Two‐Dimensional Chromatography of Complex Polymers, 7 – Detailed Study of Polystyrene‐block‐Polyisoprene Diblock Copolymers Prepared by Sequential Anionic Polymerization and Coupling Chemistry

Two‐dimensional chromatographic methods were developed using LC‐CC in the first and SEC in the second dimension. These methods were applied for the investigation of PS‐b‐PI diblock copolymers synthesized by different approaches: sequential living anionic polymerization and coupling of living precursor blocks. The first dimension separates according to the individual block length of PS or PI blocks, whereas the second dimension separates with respect to the total molar masses of components. 2D‐LC analysis provides information on the purity of the reaction products, the presence of by‐products, the chemical compositions and the molar masses of all product components. The accuracy and selectivity of 2D‐LC is discussed.

     

Synthesis and Characterization of Epoxidized Styrene‐Butadiene Block Copolymers as Templates for Nanostructured Thermosets

Summary: A wide range of epoxidized styrene‐block‐butadiene block copolymers were synthesized using hydrogen peroxide in the presence of an in situ prepared methyltrioctylammoniumtetrakis(diperoxotungstate)phosphate as the catalyst system in a water/dichloroethane biphasic system. ¹H NMR and FT‐IR spectra revealed that the epoxidation procedure led mainly to 1,4‐epoxidized butadiene units. GPC analysis showed that epoxidation changed the copolymer architecture into stars with fewer arms. The copolymers showed two glass transition temperatures in accordance with the phase separation for block copolymers and, as revealed in AFM images, the self‐assembly takes place on a nanometer scale. Moreover, epoxidized styrene‐butadiene (SB) copolymers showed improved miscibility with epoxy monomers leading to self‐assembled nanostructures in the uncured state. This allows them to be used as templates for nanostructured resins.

TM‐AFM phase images for: a) 40 mol‐% epoxidized copolymer (scale bar = 300 nm); b) uncured blend containing 50 wt.‐% DGEBA in 50 mol‐% epoxidized copolymer (scale bar = 1 μm), after annealing at 80 °C in vacuum for 3 h.     

Effect of the End‐Positioning of a Lithium Sulfonate Group on the Aggregation and Micellization Behavior of ω‐Lithium Sulfonate Polystyrene‐block‐polyisoprenes

Summary: ω‐Lithium sulfonate polystyrene‐block‐polyisoprenes were synthesized by anionic polymerization high vacuum techniques and a post‐polymerization reaction with 1,1‐diphenylethylene and 1,3‐propane sultone. The solution properties of ω‐lithium sulfonate block copolymers were studied in a low polarity solvent (CCl₄) which is a good solvent for both blocks. The study revealed the formation of aggregates due to the association of the lithium sulfonate groups (SuLi). The mass, size and shape of the aggregates were studied by static and dynamic light scattering and viscometry. The aggregation number decreased as the length of the chain increased and the aggregates showed a behavior similar to that of star polymers. The micellization of ω‐functionalized block copolymers was also studied in a polar selective solvent for polystyrene, N,N‐dimethylacetamide (DMA). Micelles formed from block copolymers containing the sulfonate group at the polyisoprene end exhibited an association number which was about 80% of that of the neutral block copolymer and had a higher critical micelle concentration. The samples having the polar group at the PS end showed a similar behavior.

The structures of the end‐functionalized block copolymers studied.     

Synthesis and Hydrolysis of α,ω‐Perfluoroalkyl‐Functionalized Derivatives of Poly(ethylene oxide)

