Poly(2-oxazoline)-Containing Triblock Copolymers: Synthesis and Applications

This review focuses on poly(2-oxazoline) containing triblock copolymers and their applications. A detailed overview of the synthetic techniques is provided. Triblock copolymers solely based on poly(2-oxazoline)s can be synthesized by sequential monomer addition utilizing mono- as well as bifunctional initiators for the cationic ring-opening polymerization of 2-oxazolines. Crossover and coupling techniques enable access to triblock copolymers comprising, e.g., polyesters, poly(dimethylsiloxane)s, or polyacrylates in combination with poly(2-oxazoline) based segments. Besides systematic studies to develop structure property relationships, these polymers have been applied, e.g., in drug delivery, as (functionalized) vesicles, in segmented networks or as nanoreactors.     

大分子光引发剂的研究进展

综述了大分子光引发剂的研究现状和进展。简要叙述了大分子光引发剂的制备方法、引发机理。对小分子光引发剂和大分子光引发剂进行了对比,着重讨论了分子结构因素对引发性能的影响。     

Stable free radical polymerization of n-butyl acrylate in the presence of high-temperature initiators

Living radical polymerizations of acrylate are known to be difficult to achieve using TEMPO as a mediator. The stable free radical polymerization (SFRP) of acrylate tends to stop at low monomer conversion due to the accumulation of TEMPO in the medium as a result of unavoidable bimolecular termination. Rather than solving this problem by destroying the excess nitroxide using ascorbic acid or glyceraldehyde associated with pyridine as reported recently, high temperature initiators were used to slowly and continuously generate new radicals throughout the polymerization to consume the excess TEMPO molecules. Polymerizations of n-butyl acrylate initiated by the alkoxyamine unimer (1-benzoyloxy)-2-phenyl-2-(2′,2′,6′,6′-tetramethyl-1′-piperidinyloxy)ethane (BST) were performed between 130 °C and 134 °C in the presence of a series of high temperature peroxide and azo initiators. The best results in this study were obtained by the continuous addition of small amounts of di-tert-amyl peroxide throughout the polymerization. Under these conditions, the acrylate polymerizations fulfilled the criteria of a controlled polymerization process although the molecular weight distributions were slightly broad (M_w/M_n ∼ 1.5).     

Synthesis of polymethylmethacrylate in THF solution with phosphorous containing initiators

For the synthesis of polymethylmethacrylate, tetraphenyl biphosphine (TPhBP) with a thermally and photochemically unstable P‐P bond was employed. Under the influence of UV light, this bond split to two relatively stable biphenylphosphine radicals, which are able to react with the monomer. The resultant macroinitiators were isolated and were used for further polymerization with the same or another monomer to synthesize block‐copolymers. Controlled polymerization of methyl methacrylate with tetraphenyl biphosphine took place in the absence of oxygen by UV irradiation in THF solution. For MMA alone an insignificant portion photo‐ (0.3%) and thermal‐ (2%) polymerization were detected. Using selected quantity of the initiator, macroinitiators with predicted molecular weight as well as block‐copolymers were synthesized. The macroradicals were terminated by primary ‐PPh ₂ radicals, by chain transfer to initiator and by the combination of two macroradicals. We determined chain end groups by nuclear magnetic resonance spectroscopy (NMR) and the relative molecular weights of the polymers by gel permeation chromatography (GPC). The molecular weights were calculated using the ¹H NMR spectra from the ratio between the end groups signals and signals of the chain and were compared to GPC measurements. The calculated and observed molecular weights were in good agreement. At the lower concentration of initiator the molecular weight increased with conversion, while at the higher initiator concentration the molecular weight decreased with increasing conversion which could be ascribed to chain transfer to initiator. Copyright © 2001 John Wiley & Sons, Ltd.     

PROPERTIES OF POLYURETHANE ELASTOMERS BASED ON POLY (OXYETHYLENE-co-OXYTETRAMETHYLENE) DIOL*

Polyurethane elastomers derived from 4,4'-methylene bis(phenyl isocyanate), butylene glycol and the lowoxyethylene moiety content copolyether of tetrahydrofuran and ethylene oxide, obtained by copolymerization usingheteropolyacid-ethylene oxide initiator system, were prepared. The polyurethanes exhibited an increased water absorptionand much better low temperature resilience, which remained high even at a temperature of -3℃, while that of polyurethanebased upon poly(tetramethylene ether) glycol of the same molecular weight, i.e. being 2000, and molecular weightdistribution, i.e. in the range of 1.4-1.5, starts to decrease dramatically at 5℃.     

