Gel Formation and Molecular Characteristics of Poly(N‐isopropylacrylamide) Prepared by Free‐Radical Redox Polymerization in Aqueous Solution

The molecular characteristics of poly(N‐isopropylacrylamide) (PNIPA), prepared by free‐radical polymerization using an aqueous redox initiator and reaction conditions comparable to those used in the synthesis of nanocomposite gels, were investigated by altering the monomer concentration ([NIPA]) and the polymerization temperature (T_p) across the transition temperature (LCST). When T_p<LCST, there is a critical [NIPA] (=n*) above which PNIPA partially forms gels in the absence of a chemical crosslinker, and the gel fraction increases with increasing [NIPA] and decreasing T_p. In the range of n<n*, the molecular weight of soluble PNIPA correlated well with [NIPA]. When T_p>LCST, gels were not formed regardless of [NIPA]. The structure and mechanism of formation of self‐crosslinked PNIPA gels are discussed.

     

Acetyl Halide Initiator Screening for the Cationic Ring‐Opening Polymerization of 2‐Ethyl‐2‐Oxazoline

Kinetic investigations on the cationic ring‐opening polymerization of 2‐ethyl‐2‐oxazoline were conducted using acetyl chloride, acetyl bromide, and acetyl iodide as initiators. Various polymerization temperatures ranging from 80 to 220 °C were applied under microwave irradiation. The resulting polymerization mixtures were characterized with GC and GPC for the determination of monomer conversion and molecular weight distribution, respectively. Well defined polymers with narrow molecular weight distributions ( = 6 000 Dalton, PDI ≈ 1.10) were obtained with all three initiators.

     

Radical Polymerisation of 1H,1H,2H,2H‐perfluoro‐3,5‐alkyldiynol and 1H,1H‐perfluoro‐2,4‐alkyldiynol Acrylates and Methacrylates: A New Family of Fluorinated Polymers

Summary: The preparation of a new family of acrylic esters with a perfluorinated chain directly bonded to one or two conjugated triple bonds from 1H,1H,2H,2H‐perfluoro‐3,5‐alkyldiynol and 1H,1H‐perfluoro‐2,4‐alkyldiynol is reported. Their solution homopolymerisation was studied in the presence of a radical initiator and it was found that the polymerisation process bore more similarity to that observed with the analogous hydrocarbonated monomers containing triple bonds rather than that observed with polyfluorinated monomers without any alkyne groups. Furthermore, under the reaction conditions used, the triple bonds remained unchanged as there was no cross‐linking process. The homopolymers obtained were characterised by NMR, infrared spectroscopy and gel permeation chromatography whenever possible.

Structure of new fluorinated homopolymers.     

Cycle Formation in the Polycondensation of Potassium 3‐Bromomethyl‐5‐methylbenzoate

Potassium 3‐bromomethyl‐5‐methylbenzoate 2K , preformed or obtained from 3‐bromomethyl‐5‐methylbenzoic acid 2 by exchange reaction with potassium undecanoate 5K , was polymerized in bulk or in 1‐methyl‐2‐pyrrolidone (NMP), in the presence or not of a trifunctional core molecule. GPC analyses of the reaction products have shown that small cyclic oligomers (usually from C₂ to C₆ ) are formed in all the cases. MALDI‐TOF MS of polymers have shown that, in the range of masses examined, those prepared by polymerization of 2K in bulk or in NMP are almost exclusively formed of cycles. In the polymer obtained by polymerization, in NMP, of 2 in the presence of 5K linear and cyclic species coexist. In the polymer resulting from the polymerization of 2K in the presence of 2,4,6‐tris(bromomethyl)mesitylene (TBMM) the expected three‐arm stars are predominant and the content in cycles is relatively low and obviously the lowest of all the other prepared polymers. The formation of cycles (large and/or small) is an inherent component of the polycondensation of 2K . Cyclization strongly contributes to the limitation of molar masses of the reaction products.

MALDI‐TOF mass spectrum (reflector mode) of PB2 (run 3, Table 1 ). Symbols: ?, C_n; × , (C_p[D])L_qOCH₃ and + , (C_p[D])L_qBr.     