Summary: Two series of novel α,ω‐perfluoroalkyl terminated esters of poly(ethylene oxide) (PEO) (R_F‐PEO) having the general structure C_mF_2m+1? COO? (CH₂? CH₂? O)_n? OC? C_mF_2m+1, with m = 1,2,3,4 or 5 have been synthesized. The influences of the PEO molar mass, the length of the perfluoroalkyl group (R_F) and temperature on the cleavage of the ester bridge in aqueous solution and the effect of the hydrolysis process on the size of aggregates formed in water were studied. According to ¹H and ¹⁹F NMR measurements the degree of functionalization obtained (up to 96 mol‐%) increases with the decrease in the length of the R_F group. All of the derivatives showed ester cleavage in water in short time scales. The rates of hydrolysis of the ester bridge in aqueous solution were determined from pH‐measurements. It was verified that the rate law for hydrolysis corresponds to a pseudo‐first order type. The hydrolysis kinetic constant k increased with a decrease in the length of the R_F group ranging from 0.2 × 10^?3 s^?1 for the longest R_F group (C₅F₁₁? ) up to 1.2 × 10^?2 s^?1 for the shortest R_F group (CF₃? ). The value of k depended almost exclusively on the length of the perfluoroalkyl chain and was independent of the length of the PEO backbone (1 000 or 2 000 g · mol^?1), as long as no additional phenomenon such as phase separation was present. It was also found that the change in the value of k with temperature followed a non‐Arrhenius pattern and there was an evident relationship between the non‐linearity in the ln k vs. 1/T relation with increasing temperatures and the occurrence of a macroscopic phase separation of LCST type. Dynamic light scattering measurements showed the coexistence of unimers with associated species with apparent hydrodynamic radii (R_h) of approximately 20–45 nm for all samples in aqueous solutions. These species might correspond to aggregates of a few micelles. For some samples also larger aggregates were found with R_h in the 100–500 nm range, which might be attributed to clusters of micelles.

     

Synthesis and Self‐Assembly of Functionalized Cyclooctene Block Copolymers via ROMP

A novel block copolymer consisted of MACIT and HCO segments was synthesized via ROMP in the ionic liquid [bmim][PF₆] with good control over the polymerization process. The molecular weight of the block copolymer was estimated by ¹H NMR, and the molar composition ratio of repeating units in the MACIT block to those in the HCO block was 100:120 (120:120 in feed). The micellar characterization was carried out by DLS, AFM, and TEM. The hydrodynamic diameter of the micelles, measured by DLS, was 129 ± 0.09 nm with a narrow distribution (PDI = 0.034). The TEM image showed spherical micelles.

     

A Novel Macroinitiator for the Synthesis of Triblock Copolymers via Atom Transfer Radical Polymerization: Polystyrene‐block‐poly(bisphenol A carbonate)‐block‐polystyrene and Poly(methyl methacrylate)‐block‐poly(bisphenol A carbonate)‐block‐poly(methyl methacrylate)

Summary: Bis(hydroxy)telechelic bisphenol A polycarbonate (PC) was prepared via melt polycondensation of bisphenol A (BPA) and diphenyl carbonate (DPC) using lanthanum(III ) acetylacetonate as a catalyst for transesterification. Subsequently, the polycarbonate was converted to a bifunctional macroinitiator for atom transfer radical polymerization (ATRP) with the reagent, α‐chlorophenylacetyl chloride. The macroinitiator was used for the polymerization of styrene (S) and methyl methacrylate (MMA) to give PS‐block‐PC‐block‐PS and PMMA‐block‐PC‐block‐PMMA triblock copolymers. These block copolymers were characterized by NMR and GPC. When styrene and methyl methacrylate were used in large excess, significant shifts toward high molecular weights were observed with quantitative consumption of the macroinitiator. Several ligands were studied in combination with CuCl as the ATRP catalyst. Kinetic studies reveal the controlled nature of the polymerization reaction for all the ligands used.

Formation of a bifunctional ATRP macroinitiator by esterification of bis(hydroxy)telechelic PC with α‐chlorophenylacetyl chloride.     

Synthesis of Diverse Asymmetric α,ω‐Dienes Via the Passerini Three‐Component Reaction for Head‐to‐Tail ADMET Polymerization

The Passerini three‐component reaction is applied to synthesize, in a one‐step procedure, diverse asymmetric α,ω‐dienes containing an acrylate and a terminal olefin. Such monomers are well known to undergo head‐to‐tail acyclic diene metathesis (ADMET) polymerization due to the high cross‐metathesis selectivity between acrylates and terminal olefins. Additionally, amphiphilic block copolymers are synthesized using a monofunctional PEG₄₈₀ monoacrylate, which acts as a selective chain‐transfer agent during the polymerization process. Thus, control over the molecular weight of the amphiphilic ADMET polymers is shown by using different ratios of mono­mer and chain‐transfer agent. All the polymers are thoroughly characterized, and their ability to form nanoparticles in aqueous solution is studied.

