The structures and synthesis of polyzwitterions ("polybetaines") are reviewed, emphasizing the literature of the past decade. Particular attention is given to the general challenges faced, and to ...successful strategies to obtain polymers with a true balance of permanent cationic and anionic groups, thus resulting in an overall zero charge. Also, the progress due to applying new methodologies from general polymer synthesis, such as controlled polymerization methods or the use of "click" chemical reactions is presented. Furthermore, the emerging topic of responsive ("smart") polyzwitterions is addressed. The considerations and critical discussions are illustrated by typical examples.
The widespread occurrence of zwitterionic compounds in nature has incited their frequent use in designing biomimetic materials. Therefore, zwitterionic polymers are a thriving field. A particular ...interest for this particular polymer class has currently focused on their use in establishing neutral, low-fouling surfaces. After highlighting strategies to prepare model zwitterionic surfaces as well as those that are more suitable for practical purposes relying strongly on radical polymerization methods, we present recent efforts to diversify the structure of the hitherto quite limited variety of zwitterionic monomers and of the derived polymers. We identify key structural variables, consider their influence on essential properties such as overall hydrophilicity and long-term stability, and discuss promising targets for the synthesis of new variants.
A series of biomass-derived levoglucosenyl alkyl ethers (alkyl = methyl, ethyl, n-propyl, isopropyl, and n-butyl) were synthesized and polymerized by ring-opening olefin metathesis polymerization ...using the Grubbs catalyst C793 at room temperature. Polymerizations were successfully performed in conventional solvents such as 1,4-dioxane and dichloromethane as well as in polar aprotic “green” solvents such as 2-methyltetrahydrofuran, dihydrolevoglucosenone (Cyrene), and ethyl acetate. The prepared polyacetals with degrees of polymerization of ∼100 exhibit Schulz–Flory-type molar mass distributions and are thermoplastic materials with rather low glass transition temperatures in the range of 43–0 °C depending on the length of the alkyl substituent. Kinetic studies revealed that the polymerization proceeded rapidly to a steady state with a certain minimum monomer concentration threshold. When the steady state was reached, just about half of the Ru catalyst had been effective to initiate the polymerization, indicating that the initiation step was a slow process. The remaining catalyst was still active and did no longer react with monomers but with in-chain double bonds, cutting the formed polymer chains into shorter fragments. In the long term, all catalyst was consumed and propagating Ru chain ends were deactivated by the elimination of Ru from the chain ends to form inactive chains with terminal aldehyde groups.
A nonionic double hydrophilic block copolymer with a long permanently hydrophilic and a small thermoresponsive block is synthesized by reversible addition–fragmentation chain-transfer polymerization ...(RAFT). By employing a specifically designed chain-transfer agent, the polymer is functionalized with complementary end groups which are suited for Förster resonance energy transfer (FRET). The end group attached to the permanently hydrophilic block of poly(N,N-dimethylacrylamide) pDMAm is designed as a permanently hydrophobic segment (“sticker”) comprising a long alkyl chain and the 4-aminonaphthalimide fluorophore. The other end attached to the thermoresponsive block of poly(N-isopropylacrylamide) pNiPAm incorporates a coumarin fluorophore. The temperature-dependent self-assembly of the twofold fluorescently labeled copolymer is studied in pure aqueous solution as well as in an o/w microemulsion by several techniques including turbidimetry, dynamic light scattering (DLS), and fluorescence spectroscopy. It is compared to the behaviors of the analogous twofold-labeled pDMAm and pNiPAm homopolymer references. The findings indicate that the block copolymer behaves as a polymeric surfactant at low temperatures, with one relatively small hydrophobic end block and an extended hydrophilic chain forming “hairy micelles”. At elevated temperatures above the LCST phase transition of the pNiPAm block, however, the copolymer behaves as an associative telechelic polymer with two nonsymmetrical hydrophobic end blocks, which do not mix. Thus, instead of a network of bridged “flower micelles”, large dynamic aggregates are formed. These are connected alternatingly by the original micellar cores as well as by clusters of the collapsed pNiPAm blocks. This type of structure is even more favored in the o/w microemulsion than in pure aqueous solution, as the microemulsion droplets constitute an attractive anchoring point for the hydrophobic dodecyl sticker but not for the collapsed pNiPAm chains.
