Monomer Addition.
Section 3.2.3 reported the chain extension, or self-blocking, of linear p(HPMAx)
generating a block copolymer with low molecular weight distribution, with no experimental difficulty. This section aims to investigate whether chain ends in branched copolymerisations still remain active after sequential addition of additional monomer feeds. This may be done in three ways, one of which would be to copolymerise chains of branched polymers and after reaching high conversion, add another batch of monomer and brancher to give a doubly branched copolymer. This reaction was not performed as it was highly likely that a gelled network would result, due to previous experiments where insoluble copolymer mass was formed after leaving copolymerisations which had reached > 100 % conversion.
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Scheme 3.6 represents some of the different architectures which could be generated by varying the order of monomer and brancher addition. A) may be seen as an “arms-first” approach, B) and C) both could be seen as a “core-first” approach to generate branched block copolymers where maroon spheres represent the outer (or second) block of monomer. B) was attempted several times; despite successful addition of the second linear block, all reactions gelled. The exact reason for this is unclear but the presence of unreacted pendant vinyl groups which become available when the second monomer feed is added may lead to further branching and gelation. This would occur if, rather adding and an essentially linear chain to the branched polymer end groups, the longer chain ends are able to access hidden pendant reactive functionality. It may also be that the polymerisation reactions were left for too long after high conversion of the second monomer feed and gelation occurred due to termination by combination reaction. It is proposed that if reactions are terminated before gelation ensues, the architecture will be more similar to those previously synthesised rather than the block copolymer depicted in Scheme 3.6(B).
Route (A) shown in Scheme 3.6, was conducted with the initial formation of
p(HPMA30) chains (red spheres) and, after achieving relatively high conversion, a
second batch of HPMA monomer (equivalent to a DPn = 80 monomer units)
containing EGDMA brancher (green spheres) at an initiator:brancher ratio of 1:0.95 was added. The second aliquot of monomer is represented in Scheme 3.6(A) by maroon spheres and a controlled copolymerisation was expected, however, the resulting copolymer was also expected to form a dramatically different architecture when compared to the formation of a branched copolymer from the direct copolymerisation of HPMA and EGDMA with a primary chain target DPn = 80
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monomer units. The synthesis of a soluble branched copolymer was expected, as only a small quantity of brancher was added and hypothetically, the introduction of a short chain of p(HPMA) prior to the addition of brancher should have no negative impact on the polymerisation and crosslinking should not occur. 1H NMR spectroscopy and GPC analysis were used to monitor the reaction over time, with a sample taken just before addition of the second monomer batch to ensure relatively high conversion had been reached. Triple detection GPC (THF eluent) was used to analyse the samples, as shown in Figure 3.25.
Figure 3.25: GPC chromatograms at varying points during a branched chain extension using methanolic ATRP to form p((HPMA30)-b-((HPMA50)-co-
EGDMA0.95)).
It is clear from Figure 3.25 that high molecular weight material is formed, due to brancher introduced within the second batch of monomer. Molecular weight distribution increases as conversion increases suggesting that branching dominates at extremely high conversion. Again, as previously noted, a linear peak appears at high
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retention volume, suggesting a proportion of these copolymer chains are not branched, even at high conversion. Importantly, no linear polymer from the initial formation of the linear p(HPMA30) chains are observed in the final polymer. This
suggests a very efficient propagation of the second mixed monomer:brancher feed without termination of the initial propagating chains. The presence of linear polymer in the final branched polymer sample must therefore be derived from the addition of the second HPMA monomer feed but without successful intermolecular branching of these chains.
Table 3.4 displays the data obtained during this reaction where no theoretical molecular weights could be calculated as EGDMA incorporation is statistical. Very high number average and weight average molecular weight was obtained (Mn =
185000 g mol-1, Mw = 1128500 g mol-1) alongside a molecular weight distribution of
143 Sample Mn (g mol-1) (theoretical)* Mn (g mol-1) (GPC) Mw (g mol-1) (GPC) Mw/Mn Conversion (%) p(HPMA30) 3500 3300 3600 1.09 81 p(HPMA30)-b- p((HPMA50)-co- EGDMA0.95) / 25400 51200 2.10 57 p(HPMA30)-b- p(HPMA50)-co- EGDMA0.95) / 50000 245000 4.90 78 p(HPMA30)-b- p(HPMA50)-co- EGDMA0.95) / 185000 1128500 6.10 99
Table 3.4: Data for the branching chain extension of p(HPMA) using a statistical copolymerisation of HPMA and EGDMA utilising methanolic ATRP. *Theoretical
values are based on the measured conversion values using 1H NMR (DMSO-d6).
Similar to Section 3.2.3 where, linear block copolymers, synthesised from a self- blocking experiment with an overall target DPn = 80 monomer units, were compared
to p(HPMA80) synthesised in a single reaction, Figure 3.26 shows the GPC
chromatograms (RI detector response) of p((HPMA80)-co-EGDMA0.95) overlaid with
the same chromatogram from p(HPMA30)-b-p((HPMA50)-co-EGDMA0.95). The
branched self-blocked copolymer begins to elute at a lower retention volume in comparison to the branched copolymer prepared with EGDMA being present at all
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times. Previous analysis of the RI chromatograms of branched polymers synthesised during this study have shown that a significant fraction of linear polymer is present in the final branched copolymers. The overlaid chromatograms in Figure 3.26 also suggest the presence of linear polymers, however, despite the nominal equivalent chain length of the linear polymers (DPn = 80 monomer units), when EGDMA is
added in the second monomer feed, the linear fraction of the polymer distribution can be seen at a lower elution volume.
Figure 3.26: Overlaid Refractive Index chromatograms from GPC analysis (THF eluent) of p((HPMA80)-co-EGDMA0.95) (solid line) and p(HPMA30)-b-p(HPMA50)-
co-EGDMA0.95) (dashed line).
The presence of longer DPn linear chains may contribute to the observed decrease in
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for p((HPMA80)-co-EGDMA0.95). The reason for the higher molecular weight
fraction in the chain extended copolymer is not clear; as this reaction was only performed once, it may be that temperature and/or experimental protocol were slightly different. Homogeneity of the reaction mixture will be altered as the second monomer feed is added, and as diffusion may be hindered it may lead to less than ideal EGDMA incorporation.
Figure 3.27 shows the GPC chromatograms for p(HPMA30)-b-p((HPMA50)-co-
EGDMA0.95) and p((HPMA120)-co-EGDMA0.95), where near complete overlap of
peaks at approximately 18 mL is noted.
Figure 3.27: Overlaid Refractive Index chromatograms from GPC analysis (THF eluent) of p((HPMA120)-co-EGDMA0.95)) (solid line) and p(HPMA30)-b-
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The data suggests that the linear material present within the chain extended copolymer is much more likely to have a DPn of approximately 120 monomer units,
suggesting that despite an overall target DPn of 80 monomer units, the generation of
linear material of a higher chain length is formed. It is possible that these are lightly branched chains or possibly dimers, produced by linear chains of DPn = 80 monomer
units and enhanced incorporation of EGDMA is seen through this approach.