Comparison of Structure,Morphology, and Properties of Miktoarms Star Styrene-Butadiene Rubber and Star-Shaped Styrene-Butadiene Rubbe/Polybutadiene Rubber Blends

Four miktoarms star-shaped polybutadiene-Sn-poly(styrene-butadiene) rubber (MSS-PB-PSBR) with 1,1-diphenylhexyl at the ends of the arms were prepared by two different coupling techniques. One technique was a one-step technology, from which two miktoarms star styrene-butadiene rubbers, called AMSS-PB-PSBR, were obtained in which the four arm stars had varying ratios of PB:PSBR arms; another was a two-step technology, from which another two miktoarms star styrene-butadiene rubbers, called BMSS-PB-PSBR, were obtained in which all consisted of PB-Sn-(PSBR)₃ stars. The molecular structure parameters and morphology-properties of the four MSS-PB-PSBR were determined and studied, and compared with that of a star-shaped styrene-butadiene rubber (S-SSBR)/poly butadiene rubber (PBR) blend. The results showed that the total coupling efficiency (the ratio of the total number of polymer chains (arms) coupled by SnCl₄ to that of the total number of polymer chains) of the MSS-PB-PSBR was higher than 60%. However, the coupling efficiency of the polybutadiene arms of BMSS-PB-PSBR was obviously higher than that of the AMSS-PB-PSBR. Compared with the S-SSBR/PBR blend, MSS-PB-PSBR had a more uniform distribution of the PB phase and a smaller phase size of PB. It was found that MSS-PB-PSBR composites filled with carbon black (CB) had a lower Payne effect than the S-SSBR/PBR/CB composite, with the BMSS-PB-PSBR/CB composites being especially lower. The BMSS-PB-PSBR/CB composites had higher mechanical properties and lower rolling resistance than the AMSS-PB-PSBR/CB composites due to the high coupling efficiency of the polybutadiene arms; the results indicated that the two-step technology was better than the one-step technology for preparing the tread material of “green” tires.     

Atom Transfer Radical Polymerization of Styrene Using a Bifunctional Initiator

A bifunctional alkyl halide, namely l, 2-bis(2′-bromobutyryl) ethane (BBrBE), was synthesized and used to initiate the bulk atom transfer radical polymerization (ATRP) of styrene (St) at 110°C in the presence of CuBr/2,2′-bipyridyl. The narrow polydispersity of polystyrene (PSt) with precisely two arms could be synthesized. The initiate ability of the two active bromide functional groups at both sides of BBrBE for St and the propagation ability of the two arms were confirmed to be similar by the characterization of the individual arms obtained upon hydrolysis of the ester link between the core and the branches.     

Polymerization of Styrene by Neutral Ni (II) Acetylide Complex

Over the past few years, the utilization of late transition metal-based soluble complexes as styrene polymerization catalysts has received considerable attention1. Various systems have been explored. For example, cationic h3-allylnickel complexes alone2 or modified by P (III) ligands3 as well as a few other systems (e. g., cationic h3-benzylic nickel complexes4) are active homogeneous catalysts for the low molecular weight polymerization of styrene by simple cationic mechanism2,3,4. Neut…     

Photopolymerization reactions initiated by a visible light photoinitiating system: Cyanine dye/borate salt/1,3,5‐triazine

New three‐component photoinitiating systems consisting of a cyanine dye, borate salt, and a 1,3,5‐triazine derivative were investigated by measuring their photoinitiation activities and through fluorescence quenching experiments. Polymerization kinetic studies based on the microcalorimetric method revealed a significant increase in polymerization rate when the concentration of n‐butyltriphenylborate salt or the 1,3,5‐triazine derivative were increased. The photo‐induced electron transfer process between electron donor and electron acceptor was studied by means of fluorescence quenching and SrEt change of the fluorescence intensity. The experiments performed documented that an increase of the n‐butyltriphenylborate salt concentration dramatically increases the rate of dye fluorescence quenching, whereas the increasing of the 1,3,5‐triazine derivative concentration slows down the consumption of the dye. We conclude that the primary photochemical reaction involves an electron transfer from the n‐butyltriphenylborate anion to the excited singlet state of the dye, followed by the reaction of the 1,3,5‐triazine derivative with the resulting dye radical to regenerate the original dye. This reaction simultaneously produces a triazinyl radical anion derived from the 1,3,5‐triazine, which undergoes the carbon‐halogen bond cleavage yielding radicals active in initiation of a free radical polymerization chain. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 3626–3636, 2007