Sunirmal
Pal
,
Saswati
Ghosh Roy
and
Priyadarsi
De
*
Polymer Research Centre, Department of Chemical Sciences, Indian Institute of Science Education and Research-Kolkata, PO: BCKV Main Office, Mohanpur-741252, Nadia, West Bengal, India. E-mail: p_de@iiserkol.ac.in
First published on 25th October 2013
Here we report the design and synthesis of polymers containing cholic acid pendant groups, a major bile acid synthesized in the liver from cholesterol. Random copolymers derived from 2-(methacryloyloxy)ethyl cholate (MAECA) with polyethylene glycol methyl ether methacrylate (PEGMA) and N,N-dimethylaminoethyl methacrylate (DMAEMA) have been synthesized by reversible addition–fragmentation chain transfer (RAFT) polymerization. Compositions of co-monomers in the copolymers were determined by 1H NMR spectroscopy, which were further used to determine reactivity ratios of the monomers using the extended Kelen–Tüdös method at high conversions. Copolymers from MAECA with PEGMA (P(MAECA-co-PEGMA)) series displayed lower critical solution temperatures (LCSTs) up to 23% MAECA content in the copolymer, which was studied by UV-visible spectroscopy. Both thermo- and pH-responsiveness were observed for the P(MAECA-co-DMAEMA) copolymers with an MAECA unit in the copolymer up to 11.8%. The critical aggregation concentration (CAC) of water soluble copolymers was determined by fluorescence spectroscopy. Self-assembly of random copolymers was investigated by 1H NMR spectroscopy, dynamic light scattering (DLS), atomic force microscopy (AFM) and scanning electron microscopy (SEM), which indicated micellar structures in water from cholic acid based random copolymers. The polymeric aggregates were further explored for their drug encapsulation capabilities in aqueous medium using nile red as a hydrophobic model compound.
Among various CRP methods (ATRP, nitroxide-mediated radical polymerization and reversible addition–fragmentation chain transfer (RAFT) polymerization), RAFT is the most versatile technique.12,13 Amphiphilic diblock copolymers, in which the hydrophobic block was a cholesteryl-based smectic liquid-crystalline polymer and the hydrophilic poly(N,N-diethylacrylamide) block, or a copolymer containing acrylic acid moieties and poly(ethylene oxide) side chains were synthesized by RAFT polymerization.14 To date thermo- and pH-sensitive copolymers containing bile acid derivatives have been synthesized by the FRP process and their phase separation behaviour in aqueous solutions and thermosensitivity were studied.15 However, to the best of our knowledge there is no report on bile acid containing random copolymer synthesis via a CRP route. CRP provides preparation of well-defined polymers with controlled molecular weight, polydispersity, chain architecture, etc., which are difficult to achieve by FRP. Herein we have synthesized a 2-(methacryloyloxy)ethyl cholate (MAECA) monomer and copolymerized using a RAFT technique with polyethylene glycol methyl ether methacrylate (PEGMA) and N,N-dimethylaminoethyl methacrylate (DMAEMA) to produce thermosensitive poly(2-(methacryloyloxy)ethyl cholate-co-polyethylene glycol methyl ether methacrylate) (P(MAECA-co-PEGMA)), and dual thermo- and pH-responsive poly(2-(methacryloyloxy)ethyl cholate-co-N,N-dimethylaminoethyl methacrylate) (P(MAECA-co-DMAEMA)) copolymers. We have chosen cholic acid because the most abundant bile salts in humans are cholate, chenodeoxycholate and deoxycholate.16 Depending on the hydrophobic MAECA content in the copolymer chain, these bile acid containing copolymers self-assemble to spherical micellar structures in aqueous solution, which could be used for guest molecule encapsulation. Random copolymer based self-assembled nano-structures are advantageous over amphiphilic block copolymers because they can be achieved in a single polymerization step unlike block copolymers, which are generally used to generate nano-structured assemblies.17
