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Novel well-defined block copolymers via atom transfer radical polymerization and their aggregation behaviors
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Type
Thesis
Author
Yu, Hui
Supervisor
Gan, Leong Huat
Abstract
Various experimental conditions have been explored in an attempt to synthesize welldefined homopolymers of N-Acryloyl N’-phenyl piperazine (AcrNPhP) via atom transfer radical polymerization (ATRP). Varying the solvent, polymerization temperature, types of ligands, and initiators failed to yield well-defined polymer. It is concluded that controlled/”living” polymerization of AcrNPhP could not be achieved via the ATRP technique. The main reasons for the lack of control are believed to be similar to those offered for the unsuccessful ATRP of (meth)acrylamides: the copper cations complex to the amide group of the forming polymer chain ends. This stabilizes the radical and retards the deactivation step in ATRP, resulting in the formation of an unacceptable high concentration of radicals which leads to large termination reactions. In addition, the active Br chain end could be lost through a cyclization involving nucleophilic Br displacement by the penultimate amide nitrogen.
Well-defined diblock copolymers of 2- (dimethylamino)ethyl methacrylate (DMAEMA) and AcrNPhP (poly(DMAEMA-b-AcrNPhP)) with different Mn were successfully synthesized via the ATRP technique. The aggregation behaviors of the diblock copolymers were studied by laser light scattering (LLS) measurements. The following results were obtained for poly(DMAEMA189- b-AcrNPhP36) in aqueous solutions: Rh = 65 nm; Rg = 83 nm; Mw = 1.53 × 106 g mol–1; thus Nagg = ~ 40 and A2 = 1.70 × 10–4. The value of Rg/Rh = 1.27 indicates that swollen micelles are formed. For poly(DMAEMA140-b-AcrNPhP17) in aqueous solutions, Rh = 40 nm; Rg = 29 nm, hence Rg/Rh = 0.73; Mw = 5.1 × 105 g mol–1, hence Nagg = ~20. The results indicate that poly(DMAEMA140-b-AcrNPhP17) aggregates into compact spherical core-shell structures. In addition, poly(DMAEMA189-b- AcrNPhP36) was capable of forming regular honeycomb-patterned structure film. The honeycomb-patterns were seen clearly by laser confocal microscopy.
The diblock copolymer of poly(EO113-b-AcrNPhP11) and triblock copolymer of poly(AcrNPhP15-b-EO455-b-AcrNPhP15) were also successfully synthesized via the ATRP technique. The aggregation behavior of poly(EO113-b-PAcrNPhP11) in methanol solution was studied by LLS measurements with the following results: Rh = 14 nm; Rg = 16 nm; Mw = 9.36 × 104 g mol1, thus Nagg = 12 and A2 = –5.52 × 10–4 cm3 mol g–2. The value of Rg/Rh = 1.14 suggests that the micelle possesses a swollen core- shell structure. The LLS results for the triblock copolymer poly(AcrNPhP15- b-EO455-bAcrNPhP15) in methanol solution were: Rh = 35 nm; Rg = 29 nm; Mw = 1.65 × 106 g mol–1, hence Nagg = 62 and A2 = –1.4 × 10–5 cm3 mol g–2. Thus Rg/Rh = 0.83 which suggests the formation of a flower micelle with a core-shell structure. The negative A2 values in both cases indicate that methanol was not a good solvent for these two polymers.
A well-defined water-soluble double-[60]fullerene-capped block copolymer [C60-DMAEMA68-C(CH3)2COOCH2]2 was successfully synthesized via ATRP. TGA and spectrophometric analyses confirmed that mono-addition to C60 had occurred. In aqueous solution [C60-DMAEMA68- C(CH3)2COOCH2]2 self-assembled into flower micelles. The aggregation behaviors in unbuffered water and in acidic solution at pH 3 were studied in detail by LLS. In unbuffered distilled water, Rh = 31 nm; Rg = 34; thus Rg/Rh = 1.1, Mw = 5.58 × 106 g mol–1, hence Nagg = 242, and A2 = 1.7 × 10–4 cm3 mol g–2.
Whereas in pH 3 solution, Rh = 88 nm; Rg = 64, thus Rg/Rh = 0.73; Mw = 1.87 × 106 g mol–1 hence Nagg = 81 and A2 = 9.8× 10–4 cm3 mol g–2. It can be seen that more polymer molecules (∼3 times more) were involved in the formation of micelles in unbuffered distilled water vis-a-vis at pH 3, but the micelles were not as compact as those at pH 3 as indicated from their Rg/Rh values. Interestingly, the size of the micelles at pH 3 was larger. This is not totally unexpected as the corona of the micelles would be highly stretched at low pH owing to charge repulsion resulting from protonation of the amino groups. Similar aggregation behaviors were observed in THF solution, in which case, Rg/Rh was 0.88 and the aggregation number was 28. The LLS results were supported by AFM and TEM studies. The low CMC values of 28.6 mg dm–3 in unbuffered distilled water and 176.8 mg dm–3 at pH 3 augur well for the micelles as potential vehicles for drug/gene delivery application.