     

Polymer Loops vs. Brushes on Surfaces: Adsorption,Kinetics, and Viscoelastic Behavior of α,ω‐Thiol Telechelics on Gold

Telechelic thiopolymers were explored towards formation of doubly‐bound (polymer loops) vs. singly‐bound (polymer brushes) chains on gold substrates. The conformation, adsorption kinetics, and viscoelastic properties of the α,ω‐dithiol and monothiol PS telechelics were investigated by means of XPS and QCM‐D techniques. The oxidation behavior of the free thiol proved to be important for distinguishing doubly‐bound vs. singly‐bound chains. The results show a critical dependence of $\overline {M} _{{\rm n}} $ on the ability to obtain polymer loops. Comparison with unperturbed dimensions obtained by scaling theory show that the loop is less stretched and occupies more lateral space than the brush. The results are important in demonstrating the different and perhaps superior properties of polymer loops vs. singly‐bound polymer brushes.

     

Synthesis and Self‐Assembly Behavior of Organic–Inorganic Poly(ethylene oxide)‐block‐Poly(MA POSS)‐block‐Poly(N‐isopropylacrylamide) Triblock Copolymers

In this work, the synthesis of 3‐methacryloxypropylheptaphenyl POSS, a new POSS macromer (denoted MA‐POSS) is reported. The POSS macromer is used to synthesize PEO‐b‐P(MA‐POSS)‐b‐PNIPAAm triblock copolymers via sequential atom transfer radical polymerization (ATRP). The organic‐inorganic, amphiphilic and thermoresponsive ABC triblock copolymers are characterized by means of nuclear magnetic resonance spectroscopy (NMR) and gel permeation chromatography (GPC). Differential scanning calorimetry (DSC) and atomic force microscopy (AFM) show that the hybrid ABC triblock copolymers are microphase‐separated in bulk. Cloud point measurements show that the effect of the hydrophiphilic block (i.e. PEO) on the LCSTs is more pronounced than the hydrophobic block (i.e. P(MA‐POSS)). Both transmission electron microscopy (TEM) and dynamic light scattering (DLS) show that all the triblock copolymers can be self‐organized into micellar aggregates in aqueous solutions. The sizes of the micellar aggregates can be modulated by changing the temperature. The temperature‐tunable self‐assembly behavior is interpreted using a combination of the highly hydrophobicity of P(MA‐POSS), the water‐solubility of PEO and the thermoresponsive property of PNIPAAm in the triblock copolymers.     

Aggregation and Crosslinking of Poly(N,N‐dimethylacrylamide)‐b‐pullulan Double Hydrophilic Block Copolymers

The self‐assembly of polymers is a major topic in current polymer chemistry. In here, the self‐assembly of a pullulan based double hydrophilic block copolymer, namely pullulan‐b‐poly(N,N‐dimethylacrylamide)‐co‐poly(diacetone acrylamide) (Pull‐b‐(PDMA‐co‐PDAAM)) is described. The hydrophilic block copolymer induces phase separation at high concentration in aqueous solution. Additionally, the block copolymer displays aggregates at lower concentration, which show a size dependence on concentration. In order to stabilize the aggregates, crosslinking via oxime formation is described, which enables preservation of aggregates at high dilution, in dialysis and in organic solvents. With adequate stability by crosslinking, double hydrophilic block copolymer (DHBC) aggregates open pathways for potential biomedical applications in the future.     

Facile and Efficient Synthesis of Fluorescence‐Labeled RAFT Agents and Their Application in the Preparation of α‐,ω‐ and α,ω‐End‐Fluorescence‐Labeled Polymers

The novel fluorescence‐labeled RAFT agents 2‐(4‐benzyl)benzoxazole benzodithioate (BBB) and 2‐(4‐benzyl)benzoxazole 9H‐carbazole‐9‐carbodithioate (BBCC) were synthesized via a nucleophilic substitution reaction between commercially available 2‐(4‐(chloromethyl)phenyl)benzoxazole (CMPB) and excessive carbodithioate or dithiocarbamate salts. BBB bearing benzoxazole at R‐group and BBCC carrying benzoxazole and carbazole at R‐ and Z‐group can be applied to directly obtain α‐ and α,ω‐end‐fluorescence‐labeled polymers, respectively. Considering the mild reaction conditions and readily available dyes, the reactions between CMPB and dithiocarbamate or carbodithioate salts should be a very simple and promising approach to synthesize fluorescent RAFT agents and polymers.     