CTA | [M]![]() ![]() ![]() ![]() |
Time (min) | Conv.a (%) | M n,theo (g mol−1) | M n,NMR (g mol−1) | M n,GPC (g mol−1) | PDId |
---|---|---|---|---|---|---|---|
a Determined by gravimetric analysis on the basis of the amount of monomer feed. b M n,theo = ([MAECA]/[CTA] × molecular weight (MW) of MAECA × conversion) + (MW of CTA). c Calculated by 1H NMR chain-end analysis. d Obtained by GPC. | |||||||
BDB | 25![]() ![]() ![]() ![]() |
330 | 81 | 10![]() |
— | 58![]() |
1.94 |
DMP | 25![]() ![]() ![]() ![]() |
330 | 64 | 8600 | — | 45![]() |
1.67 |
CDP | 25![]() ![]() ![]() ![]() |
330 | 85 | 11![]() |
— | 14![]() |
1.34 |
CTP | 25![]() ![]() ![]() ![]() |
330 | 85 | 11![]() |
12![]() |
9700 | 1.20 |
CTP | 25![]() ![]() ![]() ![]() |
450 | 69 | 9250 | 9100 | 10![]() |
1.14 |
CTP | 25![]() ![]() ![]() ![]() |
210 | 67 | 9000 | 9400 | 9700 | 1.15 |
CTP | 15![]() ![]() ![]() ![]() |
330 | 81 | 6600 | — | 6500 | 1.14 |
CTP | 40![]() ![]() ![]() ![]() |
330 | 57 | 12![]() |
12![]() |
10![]() |
1.18 |
CTP | 60![]() ![]() ![]() ![]() |
330 | 51 | 16![]() |
14![]() |
17![]() |
1.32 |
To explore the controllability of the RAFT technique for the polymerization of MAECA, we performed polymerization reactions at different [MAECA]/[CTP] ratios such as 15:
1, 25
:
1, 40
:
1 and 60
:
1, while the [CTP]/[AIBN] ratio was kept constant at 5
:
1. The GPC traces of four homopolymers in Fig. 1A show major unimodal peaks and chromatograms shifted towards lower elution volume with increasing [MAECA]/[CTP] ratios. We observed narrow PDIs (<1.32) and a liner relationship between Mn,GPC and Mn,theo values for these polymers (Fig. 1B and Table 1). Note that in the high molecular weight region of the GPC traces, a small shoulder was observed for all the PMAECA homopolymers, which could be due to the partial recombination of propagating polymer chains. Also, formation of aggregates during the GPC experiments in polar DMF solvent may give rise to a shoulder in higher molecular weight regions. The influence of the initiator concentration was also studied in which the molar feed ratio of CTP to initiator was varied from 1
:
0.1 to 1
:
0.3, while the [MAECA]/[CTP] ratio was maintained at 25
:
1. Generally, the rate of polymerization depends on the availability of radicals for initiation and Table 1 shows that the polymerization rate increases with decreasing [CTP]/[AIBN] ratio, as expected.
![]() | ||
Fig. 1 (A) GPC traces of PMAECA as a function of [MAECA]/[CTP] ratios and (B) variation of Mn and PDI with the [MAECA]/[CTP] ratios for the RAFT polymerization of MAECA at 70 °C in DMF. |
The structure of PMAECA was confirmed by 1H NMR spectroscopy. In the 1H NMR spectrum of PMAECA in Fig. 2A, the vinyl peaks of the MAECA monomer completely disappeared, and signals at 3.93–4.34 (4H, 5 & 6-H), 3.78 (1H, 7′-H), 3.60 (1H, 12′-H), 3.17 (1H, 3′-H), 0.76–0.98 (9H, 3, 21′, 19′-H), 0.58 (3H, 18′-H), 1.60–1.82 (main chain methylene protons) and 0.75–0.99 ppm (main chain methyl protons) were observed. The phenyl moiety of the CTP unit was incorporated into the polymer chain ends, which showed characteristic weak resonance signals at 7.44–7.84 ppm. The Mn,NMR value of PMAECA was determined by considering the integration value of phenyl protons (7.44–7.84 ppm) at the polymer chain ends and –C(O)–O–CH2–CH2–O–C(O)– protons at 3.93–4.34 ppm from the MAECA units. The Mn,NMR values are summarized in Table 1, which match reasonably well with the Mn,theo values indicating PMAECA chain ends with the fragments of CTP as expected according to the typical RAFT mechanism.
