Copolymerisation of PAMPS with N-hydroxyethyl acrylamide (HEAm) and 4- acryloylmorpholine (NAM) were further studied to synthesise multiblock copolymers. These monomers were chosen as representative of acrylamide monomers because of their hydrophilicity and good reactivity. Octablock copolymer PAMPS-b-PHEAm and tetrablock copolymer PAMPS-b-PNAM were synthesised with an average DP of 10 for each block (Table 3-4). Chain extension of PAMPS macroCTA with HEAm or NAM required a longer reaction time to reach full monomer conversion than chain extension with AMPS®2405 itself (1.5 hours versus 1 hour), which can be attributed
[103]
Table 3-4: Block copolymers synthesised by aqueous RAFT polymerisation using AMPS®2405
and HEAm targeting an overall DP of 80.
Polymer Block Multiblock composition Conv.
(%)a Mn,th (g/mol)b Mn,SEC (g/mol)c Ð c 53 1 PAMPS10 99 2,600 5,500 1.09 - 2 PAMPS10-b-PHEAm10 99 3,700 4,000 1.25
- 3 PAMPS10-b-PHEAm10-b-PAMPS10 > 99 6,000 9,100 1.13
- 4 [PAMPS10-b-PHEAm10]2 > 99 7,100 8,400 1.18
- 5 [PAMPS10-b-PHEAm10]2-b- PAMPS10 > 99 9,400 13,100 1.18
- 6 [PAMPS10-b-PHEAm10]3 > 99 10,500 11,800 1.25
- 7 [PAMPS10-b-PHEAm10]2-b- PAMPS10 > 99 12,800 17,700 1.30
72 8 [PAMPS10-b-PHEAm10]4 > 99 13,900 16,700 1.48
18 1 PAMPS20 99 4,800 6,000 1.09
- 2 PAMPS20-b-PHEAm20 > 99 7,100 6,000 1.13
- 3 PAMPS20-b-PHEAm20-b-PAMPS20 > 99 11,700 10,000 1.14
73 4 [PAMPS20-b-PHEAm20]2 > 99 14,100 10,400 1.16
74 - PAMPS40-b-PHEAm40 > 99 13,600 8,300 1.35
75 - PAMPS40-co-PHEAm40 > 99 13,900 13,900 1.13
53 1 PAMPS10 99 2,500 5,400 1.09
- 2 PAMPS10-b-PNAM10 99 4,000 1,800 1.50
- 3 PAMPS10-b-PNAM10-b-PAMPS10 > 99 6,200 9,100 1.10
76 4 [PAMPS10-b-PNAM10]2 > 99 7,600 4,000 1.41
a Conversions were determined by 1H NMR spectroscopy, using Equation 1; b Theoretical M
n values were calculated using
Equation 2; c Experimental M
n and Ð values were determined by size-exclusion chromatography in 20 % MeOH / 80 % 0.1M
[104]
1H NMR spectroscopy was used to confirm full monomer conversion between each
monomer addition (Table S 3-4 and Figure S 3-7). Monomodal distributions were obtained after each block addition, with dispersities ranging from 1.09, for the first block, to 1.48 (Polymer 72) for the last block of the octablock copolymer of AMPS®2405 and HEAm.
Figure 3-14: Stepwise characterisation of the chain extension of Polymer 72 [PAMPS10-b-
PHEAm10]4.
A) DMF SEC chromatograms with conventional calibration; C) Molar mass and dispersity versus the number of blocks; B) Aqueous SEC molecular weight distribution calculated with conventional calibration; D) Molar mass and dispersity versus the number of blocks.
While the general trend shows a linear evolution of the experimental molecular weight with increasing number of blocks (Figure 3-14 – B and D), a shift towards lower molecular weight when PAMPS10 was chain extended with HEAm, followed by a shift
towards higher molecular weight when macroCTA (PAMPS10-b-PHEAm10) was
further chain extended with AMPS®2405. This “step effect” can be attributed to
differences in the nature of the two monomers (electrolyte versus neutral).122 As
PAMPS is a negatively charged polyelectrolyte, electrostatic interactions are expected to expand the polymer more than in the case of neutral polymer segments (i.e, NAM and HEAm), accounting for the irregular variation of hydrodynamic volumes, and therefore different molecular weights observed. The difference of molecular weights
B) A)
[105]
of the two monomers (MAMPS®2405 = 229.2 g/mol and MHEAm = 115.1 g/mol) is
expected to further enhance this phenomenon. This is in accordance with an observation made by McKenzie et al. for the synthesis of a hexablock copolymer of ethyl acrylate (EA) and methyl acrylate (MA).177 To verify this hypothesis each block
was analysed using DMF SEC with a polar column where a shift for each peak towards higher molecular weight was observed (Figure 3-14 – A). The molecular weights determined for each block by DMF-SEC were significantly higher than the theoretical one. Again this is attributed to the difference of hydrodynamic volume between our polymers and the PMMA standards used to calibrate the DMF SEC.