Molar Mass and Microstructure Analysis of PI‐b‐PMMA Copolymers by SEC‐NMR

The online coupling of size‐exclusion chromatography and NMR is used to characterize block copolymers consisting of polyisoprene (PI) and poly(methyl methacrylate) (PMMA) regarding their distributions of molar mass (MMD) and chemical composition (CCD). Using the CCD, $\overline{M}_{\rm n}$ and $\overline{M}_{\rm w}$ are calculated on the basis of PI and PMMA homopolymer calibrations. The microstructure distribution of PMMA and the distribution of isomeric units of PI in dependence of molar mass is also demonstrated. Furthermore, a simulation analysis is presented for a bimodal eluting sample. It allows for full separation, quantification and molar mass determination of the coeluting co‐ and homopolymer fractions. The quantification of the fractions is verified by liquid chromatography at critical conditions.     

Nitroxide‐Mediated Radical Polymerization with Bisaminooxy Compounds as Initiators – Controlled Biradical Polymerization

Summary: The bisaminooxy compounds Bis‐TEMPO and Bis‐TIPNO derived from 2,2,6,6‐tetramethyl‐piperidine‐1‐oxyl (TEMPO) and 2,2,5‐trimethyl‐4‐phenyl‐3‐azahexane‐3‐oxyl (TIPNO) were applied as “biradical initiators” for the nitroxide‐mediated radical polymerization (NMRP) of styrene and n‐butyl acrylate. It was shown by comparison with analogous alkoxyamines as unimolecular initiators and mixing experiments of mono‐ and biradical species, that in the case of the biradical initiators chain growth occurs at both sides under NMRP conditions. This enables a two‐step synthesis of A‐B‐A‐triblock copolymers. Kinetics and molecular mass development were investigated for the controlled biradical polymerization of styrene at different initiator concentrations, temperatures, and with addition of acetic anhydride as accelerator. For the controlled biradical polymerization of n‐butyl acrylate with Bis‐TIPNO, the effect of added free nitroxide relative to the initiator concentration was studied. The poly(styrene‐block‐n‐butyl acrylate‐block‐styrene) copolymers with higher block length prepared by this method show two glass transition temperatures, which indicates microphase separation of the polymer blocks.

Structure of poly(styrene‐block‐n‐butyl acrylate‐block‐styrene), synthesized by nitroxide‐mediated radical polymerization with Bis‐TIPNO as initiator.     

Amphiphilic Block Copolymers Containing Regioregular Poly(3‐hexylthiophene) and Poly(2‐ethyl‐2‐oxazoline)

Poly(3‐hexylthiophene)‐block‐poly(2‐ethyl‐2‐oxazoline) amphiphilic rod–coil diblock copolymers have been synthesized by a combination of Grignard metathesis (GRIM) and ring‐opening cationic polymerization. Diblock copolymers containing 5, 15, and 30 mol‐% poly(2‐ethyl‐2‐oxazoline) have been synthesized and characterized. The synthesized rod–coil block copolymers display nanofibrillar morphology where the density of the nanofibrills is dependent on the concentration of the poly(2‐ethyl‐2‐oxazoline) coil segment. The conductivity of the diblock copolymers was lowered from 200 to 35 S · cm^?1 with an increase in the content of the insulating poly(2‐ethyl‐2‐oxazoline) block. By contrast, the field‐effect mobility decreased by 2–3 orders of magnitude upon the incorporation of the poly(2‐ethyl‐2‐oxazoline) insulating segment.

     

Control of Molecular Weight and Tacticity in Stereospecific Living Cationic Polymerization of α‐Methylstyrene at 0 °C Using FeCl3‐Based Initiators: Effect of Tacticity on Thermal Properties

The successful stereospecific living cationic polymerization of α‐methylstyrene (α‐MeSt) using FeCl₃‐based initiator systems at 0 °C is reported. Linear first‐order ln([M]₀/[M]) vs. time and linear molecular weight vs. conversion plots suggest that the polymerization is living in nature, which is further confirmed from successful chain‐extension experiments. Poly(α‐methylstyrene)s of varying syndiotacticities (59.1% to 79.2%) and controllable molecular weights (4300–32 100 g mol^?1) with moderately narrow polydispersity indices (PDIs ≈1.3) are synthesized simply by varying the monomer‐to‐initiator ratio ([M]₀/[I]₀). A possible mechanism for this stereospecific polymerization is proposed. The glass‐transition temperature and thermal‐decomposition temperature depend on the syndiotacticity of poly(α‐methylstyrene).