Another well-defined water-soluble double-end-capped-C60 ABA triblock polymer, C60-DMAEMA60-b-EO105-b-DMAEMA60-C60 was also successfully prepared via ATRP. TGA and spectrophometric analysis confirmed that mono-addition to C60 had occurred. In aqueous solutions, C60-DMAEMA60-b-EO105-b-DMAEMA60-C60 formed well-defined flower micelles. In unbuffered distilled water, Rh = 109 nm; Rg = 112; thus Rg/Rh = 1.03, Mw = 0.76 × 106 g mol–1, hence Nagg = 30 and A2 = 3.0× 10–4 cm3–2. In acidic solution at pH 1.7, Rh = 128 nm; Rg = 109; thus Rg/Rh = 0.85, Mw = 1.42 × 106 g mol–1, hence Nagg = 55 and A2 = 5.8 × 10–4 cm3 mol g–2. In THF, Rh = 43 nm; Rg = 45; thus Rg/Rh = 1.05, Mw = 0.44 × 106 g mol–1, hence Nagg = 17 and A2 = 4.1 × 10–4 cm3 mol g–2. Compared to [C60-DMAEMA68-C(CH3)2COOCH2]2, the micellar structure of C60- DMAEMA60-b-EO105-b-DMAEMA60-C60 was less affected by changing the medium from a near neutral solution to an acidic solution. The presence of the EO block increases the polymer/solvent compatibility in unbuffered solution, as indicated from the more positive A2 value, thus favors the formation of micelles. The size of the micelles increased only slightly at pH 1.7 as the EO block forms a large part of the corona. Hence the effect of charge repulsion resulting from the protonation of amino groups of the DMAEMA blocks only caused a slight expansion. The micellar aggregates were also examined by AFM and TEM microscopies. After calcination at 500 °C under N2, the micellar core consisting of C60 could clearly be seen by AFM. The low CMC value of 2.1 mg dm–3 for C60-DMAEMA60-b- EO105-bDMAEMA60-C60 in distilled water shows that the micelles are suitable candidate as potential vehicle for drug/gene delivery application. Furthermore, the micelles are expected to have added advantage of extending circulation lifetime in vivo, owing to the ‘stealth’ property of PEO at the corona, which will prevent/minimize protein adhesion.
For the two C60-containing block copolymers synthesized, only one decay mode was observed in aqueous solutions, indicating the existence of only one scattering species in each case. This is in contrast with the previous report of more than one decay mode for other C60-containing polymers in aqueous solutions, which have been attributed to the presence of unimers or/and a mixture of different aggregate structures. The better solubility imparted by the poly(DMAEMA) block and the well-defined chain length as the result of the controlled ATRP are the two main factors responsible for the formation of well-defined micellar aggregates for the two amphiphilic double-C60-capped block copolymers observed in this study.
Well-defined diblock copolymers of 2- (dimethylamino)ethyl methacrylate (DMAEMA) and AcrNPhP (poly(DMAEMA-b-AcrNPhP)) with different Mn were successfully synthesized via the ATRP technique. The aggregation behaviors of the diblock copolymers were studied by laser light scattering (LLS) measurements. The following results were obtained for poly(DMAEMA189- b-AcrNPhP36) in aqueous solutions: Rh = 65 nm; Rg = 83 nm; Mw = 1.53 × 106 g mol–1; thus Nagg = ~ 40 and A2 = 1.70 × 10–4. The value of Rg/Rh = 1.27 indicates that swollen micelles are formed. For poly(DMAEMA140-b-AcrNPhP17) in aqueous solutions, Rh = 40 nm; Rg = 29 nm, hence Rg/Rh = 0.73; Mw = 5.1 × 105 g mol–1, hence Nagg = ~20. The results indicate that poly(DMAEMA140-b-AcrNPhP17) aggregates into compact spherical core-shell structures. In addition, poly(DMAEMA189-b- AcrNPhP36) was capable of forming regular honeycomb-patterned structure film. The honeycomb-patterns were seen clearly by laser confocal microscopy.
The diblock copolymer of poly(EO113-b-AcrNPhP11) and triblock copolymer of poly(AcrNPhP15-b-EO455-b-AcrNPhP15) were also successfully synthesized via the ATRP technique. The aggregation behavior of poly(EO113-b-PAcrNPhP11) in methanol solution was studied by LLS measurements with the following results: Rh = 14 nm; Rg = 16 nm; Mw = 9.36 × 104 g mol1, thus Nagg = 12 and A2 = –5.52 × 10–4 cm3 mol g–2. The value of Rg/Rh = 1.14 suggests that the micelle possesses a swollen core- shell structure. The LLS results for the triblock copolymer poly(AcrNPhP15- b-EO455-bAcrNPhP15) in methanol solution were: Rh = 35 nm; Rg = 29 nm; Mw = 1.65 × 106 g mol–1, hence Nagg = 62 and A2 = –1.4 × 10–5 cm3 mol g–2. Thus Rg/Rh = 0.83 which suggests the formation of a flower micelle with a core-shell structure. The negative A2 values in both cases indicate that methanol was not a good solvent for these two polymers.
