Cationic ROP

By utilizing a combination of RAFT and cationic ROP, the synthesis of [poly(methyl methacrylate)][poly(l,3-dioxepane)][polystyrene] miktoarm star terpolymers was achieved [182], The approach involved the synthesis of PS functionalized with a dithiobenzoate group by RAFT polymerization and subsequent reaction with hydroxyethylene cinnamate (Scheme 98). The newly created hydroxyl group was then used for the cationic ring opening polymerization of 1,3-dioxepane (DOP). The remaining dithiobenzoate group was used for the RAFT polymerization of methyl methacrylate. 

A third example combines cationic ROP and ATRP for the synthesis of (polytetrahydrofurane)(poly-l,3-dioxepane)(PS) miktoarm stars (Scheme 99). The initiating sites for the above polymerization were created step-by-step from amino-succinic acid (Scheme 99). 

Fig.3A-D. Use of ring-opening polymerization (ROP) for neobiopolymer synthesis A General mechanism of cationic ROP. B Okada s use of cationic ROP to generate polymers with carbohydrate substituents at the terminus. C General mechanism for anionic ROP. D Okada s application of anionic ROP to generate carbohydrate-substituted polymers... 

The oxacyclobutane derivative, 3,3-bischloromethyloxacyclobutane, is polymerized (structure 5.26) using cationic ROP giving a water-insoluble, crystalline, corrosion-resistant polymer sold under the trade name of Penton. 

Cationic ROP of lactones has been known for a long time but is not very popular due to its poor control of the molecular parameters. In 1984, Penczek and coworkers reported the cationic polymerization of sCL and p-propionolactone... 

Until the middle of the 1980s, it was accepted that the mechanism for the cationic ROP initiated by alkylating agents was based on the reaction of the cation with the endocyclic oxygen, followed by cleavage of the acyl-oxygen bond (Fig. 16). 

Under certain conditions, irreversible chain-breaking reactions are absent and cationic ROPs of cyclic ethers proceed as living polymerizations. These conditions are found for polymerizations initiated with acylium and l,3-dioxolan-2-ylium salts containing very stable counterions such as AsFg, PFg, and SbClg or with very strong acids (fluorosulfonic and... 

Cationic ROP of ethylene oxide is not useful for the synthesis of linear polymer, but is used to produce crown ethers. Propylene oxide gives less cyclic dimer than does ethylene oxide for steric reasons cyclic tetramer predominates. 

Activated monomer polymerization competes with conventional cationic ROP. Initiation in conventional ROP involves the reaction of protonated monomer with unprotonated monomer (Eq. 7-31a), whereas initiation in AM ROP involves the reaction of protonated monomer with alcohol (Eq. 7-3 lb). 

Cationic ROP of lactones in the presence of an alcohol proceeds by an activated monomer mechanism similar to that for cyclic ethers (Sec. 7-2b-3-b) [Endo et al., 2002 Lou et al., 2002]. Propagation proceeds by nucleophilic attack of the hydroxyl end group of a propagating chain on protonated (activated) monomer ... 

Spiroorthocarbonates and spiroorthoesters are stable toward bases and nucleophiles but easily undergo cationic ROP. Cationic ROP of a spiroorthocarbonate (LXX) proceeds by initial formation of carbocation LXXI in which the carbocation center is stabilized by... 

Among the cyclic esters, 4-, 6-, and 7-membered rings form polyesters when reacted with cationic catalysts [18,23-25]. The cationic ROP involves the formation of a positively charged species which is subsequently attacked by a monomer (Scheme 2). The attack results in a ring-opening of the positively charged species through an Sn2 -type process. 

The cationic polymerization is difficult to control and often only low-molecular weight polymers are formed. When the bulk and solution polymerization of l,5-dioxepan-2-one (DXO) with cationic initiators were studied, the highest molecular weight achieved was about 10,000 [23]. More detailed reviews on cationic ROP have been published by Penczek and coworkers [26,27]. 

The structure of the active propagation center in the cationic ring-opening polymerization (ROP) of cyclosiloxanes is still controversial. Trisilyloxonium ions generated from hexamethyl-cyclotrisiloxane, D3, and octamethylcyclotetrasiloxane, D4, were observed at low temperature by Olah et al. [I] and their participation as intermediates in cationic ROP of cyclosiloxanes was postulated. However, the main objection against this mechanistic concept is a relatively low rate of polymerization when an initiator able to form persistent tertiary oxonium ions is used [2]. 

Studies of the polymerization of a monomer able to form more stable tertiary trisilyloxonium ions could throw more light on this problem. Octamethyl-l,4-dioxatetrasilacyclohexane ( 02) may be such a model monomer. This monomer easily undergoes the cationic ROP initiated by a strong... 

Scheme 36. Sensitized cationic ROP of cyclohexene oxide followed by the TEMPO-mediated CRPofSt [281]...

Carbathioferrocenophane, 47, underwent cationic ROP with methyl triflate and boron trifluoride etherate, but was resistant to both anionic and transition-metal-catalyzed ROP.100101 However, cationic ROP initiated with methyl triflate caused oxidation of the iron centers in the polymers. Me+ and H+ were determined to initiate cationic ROP for the methyl triflate and boron trifluoride, respectively.101 Scheme 2.12 shows a proposed mechanism of the methyl triflate, 48, initiated ROP of [2]carbathioferrocenophane, 47. 

This chapter will first discuss the living carbocationic polymerization of the three most important monomer classes isobutene, vinyl ethers, and styrenics. The second part of the chapter will focus on living cationic ROP of cyclic ethers, cyclic imines, and cyclic imino ethers. For more detailed discussions on carbocationic polymerizations [8-14] and cationic ROPs [15-18] in general, the readers are referred to previous literature [19]. 

In contrast to these initial reports on the living CROP of tetrahydrofuran which were performed without additional solvents, Penczek and coworkers demonstrated that the solvent plays an important role in the cationic ROP of tetrahydrofuran since it controls the proximity and stability of the ion pair at the living chain end [100, 101]. The polymerization rate increases in more polar solvents because of stabilization of the ion pair, whereby it was demonstrated that the methyl-triflate-initiated CROP of tetrahydrofuran involves an equilibrium between the cationic propagating oxonium species and the covalent triflic acid adduct, which can be shifted by the solvent choice as depicted in Scheme 8.19. Nonetheless, as a result of the much higher reactivity of the cationic propagating species, the polymerization rate is almost exclusively determined by the concentration of oxonium ions. 

In contrast to the previously discussed cationic ROPs, the nucleophilicity of the cyclic imino ether monomers is much higher compared to the resulting poly(cyclic imino ether)s. This decrease in nucleophilicity is due to the isomerization of the imino ether moiety into an amide during the CROP as depicted in Scheme 8.25. As a result, the CROP of a wide range of cyclic imino ethers can be performed in a living manner since chain transfer to polymer side reactions are less likely to happen. Moreover, the R-group attached to the 2-position of the monomer determines the polyamide side chains and strongly influences the polymer properties. 

In contrast to anionic ROP, cationic ring-opening polymerisations are far less utilised due to the poor control over the reaction parameters. Cationic ROP was first reported by Penczek et al. for the polymerisation of -caprolactone and /3-propionolactone [52]. Afterwards, various initiating systems have already been reported on which can be divided in four classes including alkylating... 

In case of Lewis acid initiation, a cationic ROP will only occur if the counterion is not too nucleophilic [54]. When nucleophilic counterions are present, the metal-insertion mechanism will be followed instead, as already reported for several catalytic systems including ZnCl2 [55], TiCl4 and AICI3 [56]. 

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