA:DoPAT was calculated
to be 505.6, the degree of polymerisation was found to be:
𝐷𝑃 = 506 × 0.8
𝐷𝑃 = 406
From this, the molecular mass of the polymer product could be determined via:
𝑀𝑜𝑙𝑒𝑐𝑢𝑙𝑎𝑟 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑝𝑜𝑙𝑦𝑚𝑒𝑟 = 𝑀𝑊(𝐷𝑜𝑃𝐴𝑇) × 𝐷𝑃
= 350.6 × 406
= 142 𝑔 𝑚𝑜𝑙 −1
Calculating Dispersion
Ð=
̅̅̅̅̅
𝑀𝑤
̅̅̅̅
𝑀𝑛
SEC Elugram
3500
Relative Intensity
3000
2500
2000
1500
1000
500
0
10
11
12
13
14
15
16
17
18
19
20
Retention Time (min)
Figure 4: SEC Elugram of Polymer
The significance of the SEC Elugram is the information it provides on the weight distribution
of the polymer. In this case, we can see the polymer weight is not normally distributed base
on the asymmetric nature of the monomodal peak. The significance of only having one peak
is that it determines whether or not the termination of the polymer process occurred in one or
two methods. Based on the fact the peak is monomodal, the termination of the polymer
process likely occurred via one method only.
Cloud Point Transmitance
120
Conentration mg/mL
100
80
60
40
20
0
31,5
32
32,5
33
-20
33,5
34
34,5
Temperature ˚C
30 mg/mL
20 mg/mL
10 mg/mL
Figure 5: Cloud Point Analysis Graph
5 mg/mL
2.5 mg/mL
1.25 mg/mL
35
Figure 5 depicts the decreasing solubility of the thermoseparating polymer (PNIPAM) in the
solution as the temperature increases. The cloud point temperature (LCST) at each polymer
concentration is depicted with varying colours representing each polymer concentration for
the temperature at which transmittance reached 50%. The turbidity of each solution became
poor (cloudy) at the LCST, indicating the decreasing solubility of the polymer. This occurred
as a result of the increasing temperature and graphically represents how the polymer was
coiling and hence, precipitating out from the solution as emulsion droplets containing the
polymer, making the solution cloudier. These emulsion droplets occur as a result of PNIPAM
undergoing a reversible thermos-responsive precipitation in water i.e. as the temperature
increases the polymer extracts from the solution in the PNIPAM-filled emulsion droplets [3].
The non-linear relationship between concentration and LCST implies an inverse relationship
between polymer concentration and LCST based on the fact that Figure 5 shows the highest
temperature increase associated with the lowest concentration this being approx. 34.8 ˚C and
1.25 mg/mL respectively.
Conclusion
The aim of this experiment was to polymerise acrylamides and characterise the product
through means of UV-Vis spectroscopy, SEC exclusion and cloud point analysis. It was
revealed by comparing the UV-Vis spectrum of ZnTPP to a literature example of the same
spectrum that the product yielded in this experiment was of high purity based on the near
identical resemblance between the two spectrums. The SEC Elugram and analysis showed
there was a somewhat asymmetrical distribution of weight throughout the polymer molecule
which could suggest a good dispersion but not a perfect molecule dispersion of 1. It could
also be concluded based on the SEC Elugram that the polymerisation synthesis was
monomodal based on the fact there was only one peak shown on Figure 4, indicating just one
method in the termination step in the process. Finally, the cloud point measurement analysis
indicated the inverse relationship between concentration of polymer in solution and the
temperature of solution. It demonstrated that the polymer extracted out of the solution i.e.
became more insoluble as a result of increasing the temperature of the solution. This was
observed via measuring the turbidity of the solution at the LCST. Hence, the general trend
seen can be summarised as more polymer-containing PNIPAM emulsion droplets
precipitating out of the solution proportionately to the increasing temperature.
Works Cited
[1] U. o. Sydney, CHEM2522 Dry Lab Manual, Sydney: School of Chemistry, 2020.
[2] O. Kazuya, Y. Umetsu and K. Kamimura, "Changes in the absorption spectra and colour
of tetraphenylporphyrins after redox reactions," Journal of Chemical Research, vol. 44,
no. 9, pp. 613-617, 2020.
[3] T. E. de Oliveira, D. Mukherji, K. Kremer and P. A. Netz, "Effects of stereochemistry
and copolymerization on the LCST of PNIPAM," Journal of Chemical Physics, no. 146,
2017.