     

Hyperbranched Polyalkoxysiloxanes via AB3‐Type Monomers

We have synthesized polyethoxysiloxanes starting from the AB₃‐type monomers triethoxysilanol and acetoxytriethoxysilane. The polymers are liquid and soluble in organic solvents. ²⁹Si NMR spectroscopy and MALDI‐ToF mass spectrometry analyses show that the polymers have a hyperbranched structure with additional internal cyclization. ²⁹Si NMR spectroscopy indicates that the polymer synthesized from acetoxytriethoxysilane is less branched than the polymer synthesized from triethoxysilanol. Analysis of the molar mass and mass distribution of the polymers via size exclusion chromatography (calibrated via MALDI‐ToF MS and viscosimetry) yields a molar mass of M _n ≈ 2 kg · mol^?1 and M _w ≈ 8 kg · mol^?1 for polymers synthesized from triethoxysilanol. The molar mass of the polymers synthesized from acetoxytriethoxysilane can be controlled by variation of the polymerization time in the range of M _n ≈ 1.8–12 kg · mol^?1 and M _w ≈ 2.1–2 200 kg · mol^?1.

Photograph of a vial containing polyethoxysiloxane obtained from triethoxysilanol and a schematic drawing of the proposed molecular structure of the polymer.     

A New Insight into the Mechanism of 1,3‐Dienes Cationic Polymerization I: Polymerization of 1,3‐Pentadiene with tBuCl/TiCl4 Initiating System: Kinetic and Mechanistic Study

The cationic polymerization of 1,3‐pentadiene using a tert‐butyl chloride (^tBuCl)/TiCl₄ initiating system in CH₂Cl₂ at different reaction conditions is reported. It is shown that the reaction rate increases with the increase of the ^tBuCl/TiCl₄ molar ratio, while the molecular weight distribution becomes narrower. Well‐defined oligo(1,3‐pentadiene)s ( ≤ 3500 g mol^?1; / ≤ 3.0) are obtained at high ^tBuCl/TiCl₄ molar ratio (340) and low temperature (–78 °C). ¹H and ¹³C NMR spectroscopy studies reveal the presence of tert‐butyl head and –CH₂–Cl end groups. The number‐average functionalities (F_ns) at the α‐ and ω‐ends are calculated to be F_n(^tBu) > 1 and F_n(Cl) < 1, respectively. The general mechanism of 1,3‐pentadiene polymerization is proposed.

     

Meta‐Terphenyl Derivative/Iodonium Salt/9H‐Carbazole‐9‐ethanol Photoinitiating Systems for Free Radical Promoted Cationic Polymerization upon Visible Lights

In the present paper, new photoinitiators based on the meta‐terphenyl scaffold in combination with an iodonium salt and 9H‐carbazole‐9‐ethanol (CARET) are proposed for the free radical promoted cationic polymerization of epoxides upon visible light exposure using light emitting diodes at 405, 455, and 470 nm. Remarkably, a new high performance additive (i.e., CARET) is proposed here for cationic polymerizations. CARET shows some important advantages compared to other reported additives such as N‐vinylcarbazole or benzyl alcohol used as references. Excellent polymerization initiating abilities are found and a full picture of the involved chemical mechanisms is provided.

     

A New Insight into the Mechanism of 1,3‐Dienes Cationic Polymerization III: Polymerization of 1,3‐Pentadiene with CF3COOD/TiCl4 Initiating System: Chain‐Ends Structure and Kinetics

A novel method for the investigation of the chain‐end structure of poly(1,3‐pentadiene)s synthesized using the CF₃COOD/TiCl₄ initiating system is developed. It is shown for the first time that the content of trans‐1,2‐structures in the first monomer unit is considerably higher than the content of trans‐1,4‐structures, whereas the content of trans‐1,4‐units is substantially higher than trans‐1,2‐units for the polymer chain as a whole. Another important observation is that chain transfer to monomer is significant even at the earlier stages of the 1,3‐pentadiene polymerization (after 1 s of reaction). The very low functionality at the ω‐end (F_n (Cl) < 0.15) confirms the intensive chain transfer to monomer. This method is also applied for the estimation of the concentration of active species and the rate constant for propagation (k _p) for the cationic polymerization of 1,3‐pentadiene using the CF₃COOD/TiCl₄ initiating system: rate constants for propagation, k _p, of 1.5 × 10³ and 3.3 × 10³ L mol^?1 min^?1 are determined for 1,3‐pentadiene polymerization at 20 and –78 °C, respectively.

