In our present work, a novel controlled radical polymerization system is developed based on xanthene-9-thione(XT). It was found that the radical polymerization of styrene(St) became controlled in the presence of a small amount of XT. At the early stage of the polymerization, the polymerization rate was relatively low and the as-formed polystyrene(PS) had low number-average molecular weight(Mn) and narrow polydispersity(D). After XT was consumed, the polymerization rate increased dramatically and the Mn of PS increased gradually with polymerization proceeding. When the polymerization of St was carried out with a proper molar ratio of initiator to XT and at an appropriate temperature, shortened slow polymerization stage and good control over Mn could be achieved. To further improve the regulating ability of XT, a series of substituent groups(―CF_3, ―CH(CH_3)_2, ―N(CH_3)_2) were introduced onto the xanthene ring of XT,and the effects of these derivatives on the polymerization of St were investigated in detail. UV-Vis spectroscopy was carried out to monitor the concentration of XT during the polymerization and the chemical structure of the as-formed PS was fully characterized by 1 HNMR and ESI-MS analysis. A possible mechanism involving the formation and evolution of the cross-termination products was proposed to interpret the observed polymerization behavior. 相似文献
While miniemulsion polymerization has proven to be well‐suited for conducting living/controlled radical polymerizations, emulsion polymerizations have proven to be far more challenging. Ab initio emulsion polymerizations, in which monomer droplets are present during polymerization, have thus far not been successful with TEMPO‐mediated polymerizations, as a result of colloidal instability and coagulum formation. By selectively inhibiting polymerization in the monomer droplets, it is demonstrated that droplet polymerization is responsible for the formation of large (>1 µm) particles that can lead to coagulum formation. Furthermore, we show that coagulum‐free latexes can be produced using a TEMPO‐mediated ab initio emulsion polymerization by suppressing droplet polymerization.
Effect of a series of inhibitors as additives on atom transfer radical polymerization (ATRP) of methyl methacrylate (MMA) with FeCl2/PPh3 as catalyst system was studied, including 2,4,6-trinitrophenol (TNP), 4-methoxyphenol (4-MP), hydroquinone (HQ) and nitrobenzene (NB). It was found that TNP was the only. efficient additive for ATRP among these inhibitors. In the presence of small amounts of TNP, the polymerization proceeded rapidly after induction period to yield the polymers with controlled molecular weights and narrow molecular weight distributions (MWD). The initiating efficiency of the modified catalyst system with TNP was increased. The mechanism was proposed and confirmed by the end group analysis of the polymer. 相似文献
Bone cements based on poly(methyl methacrylate) (PMMA) have been widely used in orthopedic surgeries for fixation of prostheses and filling of bone defects. Bone cements are produced through in situ and in vivo free radical bulk polymerizations, which are highly exothermic and are subject to strong gel and glass effects. As a consequence, high temperatures may be reached during application. Furthermore, residual monomer usually remains unreacted inside the body and may cause aseptic loosening and tissue damages. 1 In a companion work in this volume, it was shown that usual free-radical polymerization models might effectively describe the bone cement preparation 2 and therefore be used for quantitative analysis of the bone cement synthesis. In this work, a theoretical investigation based on a multicell reactor model is performed to study the bone cement production and allow for future optimization of the preparation procedure. It is shown that the degree of solubility of the pre-polymer powder in the liquid monomer is the most important variable during the bone cement preparation and that this variable should be manipulated for design and control of the operation in real applications. 相似文献
It has been known that the propagation rate coefficient (kp) will change with monomer conversion, Friss, Hamielec et al.1-5 have proposed many formulas about kp to fit this change. In order to obtain the relation between Arrhenius parameters and monomer conversion, a group of kp values at the same monomer conversion must be measured. We adopted a new method through analysis of kp values measured by usual experimental techniques for this purpose. Firstly, we chose a value of monomer convers… 相似文献
This review article describes the preparation of polymer brushes by nitroxide‐mediated radical polymerization using either the ‘grafting to’ or the ‘grafting from’ approach. The use of TEMPO as a classical initiator is intensively described. More sophisticated nitroxides are also included in the discussion. Brush formation on flat surfaces such as wafers and also on particles is reported. Finally, some applications of polymer brushes are presented.
Nitroxide‐mediated polymerizations of styrene in microemulsion have been carried out at 125 °C using the cationic surfactant tetradecyltrimethylammonium bromide and the nitroxides 2,2,6,6‐tetramethylpiperidinyl‐1‐oxy (TEMPO) and N‐tert‐butyl‐N‐[1‐diethylphosphono‐(2,2‐dimethylpropyl)] nitroxide (SG1). TEMPO‐mediated polymerizations were extremely slow, with large particles (dn = 39–129 nm) and broad molecular weight distributions (MWDs). The origin of the broad MWDs was likely significant alkoxyamine decomposition and differing diffusion rates of monomer and low MW alkoxyamines (and nitroxide) between monomer‐swollen micelles and polymer particles. SG1‐mediated polymerizations proceeded at higher rates, resulting in nanoparticles (dn = 21–37 nm) and lower than for TEMPO.
As a kind of living free radical polymerizations, reversible addition-fragmentation chain transfer (RAFT) has been proved to be an important technique nowadays as it is applicable to a wide range of monomers at a wide range of temperature below 100oC. In … 相似文献
A mechanistic model is developed for high‐temperature (138 °C) styrene semibatch thermally and conventionally initiated FRP, as well as NMP with a two‐component initiating system (tert‐butyl peroxyacetate, 4‐hydroxy‐TEMPO). The model, using kinetic coefficients from literature, provides a good representation of the FRP experimental results. Implementation of a gel effect correlation to represent the change in the diffusion‐controlled termination rate coefficient with conversion improves the fit to the thermally initiated system, but is not required to represent the production of low molecular weight material ( Dalton) by conventionally initiated FRP or NMP. The low initiator efficiency found in NMP is well explained by a reaction network involving combination of free nitroxide with methyl radicals formed from initiator decomposition.
An unprecedented level of control for the radical polymerization of vinylidene fluoride (VDF), yielding well‐defined PVDF (at least up to 14 500 g mol?1) with low dispersity (≤1.32), was achieved using organometallic‐mediated radical polymerization (OMRP) with an organocobalt compound as initiator. The high chain‐end fidelity was demonstrated by the synthesis of PVDF‐ and PVAc‐containing di‐and triblock copolymers. DFT calculations rationalize the efficient reactivation of both head and tail chain end dormant species. 相似文献
