The reversible addition‐fragmentation chain transfer (RAFT) polymerization of N‐acryloylmorpholine (NAM) is performed using three dithioesters (DT) as chain transfer agents (CTA) that incorporate a morpholine (morpholine‐DT), a biotin (biotin‐DT), or a sugar (sugar‐DT) moiety in the R group. PolyNAM chains of controlled characteristics are synthesized. An unexpected behavior is observed with morpholine‐DT, described as an ‘additional retardation’, which is especially visible when low molar masses are targeted ( < 5 000 g · mol−1). In that particular case, further investigations using MALDI‐TOF mass spectrometry show the presence of terminated intermediate radicals (IRs), which corroborates the assumption based on a specific protection of IR according to the nature of the α‐chain‐end.
The layer‐by‐layer (LBL) assembly of poly(diallyldimethylammonium chloride) and poly(sodium styrene sulfonate) on poly(sulfo propyl methacrylate) brushes resulted in films with nanometer‐ and micrometer‐sized holes and ledges, observed by atomic force microscopy and scanning electron microscopy. Polyelectrolyte assembly was followed by the quartz microbalance technique. The formation of ledges and holes is explained by the interaction of the brush polymers with the incoming polyelectrolytes during the LBL assembly, inducing a spatially localized and self‐organized accumulation of the assembled polymers.
A new concept to build shape memory polymers (SMP) combining outstanding fixity and recovery ratios (both above 99% after only one training cycle) typical of chemically crosslinked SMPs with reprocessability restricted to physically crosslinked SMPs is demonstrated by covalently bonding, through thermoreversible Diels–Alder (DA) adducts, star‐shaped poly(ε‐caprolactones) (PCL) end‐functionalized by furan and maleimide moieties. A PCL network is easily prepared by melt‐blending complementary end‐functional star polymers in retro DA regime, then by curing at lower temperature to favour the DA cycloaddition. Such covalent network can be reprocessed when heated again at the retro DA temperature. The resulting SMP shows still excellent shape memory properties attesting for its good recyclability.
Conformational changes in cyclic and linear chains triggered by increasing chain stiffness are explored using MC simulations. The transition to the rigid strained objects is continuous and steeper for cycles. Circular macromolecules of contour length equal to ≈5 persistence lengths are characterized as rigid strained objects with a lowered spatial dimensionality. The coil regime is almost omitted in the static structure factor already for semiflexible chains of both architectures and the flattening of stiffer cycles is evidenced by oscillations superimposed on the rod‐like behavior of corresponding linear chains. The orientational correlation function shows qualitative difference in behavior of cyclic and linear chains.
A parallel synthetic approach toward homogeneous atom‐transfer radical polymerization of methyl methacrylate has been successfully applied by utilizing an automated synthesizer. Experimental set‐up, automated parallel synthesis and purification of the polymers via a solid‐phase extraction set‐up, as well as online and offline measurements of the molecular weights and monomer conversion are described in detail. In addition, a comparison with conventional experiments in the laboratory is provided.
Kinetic plots of ln([M]0/[M]) versus reaction time of the ATRP of MMA in p‐xylene at 90 °C conducted in an automated synthesizer (three parallel reactions, ○, ▵, ⋄) and in a conventional set‐up (▪). 相似文献
Summary: The influence of architecture on ink‐jet printability of polymer solutions is investigated by comparing linear and 6‐arm star PMMA. At comparable concentration and molecular weight, filament formation is much more pronounced for linear PMMA than for star PMMA. Visual examination of filament stretching allows estimation of the involved elongation rates, which are at high voltages sufficiently large for coil‐stretch transition of the chains, suggesting its role in filament formation.
The results obtained in this study suggest a possible role of the coil‐stretch transition of the polymer chains in filament formation. 相似文献
A novel well‐defined amphiphilic block copolymer, with the polyhedral oligomeric silsesquioxane (POSS) moiety at the junction of the two blocks of polystyrene and poly(ethylene oxide) (PEO), was designed and synthesized. First, a macroinitiator containing a POSS moiety and a PEO chain was prepared and then atom transfer radical polymerization of styrene was carried out in the presence of the macroinitiator in bulk. The polymerization results show that the process bears the characteristics of controlled/living free radical polymerizations. The structure and molecular weight of the polymers were characterized by GPC, 1H NMR, and FT‐IR spectroscopy. The self‐assembly behaviors of the polymers was investigated by TEM and SEM. It was observed that the polymers can self‐assemble into vesicles in aqueous solution.