To further demonstrate the retention of dithioester CTA moieties at the polymer chain ends, the chain extension experiment was carried out using PMAECA–macroCTA. PMAECA (Mn,GPC = 6500 g mol−1, PDI = 1.14) was used for the polymerization of MAECA at 70 °C in DMF at [MAECA]:
[PMAECA–macroCTA]
:
[AIBN] = 50
:
1
:
0.2. We obtained 77% monomer conversion after 260 min and GPC traces of PMAECA-b-PMAECA shifted toward higher molecular weight side after the chain extension (Fig. S4†). The Mn,GPC = 41
000 g mol−1 for PMAECA-b-PMAECA is somewhat higher than Mn,theo = 26
550 g mol−1 (PDI = 1.48) and GPC trace is bimodal indicating bimolecular terminations. However, the Mn,NMR = 30
100 g mol−1 value is close to the theoretical value with ∼88% CTA chain ends. This result suggests that most of the chain ends of PMAECA are capped with the fragments of CTP and could be used further for block copolymer synthesis.
To investigate the effect of cholate moieties on the solution behaviour of copolymers containing PEGMA (a thermoresponsive monomer) or DMAEMA (a dual thermo- and pH-responsive monomer) units, several RAFT copolymerizations of MAECA with PEGMA and DMAEMA were carried out using [co-monomers]:
[CTP]
:
[AIBN] = 60
:
1
:
0.2 in DMF at 70 °C (Scheme 1). We synthesised six copolymers from each of the MAECA–PEGMA and MAECA–DMAEMA series with the MAECA to PEGMA or DMAEMA feed ratios of 5
:
95; 10
:
90; 20
:
80; 30
:
70; 40
:
60 and 50
:
50. Note that CTP showed controlled RAFT polymerization of PEGMA25 and DMAEMA.26 All copolymers were soluble in DMF, THF and DMSO and few of them were soluble in aqueous medium depending on the percentage of PEGMA or DMAEMA hydrophilic units in the copolymer chain. Solubilities of all the polymers in various organic solvents and water are listed in Table S1.† To name copolymers, P(MAECA-co-PEGMA/DMAEMA), we followed the following rules (Table 2): two letters CP and CD indicate MAECA–PEGMA and MAECA–DMAEMA series, respectively; 1, 2, 3, 4, 5 and 6 stand for the different feed compositions of the MAECA monomer.