Figure 3-15: Stepwise characterisation for the chain extension of Polymer 73 [PAMPS20-b-
PHEAm20]2.
A) Aqueous SEC molecular weight distributions using conventional calibration ); B) Aqueous SEC molecular weight distributions using triple detection (RI, VS, LS detectors); C) DMF SEC chromatograms using conventional calibration.
In order to further investigate the effect of the block length on properties and for biological applications study (CHAPTER 5) a tetrablock of PAMPS and HEAm was synthesised, targeting a final DP of 80 ([PAMPS20-b-PHEAm20]2 versus [PAMPS10- b-PHEAm10]4) (Table S 3-5). Similar observations for the octablock polymer
synthesised previously were found (Polymer 72). When conventional aqueous SEC was used an overall increase of molecular weight was observed with steps (Figure 3-15), while a linear increase of molecular weight was observed when triple detection when Aqueous SEC was used. Additionally, when DMF SEC was used with conventional calibration a linear increase of molecular weight was observed (Figure 3-16). The molecular weights obtained were overestimated due to the nature of the standard for DMF SEC (i.e. PMMA).
HOOC O NH SO3-Na+ NH O OH b 20 20 S S S 3 O NH SO3-Na+ NH O OH b 20 20 b B) A) C)
[106]
Figure 3-16: Molar mass and dispersity for the chain extension of PAMPS with HEAm using
either the aqueous SEC with an RI detector only (orange), triple detection (blue) or DMF SEC with RI detector (red).
NAM, which has been widely used in the literature to synthesise well-defined multiblock homo- and copolymers due to its high reactivity, was used here as an alternative comonomer to demonstrate the robustness of the method.
Figure 3-17: Stepwise characterisation of the chain extension of Polymer 76 [PAMPS10-b-
PNAM10]2.
A) Aqueous SEC molecular weight distributions using conventional calibration; B) Molar mass and dispersity versus the number of blocks.
Using similar conditions to those used for HEAm, full monomer conversions (Table S 3-6 and Figure S 3-8) and monomodal chromatograms (Figure 3-17, Table 3-4) were obtained after each sequential monomer addition. As expected, a similar trend was seen to that of HEAm, with steps observed in the plot of the experimental
O NH SO3-Na+ N O b 10 10 S S S 3 O NH SO3-Na+ N O b 10 10 b O O HOOC A) B)
[107]
molecular weight versus the number of blocks. Similarly, this artefact was attributed to differences in the hydrodynamic volume of AMPS®2405 and NAM segments.
Finally, a diblock and a random copolymer of similar composition and molecular weight to the octablock copolymer of AMPS®2405 and HEAm were prepared for
comparison (Table 3-4, Polymer 74 and 75).
Figure 3-18: Characterisation of the copolymers synthesised with AMPS®2405 and HEAm
targeting a DP of 80 (diblock, tetrablock, octablock and random copolymer).
A) Aqueous SEC molecular weight distributions calculated by conventional calibration; B) Aqueous SEC molecular weight distributions calculated by triple detection (LS, RI and VS detectors in series); C) Theoretical and experimental Mn overlaid using either conventional
calibration or triple detection.
An induction period of 8 minutes was observed for the random copolymer synthesis (Figure S 3-9, Polymer 75), when compared to approximately 30 minutes for AMPS®2405 homopolymer synthesis (Section 2.3.2). This is likely due to the higher
consumption of the CTA with the incorporation of one monomer unit onto each BDMAT molecule, resulting in a faster monomer conversion which was obtained in 45 minutes. Figure 3-18 shows an overlay of the SEC molecular weight distributions for the copolymers synthesised. All three distributions revealed monomodal distribution with dispersities varying from 1.13 to 1.35. The narrower distribution observed for the random copolymer when compared to the block copolymers is likely due to a better distribution of both monomers along the polymer chains, lowering the repulsion between the negatively charged AMPS®2405. Using SEC with conventional
calibration, the experimental molecular weights of octablock, diblock and random copolymers were found to be 16.7, 8.3 and 13.9 kg/mol, respectively. These differences can be attributed to differences in the conformation of the three linear architectures. Indeed, spreading of the negative charge over the backbone is expected to result in a more elongated polymer due to electrostatic repulsion, which explains
[108]
why the hydrodynamic volume measured for the diblock is smaller than for its mixed counterparts. However, when the aqueous SEC coupled with triple detection was used the experimental molecular weights obtained were in good agreement with the theoretical molecular weight (Figure 3-18 – B and C). Again, this is explained in terms of differences between the hydrodynamic volume of the charged AMPS®2405
and the PEG/PEO standards used for conventional calibration. The better distribution of the two monomers in the random copolymer is expected to lessen this effect as such a phenomenon has already been reported in the literature.89
[109]