Anionic-cationic polymerization transformation

My faculty colleagues of the Institute also bring great expertise in the areas of anionic, cationic, and radical polymerization to the transformation of low-molecular-weight hydrocarbons into macromole-... 

Quite often in the ring-opening polymerization, the polymer is only the kinetic product and later is transformed to thermodynamically stable cycles. The cationic polymerization of ethylene oxide leads to a mixture of poly(ethylene oxide) and 1,4-dioxane. In the presence of a cationic initiator poly(ethylene oxide) can be almost quantitatively transformed to this cyclic dimer. On the other hand, anionic polymerization is not accompanied by cyclization due to the lower affinity of the alkoxide anion towards linear ethers only strained (and more electrophilic) monomers can react with the anion. 

Most of the methods for synthesizing block copolymers were described previously. Block copolymers are obtained by step copolymerization of polymers with functional end groups capable of reacting with each other (Sec. 2-13c-2). Sequential polymerization methods by living radical, anionic, cationic, and group transfer propagation were described in Secs. 3-15b-4, 5-4a, and 7-12e. The use of telechelic polymers, coupling and transformations reactions were described in Secs. 5-4b, 5-4c, and 5-4d. A few methods not previously described are considered here. 

B-90 and B-91, respectively.390 Another route coupled with cationic ring-opening polymerizations is accomplished for polymer B-92 with the use of a hydroxyl-functionalized initiator with a C—Br terminal, where the OH group initiates the cationic polymerizations of 1,3-dioxepane in the presence of triflic acid.329 Polyethylene oxide)-based block copolymers B-93 are obtained by living anionic polymerization of ethylene oxide and the subsequent transformation of the hydroxyl terminal into a reactive C—Br terminal with 2-bromopropionyl bromide, followed by the copper-catalyzed radical polymerization of styrene.391... 

There have been numerous literature reports on the preparation of block copolymers using CRP methods. These copolymers range from those synthesized wholly by CRP to those that involve either transformation from other living polymerization techniques (anionic, cationic, ring-opening, etc.) to CRP, or functionalization of a macromolecule that can then be used as a macroinitiator for CRP. Each of these methods will be addressed separately. Nitroxides were predominantly used for styrene containing copolymers, whereas ATRP was successful for the acrylates and methacrylates as well [42]. 

Many polymerization techniques have been combined with CRP through site transformation of the active species. These include non-living techniques like condensation (or step) and conventional free radical processes or living methods like anionic, cationic, and ring-opening polymerizations, as well as others. Early examples were undertaken perhaps just to show that two different techniques could be combined, while later examples show how elegant the combinations have become and provide a foundation for controlled synthesis of materials from any type of monomers. These types of reactions are detailed below. 

Transformation of Anionic Polymerization into Cationic Polymerization. Richards et al. (26. 27, 73-75) proposed several methods for the transformation of a living anionic polymeric chain end into a cationic one. Such a process requires three distinct stages polymerization of a monomer I by an anionic mechanism, and capping of the propagating end with a suitable but potentially reactive functional group isolation of polymer I, dissolution in a solvent suitable for mechanism (2), and addition of monomer II and reaction, or change of conditions, to transform the functionalized end into propagating species II that will polymerize monomer II by a cationic mechanism (73). 

In the anion-cation transformation reaction, the anionically generated living polymer chain is end-capped with a hahde, producing a chain that can be isolated for subsequent reaction. This can be used to initiate a cationic polymerization of a suitable monomer by activating the end with a silver or hthium salt according to the general scheme shown in Equation 5.20a to Equation 5.20c. 

Scheme 11.25 Synthesis of block copolymers by transformation of living anionic polymerization into living cationic polymerization.

An alternative method was also described for transforming an anionic polymerization process into a cationic polymerization process, assisted by organosilyl groups. Reaction of the p-tolyldimethylsilyl end group of PSt and... 