     

Fe(0) Powder/CuBr2‐Mediated “Living”/Controlled Radical Polymerization of Methyl Methacrylate and Styrene at Ambient Temperature

The combination of zero‐valent iron (Fe(0) powder) and copper(II) bromide was used to mediate the polymerization of methyl methacrylate (MMA) or styrene (St) at 25 ° C. The results demonstrated that the solvent played an important role on the polymerization rate and molecular‐weight control. The polymerization in toluene displayed a poorly controlled process with remarkably low polymerization rate. With dimethyl sulfoxide (DMSO) as solvent, the polymerization proceeded in a relatively high rate, and the number‐average molecular weights were controlled especially at higher conversion. High conversions (80%) of St could be achieved with narrow molecular weight distributions in DMSO at 25 °C. It was supposed that Fe(0) played a dual role, the activator for the generation of active radical and the reducing agent for CuBr₂.     

Cationic Bottlebrush Polymers from Quaternary Ammonium Macromonomers by Grafting‐Through Ring‐Opening Metathesis Polymerization

Cationic bottlebrush homopolymers are polymerized using a grafting‐through approach by ring‐opening metathesis polymerization (ROMP) to afford well‐defined polymers. Quaternary ammonium macromonomers (MMs) are prepared by quaternizing tertiary amine MMs synthesized by reversible addition‐fragmentation chain transfer (RAFT) polymerization. The quaternary ammonium MMs undergo ROMP to target molecular weights (M_n = 30 000–100 000 g mol^?1) and a low dispersity (? = 1.10–1.30). Halide‐ligand exchange between the third generation Grubbs catalyst (G3) and halide counter ions (bromide and iodide ions) of MMs changes the catalyst activity throughout ROMP, causing it to deviate from pseudo‐first order kinetic behavior; however, the polymerization still follows controlled behavior without significant catalyst termination. Increasing steric bulk of the MMs decreases the polymerization rate as well. Amphiphilic block copolymers are synthesized by sequential polymerization of quaternary ammonium MMs and polystyrene (PS) MMs. Using a PS macroinitiator affords block copolymers with lower ? values as compared to the less active cationic macroinitiator.     

New Thermosets Obtained by Thermal and UV‐Induced Cationic Copolymerization of DGEBA with 4‐Phenyl‐γ‐butyrolactone

Mixtures of DGEBA with 4‐phenyl‐γ‐butyrolactone (PhBL) were cationically copolymerized in the presence of ytterbium triflate or triarylsulfonium hexafluoroantimoniate as thermal or photo initiator respectively. Changes during curing and final properties of the cured materials were studied by means of DSC, FT‐IR/ATR, TMA, DMTA, TGA and densitometric measurements. The formation of a network containing polyether and poly(ether‐ester) moieties was demonstrated to take place through the ring‐opening polymerization of a spiroorthoester intermediate (SOE) formed during the copolymerization. An increase in the proportion of lactone resulted in an increase in the curing rate, a decrease in the shrinkage after gelation and in the thermal stability and glass transition temperature (T_g). A strong influence of the initiator on the curing mechanism was observed. As a consequence, the photocured materials exhibited superior thermal stability and T_g than those obtained thermally.

     

A comparative study of the properties of different distribution curves

The maximum, points of inflection and the derived peak width are enumerated for a Poisson, a Gauss and a Schulz‐Flory distribution. Starting with a (not necessarily normalized) number distribution the respective molar mass and hyper distributions are derived and the extrema and the peak width are determined as well. It turned out, that Poisson and (narrow, D < 1.1) Gauss distributions can be characterized by the fact that the peak width is an invariant quantity with respect to the number, molar mass and hyper distribution, whereas the peak width changes considerably for a Schulz‐Flory distribution. In order to demonstrate the theoretical results commercial polymer standards (prepared by anionic polymerization) were measured by size exclusion chromatography and the constancy of the peak width is demonstrated but the peak widths exceeded by far the theoretically expected values. The influence of axial broadening in size exclusion chromatography is discussed in this context and a simple correction procedure is presented for an ideal Poisson distribution. On the other hand, polymers prepared by stationary radical polymerization displayed a very marked deviation from an invariant peak width, as derived theoretically. It is demonstrated that the location of the peak maximum in the molar mass and the hyper distribution is identical with M _n and M _w, respectively.