A well-defined water-soluble double-[60]fullerene-capped block copolymer [C60-DMAEMA68-C(CH3)2COOCH2]2 was successfully synthesized via ATRP. TGA and spectrophometric analyses confirmed that mono-addition to C60 had occurred. In aqueous solution [C60-DMAEMA68- C(CH3)2COOCH2]2 self-assembled into flower micelles. The aggregation behaviors in unbuffered water and in acidic solution at pH 3 were studied in detail by LLS. In unbuffered distilled water, Rh = 31 nm; Rg = 34; thus Rg/Rh = 1.1, Mw = 5.58 × 106 g mol–1, hence Nagg = 242, and A2 = 1.7 × 10–4 cm3 mol g–2.
Whereas in pH 3 solution, Rh = 88 nm; Rg = 64, thus Rg/Rh = 0.73; Mw = 1.87 × 106 g mol–1 hence Nagg = 81 and A2 = 9.8× 10–4 cm3 mol g–2. It can be seen that more polymer molecules (∼3 times more) were involved in the formation of micelles in unbuffered distilled water vis-a-vis at pH 3, but the micelles were not as compact as those at pH 3 as indicated from their Rg/Rh values. Interestingly, the size of the micelles at pH 3 was larger. This is not totally unexpected as the corona of the micelles would be highly stretched at low pH owing to charge repulsion resulting from protonation of the amino groups. Similar aggregation behaviors were observed in THF solution, in which case, Rg/Rh was 0.88 and the aggregation number was 28. The LLS results were supported by AFM and TEM studies. The low CMC values of 28.6 mg dm–3 in unbuffered distilled water and 176.8 mg dm–3 at pH 3 augur well for the micelles as potential vehicles for drug/gene delivery application.
Another well-defined water-soluble double-end-capped-C60 ABA triblock polymer, C60-DMAEMA60-b-EO105-b-DMAEMA60-C60 was also successfully prepared via ATRP. TGA and spectrophometric analysis confirmed that mono-addition to C60 had occurred. In aqueous solutions, C60-DMAEMA60-b-EO105-b-DMAEMA60-C60 formed well-defined flower micelles. In unbuffered distilled water, Rh = 109 nm; Rg = 112; thus Rg/Rh = 1.03, Mw = 0.76 × 106 g mol–1, hence Nagg = 30 and A2 = 3.0× 10–4 cm3–2. In acidic solution at pH 1.7, Rh = 128 nm; Rg = 109; thus Rg/Rh = 0.85, Mw = 1.42 × 106 g mol–1, hence Nagg = 55 and A2 = 5.8 × 10–4 cm3 mol g–2. In THF, Rh = 43 nm; Rg = 45; thus Rg/Rh = 1.05, Mw = 0.44 × 106 g mol–1, hence Nagg = 17 and A2 = 4.1 × 10–4 cm3 mol g–2. Compared to [C60-DMAEMA68-C(CH3)2COOCH2]2, the micellar structure of C60- DMAEMA60-b-EO105-b-DMAEMA60-C60 was less affected by changing the medium from a near neutral solution to an acidic solution. The presence of the EO block increases the polymer/solvent compatibility in unbuffered solution, as indicated from the more positive A2 value, thus favors the formation of micelles. The size of the micelles increased only slightly at pH 1.7 as the EO block forms a large part of the corona. Hence the effect of charge repulsion resulting from the protonation of amino groups of the DMAEMA blocks only caused a slight expansion. The micellar aggregates were also examined by AFM and TEM microscopies. After calcination at 500 °C under N2, the micellar core consisting of C60 could clearly be seen by AFM. The low CMC value of 2.1 mg dm–3 for C60-DMAEMA60-b- EO105-bDMAEMA60-C60 in distilled water shows that the micelles are suitable candidate as potential vehicle for drug/gene delivery application. Furthermore, the micelles are expected to have added advantage of extending circulation lifetime in vivo, owing to the ‘stealth’ property of PEO at the corona, which will prevent/minimize protein adhesion.
For the two C60-containing block copolymers synthesized, only one decay mode was observed in aqueous solutions, indicating the existence of only one scattering species in each case. This is in contrast with the previous report of more than one decay mode for other C60-containing polymers in aqueous solutions, which have been attributed to the presence of unimers or/and a mixture of different aggregate structures. The better solubility imparted by the poly(DMAEMA) block and the well-defined chain length as the result of the controlled ATRP are the two main factors responsible for the formation of well-defined micellar aggregates for the two amphiphilic double-C60-capped block copolymers observed in this study.
Date Issued
2006
Call Number
QD281.P6 Yu
Date Submitted
2006