Polystyrene chain extension was studied as the formation of block copolymers and the living nature of the chains is important in the formation of amphiphilic block copolymers, which could be used in the synthesis of polymeric nanoparticles.
For a living polymerisation, with no assumed termination, all initiated and propagating polymer chains should possess an ‘active’ anion after full monomer conversion that is ready to undergo further polymerisation with the addition of more monomer. The formation of a block copolymer was therefore attempted by first polymerising styrene using sec-BuLi, as described previously, to achieve a target DPn of 50 monomer units.
After 24 hours a further volume of styrene, equivalent to 50 monomer units, was added and after another 24 hour period, another volume equivalent to 50 monomer units was added to the reaction mixture. Before each addition of monomer, samples of the reaction were taken and purified for analysis. The resulting polymer and samples were purified as before, and analysed by GPC under the same conditions previously
described. Figure 3.10 shows the GPC chromatographs of the final polymer, and overlaid are the samples of the polymer taken before each sequential addition of monomer.
Figure 3.10: GPC chromatograms of the self blocking polymerisation of polystyrene. The Mn increases with the sequential addition of monomer so for DPn50 the theoretical Mn is 5,200 gmol-1 (red), DPn50 + 50 monomer units the theoretical Mn value is 10,400 gmol
-1
(blue) and DPn50 + 50 + 50 monomer units the theoretical Mn value is 15,600 gmol-1.
As can be seen from Figure 15, the final polymer sample is multimodal with three defined species. As these appear to clearly overlay with the chromatograms of the samples with targeted DPn = 100 and DPn =50 monomer units, it would appear that a
significant amount of termination has occurred before each addition, and polystyrenes of varied chain lengths are present.
Termination may have happened during sampling, and adding the second and third batches of monomer as there is potential for air and therefore water to be introduced to the system. The time that the polymerisations were given to progress to full conversion (24 hours) may also have contributed to extra termination.
A study of the kinetics of the anionic polymerisation led to a dramatic change in the understanding of these polymerisations, and significantly changed the reaction time used experimentally for each polymerisation; a previous 24 hour reaction time was reduced to just 1 hour. With this change in synthetic method the previous chain extension experiment was repeated. A polystyrene of DPn = 150 was initially targeted
and the first addition of further monomer (equivalent to 50 monomer units) for the chain extension was added after 30 minutes, and the second monomer aliquot (equivalent to a further 50 monomer units) was added after another 30 minutes, giving a targeted polymer chain length of DPn = 250 monomer units.. Figure 3.11 shows the GPC
chromatographs of the final polymer, and overlaid are the samples of the polymer taken before each sequential addition of monomer.
Figure 3.11: GPC chromatograms of the self blocking polymerisation. Mn increases with the sequential addition of monomer so for DPn150 the theoretical Mn is 15,600 gmol-1 (red), DPn150 + 50 monomer units the theoretical Mn value is 20,800 gmol-1 (blue) and DPn150 + 50 + 50 monomer units the theoretical Mn value is 26,000 gmol-1.
Table 3.3 shows the values for the target Mn and the actual observed Mn values from
taken before each addition. The GPC chromatogram obtained showed that the final polymer was essentially monomodal, with a polydispersity of 1.20.
Table 3.3: Target Mn, Actual Mn and Ð values determined by GPC
Target DPn Target Mn (gmol-1) GPC Actual Mn (gmol-1) Ð 150 15,600 13,900 1.20 200 20,800 19,000 1.21 250 26,000 24,200 1.20
Each addition of monomer led to a linear increase in the polymer number average molecular weight as illustrated further in the graph in Figure 3.12.
To explore possible chain extensions further, which will be used in later experiments to significantly alter the architecture of branched polystyrene, a series of linear chain extension experiments were carried out. The following polystyrene DPn values were
targeted:
Initial DPn = 10 with the addition of 40 monomer units (final DPn = 50 monomer
units)
Initial DPn = 10 with the addition of 90 monomer units (final DPn = 100 monomer
units)
Initial DPn = 50 with the addition of 50 monomer units (final DPn = 100
monomer units)
Table 3.4, is a table of the Mn and Mw values of the final polymers, as well as the Mn
and Mw value of the sample taken before the second addition of monomer. This table
and the GPC chromatographs of the final polymers overlaid in Figure 3.13 illustrates the success of the self-blocking experiments.
Table 3.4: Mn and Mw values of the pre-curser sample taken before additional monomer addition and Mn, Mw and Ð values for the final self-blocking polymer synthesised by anionic polymerisation techniques.
Targeted DPn GPC Mn Mw Final Mn (gmol-1) Final Mw (gmol-1) Ð DPn10+DPn40 (Total DPn50) 1,000 1,100 5,320 5,540 1.04 DPn10+DPn90 (Total DPn100) 1,000 1,100 11,100 11,500 1.04 DPn50+DPn50 (Total DPn100) 4,810 5,070 9,280 11,100 1.09
Figure 3.13: GPC chromatograms of the self blocking polymerisation of polystyrene by anionic polymerisation techniques.