Poly(2‐vinylnaphthalene)‐block‐poly(acrylic acid) Block Copolymer: Self‐Assembled Pattern Formation,Alignment, and Transfer into Silicon via Plasma Etching

P2VN‐b‐PAA is a novel diblock copolymer which has potential as a self‐assembled nanoscale patterning material. Thin spin cast P2VN‐b‐PAA films rapidly reorganize to vertical lamellar with exposure to acetone vapor. P2VN‐b‐PAA lamellar morphology was aligned by electric field under acetone vapor at a significantly faster rate and at lower electric field strengths than other polymer systems. Observed dry etching selectivity for P2VN to PAA were comparatively high for a variety of etch gases, consistent with estimations from Ohnishi and ring parameters. Block copolymer self assembled patterns were transferred to silicon via two‐step CF₄ and SF₆ etching.

     

Controlled Synthesis of “Reverse Pluronic”‐Type Block Copolyethers with High Molar Masses for the Preparation of Hydrogels with Improved Mechanical Properties

Hydrogels based on Pluronics (EO_n/2‐PO_m‐EO_n/2, EO = ethylene oxide, PO = propylene oxide) have been frequently investigated, yet key limitations still remain, including a propensity for quick erosion and insufficient mechanical robustness. This issue can be alleviated by creating “reverse Pluronics” (PO_n/2‐EO_m‐PO_n/2), which is proposed to enable the formation of physical cross‐links via a micellar network. Until recently, however, efforts in this direction were aggravated by synthetic difficulties, specifically prohibiting the realization of poly(propylene oxide) (PPO)‐moieties with a high DP. In this study, an organocatalytic polymerization method is employed to synthesize “reverse Pluronics,” resulting in highly defined polymers (Ð_M ≤ 1.02–1.07, M_n up to 35 000 g mol^?1) with exceptionally long PPO blocks. The higher molar mass and the reverse constitution of the polyether combine to enable the generation of thermoresponsive hydrogels with a storage modulus that is increased tenfold relative to reference samples. Gelation temperature and maximum storage modulus (G′_max) are readily influenced by the choice of the polyether (down to 5 wt%). The improved mechanical properties are accompanied by an increased resistance toward erosion in water. Isotactic enrichment is presented as an additional tuning parameter for hydrogel properties.     

Poly(ethylene oxide)‐ and Poly(perfluorohexylethyl methacrylate)‐Containing Amphiphilic Block Copolymers: Association Properties in Aqueous Solution

Amphiphilic di‐ and triblock copolymers containing poly(ethylene oxide) (PEO) as the hydrophilic block and poly(perfluorohexylethyl methacrylate) (PFMA) as the hydrophobic block were synthesized by atom‐transfer radical polymerization using hydroxy‐terminated PEO as the macroinitiator. The copolymers were characterized by size exclusion chromatography and ¹H NMR spectroscopy. Self‐association in aqueous solution has been investigated using surface tension measurements, dynamic light scattering (DLS), and transmission electron microscopy (TEM). From surface tension measurements in water, a characteristic concentration (c*) can be detected, which is interpreted as the critical micelle concentration (cmc). The cmc decreases with an increase in fluoro content in the triblock copolymer up to 11 wt.‐% PFMA (solubility limit). DLS studies have been carried out for different samples above the cmc, showing small aggregates (micelles) and single chains for diblock copolymer solutions. In the case of triblock copolymers large clusters were the dominant aggregates in addition to the micelles and single chains. The effect of temperature and concentration on the micelle and cluster formation has been investigated by DLS. Micelle size was found to be resistant to any change by temperature, however, a slight but significant increase in apparent hydrodynamic radius was observed with an increase in concentration, while both temperature and concentration affected the formation of large clusters, especially in concentrated solutions. TEM has been carried out to visualize the morphology of the aggregates after transferring the solution to carbon film. The initial concentration for the preparation of TEM samples was found to have a strong influence on the morphology of the aggregates. By adding colloidal gold particles to the solutions, the typical covering by the polymer was observed by TEM.