Comparison of the peak width of molar mass distributions: Poisson, Gauss with polydispersity 1.1 and Schulz‐Flory with k = 1 and 2, respectively.     

Mechanical and Chemical Analysis of Gelatin‐Based Hydrogel Degradation

The interrelated effect of environmental pH and temperature, gelatin backbone modification and content on the tensile and degradative property of interpenetrating networks (IPNs) containing gelatin and poly(ethylene glycol) diacrylate (PEGdA) was examined. Either increasing the PEGdA content or modifying the gelatin backbone with PEG‐monoacetate ester and/or polyanions decreased the IPN elasticity at ambient room temperature (rt). Under an aqueous environment of varying pH levels and elevated temperature, the degradation of IPN tensile properties was further accelerated. IPNs showed an enhanced elasticity and strength when compared to glutaraldehyde‐fixed gelatin hydrogels. Under an aqueous condition, IPNs showed a wider range of degradation products than hydrogels cross‐linked with glutaraldehyde, as characterized with gel permeation chromatography. The nature of IPN degradation products was independent of the type of gelatin backbone modification. The presence of loaded drug, chlorohexidine digluconate, which was found to interact with PEG‐monoacetate esters of the modified gelatin backbone, resulted in unique degradation products. The tensile and chemical degradation of IPNs is a complex interrelationship of the environmental condition, time, and material modification.

Stress‐strain curves of some IPNs studied here.     

A Novel Branched Polyoxymethylene Synthesized by Cationic Copolymerization of 1,3,5‐Trioxane with 3‐(Alkoxymethyl)‐3‐ethyloxetane

Novel branched polyoxymethylene copolymers are synthesized by cationic copolymerization of 1,3,5‐trioxane (TOX) with 3‐(alkoxymethyl)‐3‐ethyloxetane (ROX) using BF₃·Et₂O as an initiator. Four oxetane derivatives with different side‐chain lengths (from 1 to 6 carbons) are tested for copolymerization. The copolymer composition is controlled by the feed ratio of ROX, and influenced by the chain length of alkyl group on ROX. The incorporation ratio and side‐chain length of the ROX unit have great influence on the thermomechanical properties and crystallinity of the copolymers.

     

Thermoresponsive Poly(2‐Oxazoline) Molecular Brushes by Living Ionic Polymerization: Modulation of the Cloud Point by Random and Block Copolymer Pendant Chains

Molecular brushes (MBs) of poly(2‐oxazoline)s were prepared by living anionic polymerization of 2‐isopropenyl‐2‐oxazoline to form the backbone and living cationic ring‐opening polymerization of 2‐n‐propyl‐2‐oxazoline and 2‐methyl‐2‐oxazoline to form random and block copolymers. Their aqueous solutions displayed a distinct thermoresponsive behavior as a function of the side‐chain composition and sequence. The cloud point (CP) of MBs with random copolymer side chains is a linear function of the hydrophilic monomer content and can be modulated in a wide range. For MBs with block copolymer side chains, it was found that the block sequence had a strong and surprising effect on the CP. While MBs with a distal hydrophobic block had a CP at 70 °C, MBs with hydrophilic outer blocks already precipitated at 32 °C.     

Demonstration of the Existence of Long‐Lived Species in the Cationic Polymerization of 1,3‐Pentadiene Initiated by Aluminium Trichloride in Non Polar Solvent

Summary: A kinetic study of the polymerization of a mixture of 1,3‐pentadiene isomers initiated by AlCl₃ was carried out in pentane at ?10 and 20 °C. A second apparent monomer order was found mainly at room temperature and was explained by a complexation of propagating active centers by the polymer, the true monomer order being first. This apparent order did not result from the presence of the two isomers in the monomer mixture. Kinetic simulations confirmed this interpretation and pointed out the fact that the interaction between the polymer and the active centers was stronger at ?10 °C, thus limiting the polymer conversion at this temperature. The novelty of these findings lies in the fact that the inactive complexed centers can be activated by reaction with the monomer, providing active species when necessary. The existence of active centers even after a long reaction time at room temperature, and the reactivation of the complexed centers, was evidenced by incremental monomer addition and by the formation of sulfonium ions after quenching the polymerization by an excess of dimethyl sulfide. The latter were characterized by ¹H NMR spectroscopy.

Complexation of propagating active centers by the polymer giving dormant species (C⁺/P), with following reactivation by a new monomer charge.