Polymer | Time (min) | Conv.b (%) | Feed | Copolymer 1H NMR | M n,theo (g mol−1) | M n,NMR (g mol−1) | M n,GPC (g mol−1) | PDId | T g (°C) |
---|---|---|---|---|---|---|---|---|---|
a [Monomer]/[CTP][AIBN] = 60![]() ![]() ![]() ![]() |
|||||||||
Molar fraction of MAECA | |||||||||
MAECA–PEGMA series | |||||||||
PPEGMA | 210 | 67 | 0.000 | 0.000 | 12![]() |
12![]() |
12![]() |
1.15 | −56 |
CP1 | 225 | 83 | 0.05 | 0.055 | 15![]() |
13![]() |
17![]() |
1.10 | −49 |
CP2 | 210 | 65 | 0.100 | 0.120 | 13![]() |
14![]() |
20![]() |
1.20 | −39 |
CP3 | 240 | 70 | 0.200 | 0.230 | 15![]() |
15![]() |
21![]() |
1.25 | −20 |
CP4 | 270 | 76 | 0.300 | 0.340 | 17![]() |
16![]() |
25![]() |
1.26 | 6 |
CP5 | 300 | 75 | 0.400 | 0.447 | 18![]() |
18![]() |
27![]() |
1.28 | 28 |
CP6 | 300 | 78 | 0.500 | 0.542 | 19![]() |
21![]() |
32![]() |
1.26 | 35 |
PMAECA | 330 | 51 | 1.000 | 1.000 | 16![]() |
16![]() |
17![]() |
1.32 | 148 |
MAECA–DMAEMA series | |||||||||
PDMAEMA | 390 | 63 | 0.000 | 0.000 | 6200 | 7400 | 7100 | 1.14 | 18 |
CD1 | 365 | 69 | 0.05 | 0.059 | 7700 | 8900 | 11![]() |
1.20 | 30 |
CD2 | 360 | 76 | 0.100 | 0.118 | 9400 | 8400 | 10![]() |
1.48 | 48 |
CD3 | 360 | 58 | 0.200 | 0.238 | 8800 | 10![]() |
14![]() |
1.29 | 73 |
CD4 | 360 | 69 | 0.300 | 0.343 | 12![]() |
14![]() |
20![]() |
1.33 | 90 |
CD5 | 360 | 78 | 0.400 | 0.450 | 15![]() |
18![]() |
27![]() |
1.30 | 103 |
CD6 | 360 | 74 | 0.500 | 0.540 | 16![]() |
19![]() |
31![]() |
1.30 | 113 |
The 1H NMR spectra of copolymers were recorded in DMSO-d6 to determine % of co-monomer units in the copolymer chain by integration of the resonance signals related to each monomeric unit (Fig. 2). The 1H NMR spectrum of PPEGMA (Fig. 2C) showed resonance signals at 1.61–1.94 ppm due to the main chain methylene protons, and α-methyl protons appeared at 0.69–1.02 ppm. The side chain methylene protons –C(O)–O–CH2– and –C(O)–O–CH2–CH2–O–(CH2–CH2–O)3/4–CH3 appeared at 4.01 and 3.41–3.65 ppm, respectively. The peak centred at 3.25 ppm was assigned to the –O–CH3 protons, and weak resonance signals appeared at 7.40–7.83 ppm were due to presence of the benzodithioate group (from CTP) at the chain ends. The presence of MAECA units in the copolymers was confirmed from the 1H NMR spectra for this series (Fig. 2B). The compositions of copolymers were determined from the integration ratio of intensities of –OCH3 protons at 3.25 ppm from PEGMA units to the –C(O)–O–CH2–CH2–O–C(O)– methylene protons at 3.93–4.34 ppm from the MAECA fragment (after subtracting the peak area contributed by the PEGMA units in this region). The 1H NMR spectrum of PDMAEMA (Fig. S5†) showed signals at 3.98, 3.42, 2.18, 1.67–1.89 and 0.73–1.01 ppm for –C(O)–O–CH2–, –C(O)–O–CH2–CH2–, –N(CH3)2, main chain methylene and α-methyl protons, respectively. Compositions of P(MAECA-co-DMAEMA) copolymers were calculated from the ratio of peak areas from the –N(CH3)2 protons at 2.18 ppm from DMAEMA units to the –C(O)–O–CH2–CH2–O–C(O)– methylene protons at 3.93–4.34 ppm from MAECA moieties (after subtracting the contribution from –C(O)–O–CH2– methylene protons at 3.98 ppm from DMAEMA units). The compositions of the copolymers determined by 1H NMR are given in Table 2. In all copolymers, we observed characteristic resonance signals in the aromatic region (7.44–7.84 ppm) due to the terminal phenyl moiety at the end of the polymer chains. The Mn,NMR values for PPEGMA and PDMAEMA were determined by comparing the integration areas from the terminal phenyl group at 7.44–7.84 ppm to the side chain –OCH3 protons at 3.25 ppm from PEGMA units and –N(CH3)2 protons at 2.18 ppm from DMAEMA units, respectively. The Mn,NMR values of P(MAECA-co-PEGMA) copolymers were calculated by comparing the integration values of chain end phenyl protons