Decay‐rate distributions for PEO₁₀F5 (4.0 g · L^?1); obtained from the time correlation functions.     

Synthesis and Self‐Assembly of Amphiphilic Block Copolymers from Biobased Hydroxypropyl Methyl Cellulose and Poly(l‐lactide)

A series of well‐defined hydroxypropyl methyl cellulose‐block‐poly(l ‐lactide) (HPMC‐b‐PLLA) diblock copolymers are synthesized via UV‐initiated thiol‐ene click reaction of thiol‐terminated HPMC with different block lengths and allyl‐terminated PLLA, using 2,2‐dimethoxy‐2‐phenylacetophenone as photocatalyst. The former is obtained by coupling the reducing aldehyde endgroup of short chain HPMC with the amine group of cysteamine, and the latter by ring‐opening polymerization of l ‐lactide in the presence of allyl alcohol. Fourier transform infrared (FT‐IR), nuclear magnetic resonance (¹H NMR), and diffusion ordered spectroscopy NMR confirm the successful coupling of both blocks. The molar mass of the resulting copolymers ranges from 7000 to 12 800 g mol⁻¹ as determined by size exclusion chromatography. The copolymers are able to self‐assemble in aqueous medium, yielding micelles of 50–100 nm with core–shell structure as evidenced by dynamic light scattering, transmission electron microscopy, and ¹H NMR. The critical micelle concentration of copolymers ranges from 0.12 to 0.15 mg mL⁻¹. Last but not the least, the copolymers exhibit thermoresponsive behavior with a lower critical solution temperature around 80 °C.

     

Model ω‐Functionalized Linear Polystyrenes with One,Two, and Three Sulfobetaine End Groups: Synthesis,Characterization, and Association Behavior

Three series of ω‐functionalized polystyrenes (PS) with different molecular weights, the first consisting of dimethylamino end‐capped PSs and the other two of ω‐branched PSs end‐capped with two and three low‐molecular‐weight (M _n ～ 500 g · mol^?1) dimethylamino ω‐functionalized polybutadienes (PB), were synthesized by high‐vacuum anionic polymerization techniques using the functional initiator ([3‐(dimethylaminopropyl)]lithium) and chlorosilane linking chemistry. The ω‐dimethylamino polymers (precursors) were molecularly characterized by size‐exclusion chromatography, low‐angle laser light scattering (LALLS), membrane osmometry, and NMR spectroscopy. The characterization results indicate a high degree of molecular and structural homogeneity. The dimethylamino end groups were transformed to the highly polar sulfozwitterionic ones (see Figure) by reaction with cyclopropanosultone. The mono‐, di‐, and tri‐zwitterion capped polymers were found by LALLS, dynamic light scattering (DLS) and viscometry, to associate in carbon tetrachloride, a good nonpolar solvent for the PS tail. In contrast, results on dimethylamino‐capped precursors show no evidence of aggregation. Aggregation numbers increase in decalin compared with those in carbon tetrachloride. At constant molecular weight of the parental PS, the degree of association increases with increasing number of functional groups and for a given number of functional groups with decreasing molecular weight of the PS tail. Temperature‐dependent light scattering measurements in decalin indicate that aggregation persists at the highest temperature investigated.

A schematic representation of the materials investigated in this study.     