Shielding effects of the surrounding arms and chains on the reactive centers taking part in RAFT four‐arm star polymerization following the Z‐group approach are calculated by means of exact enumeration of star/chain samples prepared by Monte Carlo techniques. The shielding effect, which can be relieved when using expanded core moieties, increases with increasing chain (arm) lengths. This leads to a reduction of the contact probability according to a power law with an exponent of −0.4 to −0.45. Additionally, characteristic chain properties and shape parameters are calculated as a function of the distance between the center of the star and the end of the linear chain in order to gain deeper insight into the mechanism of contact formation preceding the actual reaction.
We have calculated the segmental tensor order parameter (second moment of orientation distribution function) of flexible polymer chains in a weak good solvent with fixed end‐to‐end distance and fixed segment length. Using the Edwards model for polymer chains with excluded volume interaction, we have performed direct perturbation expansions to obtain the two‐vector correlation function in one loop order. Our results indicate that distribution of tensor order parameter is not homogeneous along the chain as expected for ideal Gaussian statistics. Segmental order is lower as compared to Gaussian chains and decays toward the chain ends. These results are in agreement with computer simulation studies.
This paper focuses on the attachment of densely grafted polymer layers (polymer brushes) to various inorganic and polymeric substrates by the “grafting to” method. A brief overview of synthesis of polymer brushes by the method is first provided, with emphasis on chemical approaches to polymer attachment. The second part of the paper covers the synthesis of polymer layers via a recently developed macromolecular anchoring layer approach. Several examples of application of the grafting technique are presented for generation of hydrophobic, hydrophilic, gradient, and switchable surfaces.
End group modification of polymers prepared by reversible addition–fragmentation chain transfer (RAFT) polymerization was accomplished by conversion of trithiocarbonate into reactive functions able to conjugate easily with biomolecules or bioactive functionality. Polymers were prepared by RAFT, and subsequent aminolysis led to sulfhydryl‐terminated polymers that reacted in situ with an excess of dithiopyridyl disulfide to yield pyridyl disulfide‐terminated macromolecules or in the presence of ene to yield functional polymers. In the first route, the pyridyl disulfide end groups allowed coupling with oligonucleotide and peptide. The second approach exploited thiol–ene chemistry to couple polymers and model compounds such as carbohydrate and biotin with high yield.
In our ongoing efforts to develop poly(2‐oxazoline)s (POx) for biomedical applications, we report on the preparation of defined, star‐like hydrophilic POx. Using pluritriflate initiators, we show, through online kinetic measurements by gas chromatography, that multiple initiating groups are of equal reactivity for the initiation of the polymerization of 2‐oxazolines. The overall polymerization rate increases linearly with the number of initiator functions per molecule. Thus, all initiating moieties are of the same reactivity and all arms grow at the same rate. This is crucial for the establishment of a meaningful structure‐property relationship for polymers of star architectures.
The use of the molecular imprinting technique to produce polymers with high specificity for a given “molecular template” has undergone a rapid and expansive evolution since the inception of the idea over half a century ago. It was only a matter of time before the seemingly inevitable “marriage” of this concept with another modern research obsession, the generation of “smart” polymers, capable of reacting quickly, accurately and reproducibly to changes in their environment. Many advances have since been made, concerning the quality and diversity of these systems and polymers responsive to temperature, pH and a host of other environmental cues now exist. This article provides a succinct overview of the process and outcomes of “smart” molecular imprinting, followed by a detailed assessment of recent developments and applications in such field.
A novel approach has been explored to prepare brain‐like polyaniline (PANI) nanostructures with many convolutions (140–170 nm in average diameter) using aniline/citric acid (CA) salt as the template and chlorine gas as the oxidant by a gas/solid reaction for the first time. The method provided here differs significantly from the traditional one in which the polymerization of PANI is usually carried out in acidic solution.
Summary: Two series of hyperbranched conjugated polymers were synthesized via an A3 + B2 type Wittig reaction. The molecular weights of the polymers were successfully tuned by simply changing the feed ratio of the monomers. Polymers with higher molecular weights presented more efficient photoluminescence, higher thermal stability and higher performance of LEDs.
Summary: Conformational energy calculations on the chain conformation in the crystalline field have been performed for various syndiotactic vinyl polymers deriving from 1,3‐diene monomers. Energy maps as a function of the independent torsion angles have evidenced for all the polymers minima corresponding to highly extended and to helical chains. Energy minimizations as a function of all the internal parameters for the s(2/1)2 and t cm symmetries have allowed the evaluation of the energy differences between chains having the two symmetries and the prediction of the values of the conformational parameters for each polymer. The results have been compared with the experimental data reported in the literature for some of the studied polymers.
pH‐responsive PHEMA‐based polymeric nanostructures were grown in a controlled manner by ATRP‐based surface‐initiated polymerization. Initiator nanopatterns were obtained on silicon wafers covered with OTS resists made by AFM scanning probe oxidation lithography. AFM images confirmed isolated grafting of stimuli‐responsive hedge and dot brush structures exhibiting dimensions corresponding to a few tens of chains.