at 7.44–7.84 ppm to the characteristic protons at 3.25 ppm (–OCH3) from PEGMA units and –C(O)–O–CH2–CH2–O–C(O)– protons at 3.93–4.34 ppm from MAECA units. For these copolymers, Mn,NMR = [(DPMAECA × MMAECA) + (DPPEGMA × MPEGMA) + molecular weight of CTP], where DPn and M are the number average degrees of polymerization and molecular weight of the monomer, respectively. Similarly, Mn,NMR values of the P(MAECA-co-DMAEMA) copolymers were determined and all Mn,NMR values are summarized in Table 2. The Mn,GPC and PDI values were determined for all copolymers from the GPC analysis (Table 2). The P(MAECA-co-PEGMA) copolymers showed unimodal RI traces with a small peak at higher molecular weight side due to the formation of dead chains via bimolecular termination reactions (Fig. S6A†). Substantial bimolecular terminations were noticed in the case of the P(MAECA-co-DMAEMA) copolymer series (Fig. S6B†). Although Mn,NMR values match well with the Mn,theo values predicted from stoichiometry and conversion (Table 2), Mn,GPC values are somewhat higher compared to the Mn,theo values and the discrepancy may be mostly due to the different hydrodynamic volume between copolymers and PMMA. Differences between Mn,NMR and Mn,theo values are higher in the case of P(MAECA-co-DMAEMA) copolymers since we observed higher bimolecular terminations (Fig. S6B†) in this series compared to the P(MAECA-co-PEGMA) copolymers (Fig. S6A†).
Copolymerization of two monomers with different properties allows the synthesis of different products by variations in the relative amounts of the two monomer units in the copolymer chain. Since knowledge of reactivity ratios allows prediction of copolymer compositions at a particular monomer feed ratio, the reactivity ratios of MAECA (rMAECA), PEGMA (rPEGMA) and DMAEMA (rDMAEMA) were determined for the copolymerization of MAECA with PEGMA and DMAEMA. To determine reactivity ratios, we have used the extended Kelen–Tüdös (extended K–T) method,27 which is applicable for living controlled copolymerization at higher conversions.28 Using monomer feed compositions, co-monomer contents in the copolymer (determined by NMR in Table 2) and conversion data, the reactivity ratios were determined by the extended K–T method. The rMAECA = 1.62 and rPEGMA = 0.71 for the MAECA–PEGMA series, and rMAECA = 1.35 and rDMAEMA = 0.65 for the MAECA–DMAEMA system were obtained, respectively. These results indicated random distribution of monomer units in both the copolymerization series and the copolymer will be moderately enriched with MAECA units.29 Although all three monomers are methacrylate derivatives, the reactivity of the bulky MAECA monomer is almost double compared to PEGMA or DMAEMA. A similar trend of the reactivity ratio was observed for the copolymerization of tert-butyl methacrylate with the bulky first-generation dendronized monomer carrying an ethylene glycol-based dendron.30
![]() | ||
Fig. 3 (A) TGA thermograms, (B) DSC traces and (C) comparison of the experimental Tg values to the theoretical Tg values calculated from the Fox relationship. |
For the copolymers, Fig. 3B and S7B† show a single Tg value in between the corresponding Tg values for the homopolymers (Table 2), which confirmed homogeneous nature of the copolymers and MAECA forms a copolymer in every composition with PEGMA or DMAEMA. The Fox equation34 was used to calculate the theoretical Tg values (Tg,theo) of copolymers in the bulk state and Tg,theo values are plotted in Fig. 3C against the MAECA composition in the copolymer. Experimental Tg values (Tg,expt) from the DSC study are also plotted in Fig. 3C as a function of the weight fractions of MAECA in two copolymer series, and we observed an excellent correlation between Tg,expt and Tg,theo.