Diblock Copolymers Based on 1,4‐Dioxan‐2‐one and ε‐Caprolactone: Characterization and Thermal Properties

Summary: Various poly(ε‐caprolactone‐block‐1,4‐dioxan‐2‐one) (P(CL‐block‐PDX)) block copolymers were prepared according to the living/controlled ring‐opening polymerization (ROP) of 1,4‐dioxan‐2‐one (PDX) as initiated by in situ generated ω‐aluminium alkoxides poly(ε‐caprolactone) (PCL) chains in toluene at 25 °C. 1 ¹H NMR, PCS and TEM measurements have attested for the formation of colloids attributed to a growing PPDX core surrounded by a solvating PCL shell during the polymerization of PDX promoted by ω‐Al alkoxide PCL chains in toluene. The thermal behavior of the P(CL‐block‐PDX) copolymers was studied by DSC; showing two distinct melting temperatures (as well as two glass transition temperatures) similar to those of the respective homopolyesters. Finally, the thermal degradation of the P(CL‐block‐PDX) block copolymers was investigated by TGA simultaneously coupled to a FT‐IR spectrometer and a mass spectrometer for evolved gas analysis (EGA). The degradation occurred in two consecutive steps involving a first unzipping depolymerization of the PPDX blocks followed by the degradation of the PCL blocks via both ester pyrolysis and unzipping reactions.

TEM observation of P(CL‐block‐PDX) block copolyesters ( = 11 600 and = 22 100) as formed by vaporization starting from a diluted suspension in toluene/TCE mixture solvent (50/50 v/v).     

Synthesis and Characterization of Block Copolymers of ε‐Caprolactone and DL‐Lactide Initiated by Ethylene Glycol or Poly(ethylene glycol)

Biodegradable copolymers were prepared by ring‐opening polymerization of sequentially added ε‐caprolactone and DL ‐lactide in the presence of ethylene glycol or poly(ethylene glycol), using zinc metal as catalyst. Polymerization was performed in bulk and yielded block copolymers with predetermined PEG/PCL/PLA segments. The obtained polymers were characterized by ¹H NMR, SEC, IR, DSC, TGA, and X‐ray diffraction. Data showed that the copolymers preserved the excellent thermal behavior inherent to PCL. The crystallinity of PLA‐containing copolymers was reduced with respect to PCL homopolymer. The presence of both hydrophilic PEG and fast degrading PLA blocks should improve the biocompatibility and biodegradability of the materials, which are of interest for applications as substrate in drug delivery or as scaffolding in tissue engineering.

Block copolymerization of ε‐caprolactone and DL ‐lactide initiated by dihydroxyl PEG.     

Photoresponsive New Polysiloxanes with 4‐Substituted Azobenzene Side‐Groups. Synthesis,Characterization and Kinetics of the Reversible Trans‐Cis‐Trans Isomerization

New polydimethylsiloxanes with p‐substituted azobenzene side‐groups were synthesized. Thin films and solutions exhibit a photochemical trans‐cis isomerization of the azobenzene groups, followed by their cis‐trans thermal relaxation in the dark. In films, relaxation rates were found to be 100–1 000 times slower than the rates of photoisomerization, the former being very sensitive to the electron‐acceptor character of the substituents. in solution, the rates of cis‐trans relaxation are lower than those obtained for the solid state. This is ascribed to the dipolar intramolecular interactions between cis chromophores, which are favored in solution.

     

Synthesis and Characterization of Comb‐Like Copolymers Based on Poly(ε‐caprolactone) and Poly(α‐olefin)

Comb‐like copolymers based on a polyolefin backbone of poly(10‐undecene‐1‐ol) (PUol) with poly(ε‐caprolactone) (PCL) side chains are synthesized in two steps. After synthesis of PUol by metallocene‐catalyzed polymerization, the side‐chain hydroxyl functionalities of this polar polyolefin are used as an initiator for the ring‐opening polymerization (ROP) of ε‐caprolactone (CL). In this context, copolymers with different lengths of PCL grafts are prepared. The chemical structure and the composition of the synthesized copolymers are characterized by ¹H and ¹³C NMR spectroscopy. It is shown that the hydroxyl end groups of PUol act effectively as initiating sites for the CL ROP. Size‐exclusion chromatography (SEC) measurements confirm the absence of non‐attached PCL and the expected increase in molar mass after grafting. The thermal and decomposition behaviors are investigated by DSC and thermogravimetric analysis (TGA). The effect of the length of the PCL grafts on the crystallization behavior of the comb‐like copolymers is investigated by DSC and wide‐angle X‐ray scattering (WAXS).