The cationic polymerization of cychc amines is well known [98-100]. Low-molecular-weight initiators such as ethyltosylate induce the polymerization of cyclic amines, such as 1-fert-butylaziridine. The concept of using a macroinitiator bearing a tosylate end group to polymerize cyclic amines prompted Kazama etal. [101] to attempt the polymerization of 1-tert-butyl aziridine, using PDMS with a terminal tosylate group. The fact that no polymerization occurred when the macroinitiator was used provided a clear demonstration of the initiative behind studying the transformation reaction between anionic and cationic polymerizations. 

A dual initiator was employed by Cramail etal to synthesize block copolymer of St and chloroethyl vinyl ether (CEVE) via anionic-to-cationic transformation. The anionic living polymerization of St was initiated by lithiopropionaldehyde diethyl acetal in the presence of tetramethylene diamine. The resulted polymer with acetal functionality was converted into the corresponding a-iodo-ether with trimethylsilyl iodide (TMSI),... 

The free-radical addition of TFE to pentafluoroethyl iodide yields a mixture of perfluoroalkyl iodides with even-numbered fluorinated carbon chains. This is the process used to commercially manufacture the initial raw material for the fluorotelomer -based family of fluorinated substances (Fig. 3) [2, 17]. Telomeri-zation may also be used to make terminal iso- or methyl branched and/or odd number fluorinated carbon perfluoroalkyl iodides as well [2]. The process of TFE telomerization can be manipulated by controlling the process variables, reactant ratios, catalysts, etc. to obtain the desired mixture of perfluoroalkyl iodides, which can be further purified by distillation. While perfluoroalkyl iodides can be directly hydrolyzed to perfluoroalkyl carboxylate salts [29, 30], the addition of ethylene gives a more versatile synthesis intermediate, fluorotelomer iodides. These primary alkyl iodides can be transformed to alcohols, sulfonyl chlorides, olefins, thiols, (meth)acrylates, and from these into many types of fluorinated surfactants [3] (Fig. 3). The fluorotelomer-based fluorinated surfactants range includes noiuonics, anionics, cationics, amphoterics, and polymeric amphophiles. 

The earlier studies in this area, mostly those of Selb and Gallot [190], were devoted to vinylpyridine (2VP and 4VP) containing block copolymers, readily accessible by anionic sequential polymerization and which could be transformed into water-soluble cationic species by quaternization or by simple protonation at low pH. The more recent development, pioneered by Armes and co-workers [57, 93, 191], concerns the micellization of amino-methacrylate-based block copolymers. 

The combination of living cationic polymerization and NCA polymerization has also been reported. Schlaad synthesized poly(2-isopropyl-2-oxazoline)-h-PBLG (16) based on an ammonium-mediated polymerization system. By adding an acid in the NCA polymerization, a primary amino (NH2)-propagating chain end could be transformed into an ammonium (NH3 ) chain end via equilibrium and this system suppresses the generation of the aminyl anion of an NCA monomer that causes byproducts via activated monomer mechanism (Scheme 13.2) (Dimitrov and Schlaad, 2003). 

The synthesis of poly(IB-b-pivalolactone (PVE)) diblock copolymers was also recently accomplished by site-transformation ofiiving cationic polymerization of IB to anionic ring-opening polymerization (AROP) of PVE, as shown in Scheme First, PIB with co-carboxylate potassium salt... 

Transformation of the cationic ring-opening polymerization of THF into ATRP of St, acrylates and methacrylates has been used to produce various block copolymers [104,105]. The ABA-type block copolymers CLB-7 to CLB-9 were prepared via termination of telechelic living PTHF with sodium 2-bro-moisopropionate, followed by ATRP of St, or MA or MMA [104]. The transformation reaction was performed by adding sodium 2-bromoisopropionate into the cationic polymerization system of THF. Due to the existence of tertiary oxonium ions, the attack of 2-bromoisopropionate anion on the a-carbon of the oxonium ion produces 2-bromoisopropionate-terminated PTHF as shown in Scheme 3.26 [104,105],... 

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