This feature article reviews our recent advancements on the synthesis, phase behavior, and micellar structures of diblock copolymers consisting of oppositely thermoresponsive blocks in aqueous ...environments. These copolymers combine a nonionic block, which shows lower critical solution temperature (LCST) behavior, with a zwitterionic block that exhibits an upper critical solution temperature (UCST). The transition temperature of the latter class of polymers is strongly controlled by its molar mass and by the salt concentration, in contrast to the rather invariant transition of nonionic polymers with type II LCST behavior such as poly(N-isopropylacrylamide) or poly(N-isopropyl methacrylamide). This allows for implementing the sequence of the UCST and LCST transitions of the polymers at will by adjusting either molecular or, alternatively, physical parameters. Depending on the location of the transition temperatures of both blocks, different switching scenarios are realized from micelles to inverse micelles, namely via the molecularly dissolved state, the aggregated state, or directly. In addition to studies of (semi)dilute aqueous solutions, highly concentrated systems have also been explored, namely water-swollen thin films. Concerning applications, we discuss the possible use of the diblock copolymers as “smart” nanocarriers.
A series of triple-thermoresponsive triblock copolymers from poly(N-n-propylacrylamide) (PNPAM, A), poly(methoxydiethylene glycol acrylate) (PMDEGA, B), and poly(N-ethylacrylamide) (PNEAM, C) was ...synthesized by sequential reversible addition−fragmentation chain transfer polymerizations. Polymers of differing block sequences, ABC, BAC, and ACB, with increasing phase transition temperatures in the order A < B < C were prepared. Their aggregation behavior in dilute aqueous solution was investigated using dynamic light scattering, turbidimetry, and NMR spectroscopy. The self-organization of such polymers was found to dependent strongly on the block sequence. While polymers with a terminal low-LCST (lower critical solution temperature) block undergo aggregation above the first phase transition temperature at 20−25 °C, triblock copolymers with the low-LCST block in the middle show aggregation only above the second phase transition. The collapse of the middle block is not sufficient to induce aggregation but produces instead stable, unimolecular micelles with a collapsed middle block, as supported by NMR and fluorescence probe data. Continued heating of all copolymers led to two additional thermal transitions at 40−55 and 70−80 °C, which could be correlated to the phase transitions of the B and C blocks, respectively. All polymers show a high tendency for cluster formation, once aggregation is induced. The carrier abilities of the triple responsive triblock copolymers for hydrophobic agents were probed with the solvatochromic fluorescence dye Nile Red. With passing through the first thermal transition, the block copolymers are capable of solubilizing Nile Red. In the case of block copolymers with sequences ABC or ACB, which bear the low-LCST block at one terminus, notable amounts of dye are solubilized already at this stage. In contrast, the hydrophobic probe is much less efficiently incorporated by the BAC triblock copolymer, which forms unimolecular micelles. Only after the collapse of the B block, when reaching the second phase transition at about 45 °C, does aggregation occur and solubilization becomes efficient. In the case of ABC and ACB polymers, the hydrophobic probe seems to partition between the originally collapsed A chains and the additional hydrophobic chains formed after the collapse of the less hydrophobic B block.
The odd–even effect, i.e., the influence of the outermost layer of polyelectrolyte multilayers (PEMs) on their swelling behavior, is investigated. For that purpose poly(styrene sodium sulfonate) ...(PSS)/poly(diallyl-dimethylammonium chloride) (PDADMAC) polyelectrolyte multilayers are studied in air with 1% relative humidity (RH), 30% RH, 95% RH, and in liquid water by ellipsometry, atomic force microscopy (AFM), and X-ray reflectometry (XRR). Since the total amount of water uptake in swollen PEMs is divided into two fractions, the void water and the swelling water, a correct evaluation of the odd–even effect is only possible if both fractions are examined separately. In order to allow measuring samples over a larger thickness regime the investigation of a larger amount of samples is required. Therefore, the concept of separating void water from swelling water using neutron reflectometry is for the first time transferred to ellipsometry. The subsequent analysis of swelling water, void water, and roughness revealed the existence of two types of odd–even effects: an odd–even effect which addresses only the surface of the PEM (surface-odd–even effect) and an odd–even effect which addresses also the bulk of the PEM (bulk-odd–even effect). The appearance of both effects is dependent on the environment; the surface-odd–even effect is only detectable in humid air while the bulk-odd–even effect is only detectable in liquid water. The bulk-odd–even effect is related to the osmotic pressure between the PEM and the surrounding water. A correlation between the amount of void water and both odd–even effects is not found. The amount of void water is independent of the terminated layer and the thickness of PEMs.