![]() | ||
Fig. 4 Phase transitions of (A) P(PMAECA-co-PEGMA) series at pH 6, (B) CD1 and CD2 at 25 °C, and (C) CD1 at different pHs. |
In the P(MAECA-co-DMAEMA) series, only CD1 and CD2 were soluble in water, where MAECA units in the copolymer chains were 5.9 and 11.8%, respectively. In order to determine the pH responsiveness of PDMAEMA, CD1 and CD2, their aqueous solutions were analyzed as a function of pH at 25 °C. PDMAEMA showed an LCST of 52 °C at pH 7.4 and pH-responsiveness at pH 12.9 at 25 °C as a result of complete deprotonation, and these values match well with the literature.39
The phase transition pH is reduced with the increasing MAECA content in the copolymer and at 25 °C we determined pH-responsiveness at 12.8 and 8.2 for CD1 and CD2, respectively (Fig. 4B). These results indicate that the hydrophobic MAECA units facilitate the aggregation of DMAEMA units in these copolymers at lower pH. The LCST of CD1 and CD2 was studied at different pHs and we determined hydrophilic to hydrophobic transitions for CD1 at 73, 63, 53, 44, 37 and 33 °C at pH values 7.4, 7.6, 7.8, 8.0, 8.2 and 8.4, respectively (Fig. 4C). Similarly, the LCSTs of 54, 43 and 31 °C were determined for CD2 at pH 7.4, 7.6 and 7.8, respectively (Fig. S8†). The transmittance decreased slowly for CD2 (Fig. S8†) at all pHs, indicating disturbed deprotonation of tertiary amine groups with increasing cholate moiety in the copolymer. The turbidity of polymers in aqueous solution depends on pH, involving hydrophobic interactions between cholate moieties and electrostatic repulsion between protonated amino groups. Amino groups are protonated below the responsive pH at a particular temperature, resulting in electrostatic repulsion among them and increased solubility. However, the above results indicate that the LCST can be tuned at a particular solution pH by employing different molar factions of monomer units in the copolymer. The above phase transitions were reversible indicating uninterrupted protonation and deprotonation of the tertiary amine functional groups in the DMAEMA moiety.
Furthermore, self-assembly behaviour of CP2, CP4 and CD2 was investigated by 1H NMR spectroscopy in two different solvents, D2O and DMSO-d6. The DMSO-d6 is a good solvent for both MAECA and PEGMA/DMAEMA units in the copolymer, whereas D2O does not solubilise the hydrophobic MAECA moiety. The 1H NMR spectra of CP2 (Fig. 5A), CP4 (Fig. S11†) and CD2 (Fig. 5B) in DMSO-d6 show all characteristic peaks from both the co-monomers, while the resonance signal in D2O disappeared at 0.60 ppm from the methyl group of hydrophobic cholate units. These results revealed that CP2, CP4 and CD2 formed higher order structure through the arrangement of hydrophilic and hydrophobic components in the copolymers in aqueous media. In the aggregates the hydrophilic units were uncovered to the outer surface and hydrophobic parts were favoured to the core, and as a result we did not observe the 0.60 ppm peak from hydrophobic cholate moieties in the core in the NMR spectra of CP2, CP4 and CD2 in D2O.
