In cationic polymerization of hexamethylcyclotrisiloxane (D3) in methylene chloride, a control of the mol wts could be observed with various initiators such as RCOCl/ SbCl5 or trifluoromethanesulfonic acid (TfOH) and its derivatives. But an important difference with usual living polymerizations is the simultaneous formation of large amounts of cyclic oligomers D3x, their weights increasing linearly from the origin. A second population of much larger DPn has been shown to consist of macrocycles (MC). It was concluded that while MC result from end-to-end ring closure of a fraction of linear macromolecules, the small D3x cycles (essentially D6) are formed independently by a selective back-biting reaction involving oxonium end-groups. However, it has been proposed again in recent publications in which various initiators were used (including TfOH) that D3x cyclics of all sizes result from end-to-end ring closure reactions between the (electrophilic) active site for propagation and a silanol end group. In the present paper, the initiator used was the mixed anhydride of trifluoroacetic and trifluoromethanesulfonic acid, which give non-nucleophilic CF3CO2(CH3)2Si end-groups. The residual acid present in the anhydride was neutralized by varying amounts of 4-methyl,2,6-di-t-butylpyridine (MDTBP). The linear increase of the HP molecular weight and the formation of large amounts of D3x oligomers were observed again. The weight ratio of D6 was larger than for TfOH initiation (1 < D6/HP < 1,5) and when a large excess of MDTBP on the acid was used, D6/HP was even higher but MC formation was completely suppressed. This confirms the difference in the mechanisms giving MC and D6 and agrees with small D3x cyclics formation involving the silyltriflate end-groups alone (and probably the derived oxonium sites). 相似文献
Increasingly precise control of polymer architectures generated by “Living” Anionic Ring-Opening Polymerization (Living AROP) is leading to a broad range of commercial advanced material applications, particularly in the area of siloxane macromers. While academic reports on such materials remain sparse, a significant portion of the global population interacts with them on a daily basis—in applications including medical devices, microelectronics, food packaging, synthetic leather, release coatings, and pigment dispersions. The primary driver of this increased utilization of siloxane macromers is their ability to incorporate the properties of silicones into organic structures in a balanced manner. Compared to organic polymers, the differentiating properties of silicones—low Tg, hydrophobicity, low surface energy, and high free molal space—logically lend themselves to applications in which low modulus, release, permeability to oxygen and moisture, and tactile interaction are desired. However, their mechanical, structural and processing properties have until recently precluded practical applications. This review presents applications of “Living” AROP derived polymers from the perspective of historical technology development. Applications in which products are produced on a commercial scale—defined as not only offered for sale, but sold on a recurrent basis—are emphasized. Hybrid polymers with intriguing nanoscale morphology and potential applications in photoresist, microcontact printing, biomimetic soft materials, and liquid crystals are also discussed. Previously unreported work by the authors is provided in the context of this review. 相似文献
Atom transfer radical polymerization (ATRP) and ring opening polymerization (ROP) were combined to synthesize various polymers with various structures and composition. Poly(ε-caprolactone)-b-poly(n-octadecyl methacrylate), PCL-PODMA, was prepared using both sequential and simultaneous polymerization methods. Kinetic studies on the simultaneous process were performed to adjust the rate of both polymerizations. The influence of tin(II) 2-ethylhexanoate on ATRP was investigated, which led to development of new initiation methods for ATRP, i.e., activators (re)generated by electron transfer (AGET and ARGET). Additionally, block copolymers with two crystalizable blocks, poly(ε-caprolactone)-b-poly(n-butyl acrylate)-b-poly(n-octadecyl methacrylate), PCL-PBA-PODMA, block copolymers for potential surfactant applications poly(ε-caprolactone)-b-poly(n-octadecyl methacrylate-co-dimethylaminoethyl methacrylate), PCL-P(ODMA-co-DMAEMA), and a macromolecular brush, poly(hydroxyethyl methacrylate)-graft-poly(ε-caprolactone), PHEMA-graft-PCL, were prepared using combination of ATRP and ROP. 相似文献
The successful activation observed when using ButP4 phosphazene base and thiophenol or bisthiols for the anionic ring opening polymerization (ROP) of di‐n‐propyl cyclopropane‐1,1‐dicarboxylate is described. Well‐defined monofunctional or difunctional polymers with a very narrow molecular weight distribution were obtained through a living process. Quantitative end‐capping of the propagating malonate carbanion was accessible by using either an electrophilic reagent such as allyl bromide or a strong acid such as HCl. Kinetics studies demonstrated a much higher reactivity compared to the conventional route using alkali metal thiophenolates.
We investigated the anionic polymerization of butadiene in d-heptane solvent using tert-butyl lithium as initiator. Two complementary techniques were used to follow the polymerization processes: 1H NMR and small angle neutron scattering (SANS). The time resolved 1H NMR measurements allowed us to evaluate quantitatively the kinetics of the processes involved. The initiation event commences slowly and then progressively accelerates. This indicates an autocatalytic mechanism. The microstructure of the first monomer units attached is to a high extent 1,2. The disappearance of initiator --- at about 10% monomer conversion --- signals the onset of the normal ∼6% vinyl content of the chain. Small angle neutron scattering was used to study the aggregation behavior of the carbon lithium head groups. It is well known that the polar head groups aggregate and form micellar structures. For dienes in non-polar solvents the textbook mechanism assumes the formation of only tetramers during the propagation reaction. By combining 1H NMR and SANS results we were able to determine quantitatively the aggregation number during all stages of the polymerization. Our measurements show the existence of large-scale structures during the initiation period. The initial degree of aggregation of more than 100 living polymer chains diminished as the polymerization progressed. In addition, even larger, giant structures with Nagg >>1000 and Rg ≈ 1000Å were found. 相似文献