The swelling and co-nonsolvency behaviors in pure H2O and in a mixed H2O/CH3OH vapor atmosphere of two different polar, water-soluble polymers in thin film geometry are studied in situ. Films of a ...zwitterionic poly(sulfobetaine), namely, poly3-((2-(methacryloyloxy)ethyl)dimethylammonio) propane-1-sulfonate (PSPE), and a polar nonionic polymer, namely, poly(N-isopropylmethacrylamide) (PNIPMAM), are investigated in real time by spectral reflectance (SR) measurements and Fourier transform infrared (FTIR) spectroscopy. Whereas PSPE is insoluble in methanol, PNIPMAM is soluble but exhibits co-nonsolvency behavior in water/methanol mixtures. First, the swelling of PSPE and PNIPMAM thin films in H2O vapor is followed. Subsequently, CH3OH is added to the vapor atmosphere, and its contracting effect on the water-swollen films is monitored, revealing a co-nonsolvency-type behavior for PNIPMAM and PSPE. SR measurements indicate that PSPE and PNIPMAM behave significantly different during the H2O swelling and subsequent exposure to CH3OH, not only with respect to the amounts of absorbed water and CH3OH, but also to the cosolvent-induced contraction mechanisms. While PSPE thin films exhibit an abrupt one-step contraction, the contraction of PNIPMAM thin films occurs in two steps. FTIR studies corroborate these findings on a molecular scale and reveal the role of the specific functional groups, both during the swelling and the cosolvent-induced switching of the solvation state.
Poly(N-isopropylmethacrylamide) (PNIPMAM) is a thermoresponsive polymer, exhibiting lower critical solution temperature (LCST) behavior in an aqueous solution. We investigate the ...temperature-dependent phase behavior of PNIPMAM solutions in D2O using turbidimetry, differential scanning calorimetry (DSC), small-angle and very small angle neutron scattering (SANS and VSANS), and Raman spectroscopy, covering a large concentration range, and compare the results from PNIPMAM with the findings from its analogue poly(N-isopropylacrylamide) (PNIPAM). We find that the PNIPMAM chains only dehydrate 2–3 °C above the macroscopic cloud point temperature, T CP. Even in the one-phase state, loosely packed, large-scale inhomogeneities and physical cross-links are observed, and the chain conformation of PNIPMAM is more compact than the one of PNIPAM. This is attributed to the attractive intermolecular interactions between the hydrophobic moieties. The phase transition of PNIPMAM is broader than the one of PNIPAM. Upon heating to the two-phase state, the PNIPMAM chains collapse and form mesoglobules. These are larger and more hydrated than those for PNIPAM. This is attributed to the steric hindrance caused by the additional methyl groups, which weaken the intrapolymer interactions in the two-phase state. Thus, the methyl groups in the backbone of the PNIPMAM chains have a significant impact on the hydration and the structural behavior around the phase transition.
The 2-fold thermoresponsive diblock copolymer PSPP498-b-PNIPMAM144, which consists of a zwitterionic polysulfobetaine (PSPP) block and a nonionic poly(N-isopropylmethacrylamide) (PNIPMAM) block, is ...prepared by consecutive RAFT polymerizations. It combines the upper and lower critical solution temperature (UCST and LCST) behaviors, respectively, of the constitutive homopolymers in aqueous solution. We investigate the temperature-dependent phase behavior and the self-assembled structures of the block copolymer in D2O by turbidimetry and by small-angle neutron scattering (SANS) in salt-free solution and in the presence of small amounts of NaCl and NaBr. For comparison, solutions of PNIPMAM homopolymer in D2O are studied as well. Turbidimetry indicates thermally induced “schizophrenic” aggregation behavior for PSPP498-b-PNIPMAM144. SANS reveals that conventional star-like core–shell micellar structures are formed above the LCST transition, whereas below the UCST-transition, structure formation is much less pronounced. This is attributed to the different types of interactions, namely hydrophobic and ionic ones, dominating in the different regimes. Despite the increased polarity contrast between the zwitterionic and the nonionic blocks, and the much wider separation of the UCST- and LCST-based cloud points, CPUCST and CPLCST, the structural features of the new PSPP498-b-PNIPMAM144 resemble the ones found previously for the also 2-fold thermoresponsive analogue PSPP432-b-PNIPAM200, for which both phase transition temperatures nearly coincide. Remarkably, the addition of small amounts of NaBr or NaCl to the solution of PSPP498-b-PNIPMAM144 causes a significant increase of CPUCST, as well as minor but notable changes in the self–assembled structures, but no gross alterations of the phase behavior.
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