In order to study the aggregation behaviour in aqueous solution, the hydrodynamic diameter (Dh) of various copolymers was determined by DLS at 25 °C with the sample concentration of 1 mg mL−1 (w/v). CP2 (2.8 and 95 nm), CP3 (3.0 and 95 nm), and CP4 (4.6 and 96 nm) displayed two peaks, one at around 3–5 nm and another peak at approximately 95 nm (Fig. 6A). The smaller size in the range of 3 to 5 nm might be attributed to the size of a single chain conformation. Larger size in the range of 95–96 nm suggests formation of aggregated particles. CP1 showed only one peak in Fig. 6A with a Dh value of 2.7 nm, confirming its unimeric structure with insufficient amount of hydrophobic units in the polymer chain for self-assembly. This result confirmed the above pyrene encapsulation studies, where CP1 was unable to increase the fluorescence intensity of pyrene with increasing polymer concentration, due to the lack of self-assembly formation capabilities. Similarly, we measured Dh values for CD1 at pH 7 and CD2 at pH 5 (Fig. 6B). Both CD1 and CD2 showed single size distribution with Dh values of ∼100 nm, indicating their self-assembly in water to yield multi-molecular aggregates.
![]() | ||
Fig. 6 DLS size distributions of (A) CP1, CP2, CP3 and CP4, and (B) CD1 and CD2 in aqueous medium at 25 °C (concentration: 1 mg mL−1). |
To further characterize the aggregates and obtain direct visualization of their size and morphology, we investigated thin films of the random copolymers by imaging techniques such as AFM and SEM. The morphology of CP2, CP3 and CP4 copolymers was found to be spherical, with an average size of 78, 71 and 76 nm, respectively (Fig. 7). These copolymers formed micellar structures in water due to the aggregation of the hydrophobic cholate component towards the core and hydrophilic PEGMA units constitute the corona of the micelles. The AFM result for CP4 was also supported by SEM analysis, where we observed ∼78 nm size with spherical structure (Fig. S12†). Similarly, CD1 gave globular shape in the SEM image with a diameter of ∼82 nm (Fig. S12†). Note that sizes estimated from the AFM/SEM study are slightly lower than DLS data as reported elsewhere.40 Also, the observed diameters by DLS and AFM analyses are much higher than the contour length of the corresponding copolymers, indicating some another sort of self-assemblies.41
![]() | ||
Fig. 7 AFM images of (A) CP2, (B) CP3 and (C) CP4. Copolymer solutions (0.2 mg mL−1) were deposited onto cover-slips. |
![]() | ||
Fig. 8 (A) CP4 (CPCP4 in the picture) loaded with nile red. (B) UV-vis and (C) corresponding fluorescence emission (λex = 550 nm) spectra of nile red solubilised by CP2, CP3 and CP4. |
Fig. 8B shows UV-vis spectra of nile red loaded CP2, CP3 and CP4, where we observed λmax values of 568, 563 and 560 nm for CP2, CP3 and CP4, respectively. Also, absorbance of copolymers gradually increased with increasing MAECA content in the copolymer. The fluorescence emission spectra in Fig. 8C suggest relatively low fluorescence intensity for CP2 (λmax = 628 nm) and CP3 (λmax = 625 nm) compared to CP4 with a λmax of 622 nm. The fluorescence intensity of CP4 is roughly 10 times higher than CP3. These results indicate that CP4 has more hydrophobic environment due to the presence of more number of MAECA units. Since the P(MAECA-co-PEGMA) copolymer series is thermoresponsive and the P(MAECA-co-DMAEMA) copolymers are both thermo- and pH-sensitive, external stimuli such as thermo- and pH-induced controlled release of hydrophobic molecules (e.g., nile red and doxorubicin) from the polymeric micelles are under investigation.43
Footnote |
† Electronic supplementary information (ESI) available: The synthesis protocol for MAECA, NMR characterization of monomers, homopolymers and various copolymers, FT-IR and ESI-MS spectra, GPC RI traces of various polymers, DSC and TGA thermograms of P(MAECA-co-DMAEMA) copolymers, phase transitions of CD2 at different pHs, fluorescence spectra and corresponding CAC determination plots, SEM images, and solubility studies of both the copolymer series. See DOI: 10.1039/c3py01317k |
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