Activated monomer cyclic ether

As far as the polymerisation of heterocyclic monomers is concerned, the situation is qualitatively similar, but quantitatively different. As a model for the active species in oxonium polymerisations, Jones and Plesch [10] took Et30+PF6 and found its K in methylene dichloride at 0 °C to be 8.3 x 10"6 M however, in the presence of an excess of diethyl ether it was approximately doubled, to about 1.7 x 10 5 M. This effect was shown to be due to solvation of the cation by the ether. Therefore, in a polymerising solution of a cyclic ether or formal in methylene dichloride or similar solvents, in which the oxonium ion is solvated by monomer, the ion-pair dissociation equilibrium takes the form... 

Considerable effort has been carried out by different groups in the preparation of amphiphihc block copolymers based on polyfethylene oxide) PEO and an ahphatic polyester. A common approach relies upon the use of preformed co- hydroxy PEO as macroinitiator precursors [51, 70]. Actually, the anionic ROP of ethylene oxide is readily initiated by alcohol molecules activated by potassium hydroxide in catalytic amounts. The equimolar reaction of the PEO hydroxy end group (s) with triethyl aluminum yields a macroinitiator that, according to the coordination-insertion mechanism previously discussed (see Sect. 2.1), is highly active in the eCL and LA polymerization. This strategy allows one to prepare di- or triblock copolymers depending on the functionality of the PEO macroinitiator (Scheme 13a,b). Diblock copolymers have also been successfully prepared by sequential addition of the cyclic ether (EO) and lactone monomers using tetraphenylporphynato aluminum alkoxides or chloride as the initiator [69]. 

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 ... 

For copolymerizations proceeding by the activated monomer mechanism (e.g., cyclic ethers, lactams, /V-carboxy-a-amino acid anhydrides), the actual monomers are the activated monomers. The concentrations of the two activated monomers (e.g., the lactam anions in anionic lactam copolymerization) may be different from the comonomer feed. Calculations of monomer reactivity ratios using the feed composition will then be incorrect. 

It is generally agreed that propagation in the cationic polymerization of cyclic ethers occurs after nucleophilic attack by the monomer oxygen atom (equation 3). Therefore, many authors attempt to explain their copolymerization data by noting that the more basic monomer has the higher reactivity with the active chain end. The order of basicity which has been established (36, 38) is ... 

Tetrahydrofuran is often used as a polymerization model for multimembered cyclic ethers. Its propagation proceeds by nucleophilic attack of the monomer oxygen on the a carbon of the active centre with a polarized bond [312] ... 

The other limitation stems from very different structure of heterocyclic monomers and thus very different reactivity of resulting active species. As already discussed, oxonium ions may initiate the polymerization of cyclic amines, but ammonium ions would not initiate the polymerization of cyclic ethers. Thus, the sequential polymerization is possible only when the first monomer is not a stronger nucleophile than the second monomer. 

In the random copolymerization process, both types of active species should be able to participate in the cross-propagation reactions. This imposes certain limitations on the choice of comonomers in the cationic polymerization of heterocyclic monomers. Onium ions, being the active species of these polymerizations, differ considerably in reactivity thus, as already discussed, oxonium ions initiate the polymerization of cyclic amines, whereas ammonium ions do not initiate the polymerization of cyclic ethers and the corresponding cross-propagation reaction would not proceed ... 

In115 116) the cationic polymerization of cyclic ethers was examined theoretically and experimentally with regard to the nature of MWD variation. A theoretical analysis was made of how MWD is affected by the depolymerization reactions, monomole-cular deactivation of active centers, recombination of active centers, chain transfer by hydroxyl-containing compounds, chain transfer to the monomer, and ether oxygen of the polymer chain, as well as via the end hydroxyl group. 

A very interesting variant of cationic polymerisation of CPL is based on the polymerisation initiated by hydroxyl compounds, at room temperature [42,43,44]. The mechanism called hydroxo-mechanism is very similar to the activated monomer mechanism developed for cyclic ethers. This kind of polymerisation is practically a living cationic polymerisation and in the particular case of CPL, using various polyols as starters, it is possible to obtain hydroxy-telechelic poly CPL) polyols, with various MW, depending on the molar ratio of CPL per polyol (reactions 8.28). 

P. Kubisa, Activated monomer mechanism in the cationic polymerization of cyclic ethers. Makromol. Chem. Macromol. Symp. 1988, 13(4), 203-210. 

The relative contribution of the specific chain growth mechanism (i.e., activated monomer vs. oxonium ion addition) may depend on ring strain of monomer, nucleo-philicity of anion and solvating power of solvent (ability to stablize ions). Many of these factors have been quantiatively determined in the polymerization of cyclic ethers and acetals, where the concentrations of the tertiary and secondary oxonium ions were simultaneously determined by the phosphine cation-trapping method (cf. Adv. Polymer Sci. 37). This method seems to be also applicable in the polymerization of siloxanes, but has not yet been evaluated. 

The attack on the endocyclic carbon leads to crosspropagation whereas attack on the exocyclic group is merely an exchange of one active species for another with expulsion of a monomer molecule. In the copolymerization of cyclic ethers, like THF and OXP, this reaction has been quantitatively studied. For the tetrahydro-furanium cation kexo/kendo = 2 10-2, whereas for the oxepanium cation kexo/kendo = 0.5 and, therefore, the exo-attack cannot be neglected 8). 

The choice of monomers is limited not only by the order of nucleophilicity. The first block should carry a living end capable to initiate the polymerization of the second monomer by addition. Up to now, only a few systems are known to meet these requirements. Thus, in the presence of stable counterions, living polymerization of DXL 109), DXP 109), XHF 110,111,112), N-t-butylaziridine 113), 1,3,3-trimethylazeti-dine 1W), conidine 115) and l-(2-phenylethyl)-2-methylaziridine 116) may proceed. The last four monomers are highly nucleophilic cyclic amines and there is not too much chance that the active species derived from this class of monomers would initiate polymerization of other, less nucleophilic monomers. Thus, attempts directed toward the preparation of block copolymers of cyclic ethers or acetals were employed... 

The cationic pohmierizations of cyclic acetals are different from the polymerizations of the rest of the cyclic ethers. The differences arise from greater nucleophilicity of the cyclic ethers as compared to that of the acetals. In addition, cyclic ether monomers, epirane, tetrahydrofuran, and oxepane, are stronger bases than their corresponding polymers. The opposite is true of the acetals. As a result, in acetal polymerizations, active species like those of 1,3-dioxolane may exist in equilibrium with macroalkoxy carbon cations and tertiary oxonium ions. By comparison, the active propagating species in polymerizations of cyclic ethers, like tetrahydrofuran, are only terdaiy oxonium ions. The properties of the equilibrium of the active species in acetal polymerizations depend very much upon polymerization conditions and upon the structures of the individual monomers. 

Cationic and anionic polymerizations of heterocyclic monomers provide many examples in which the concurrent formation of cyclics of various sizes is observed during the ring-opening polymerization. As illustrated in Scheme 1, in these systems active species follow three pathways they can react with a functional group of the monomer, of its own polymer chain, or of other chains. When the function / involved belongs to a linear polymer chain, intramolecular chain saambling or intermolecular macrocycle formation takes place, as observed in the cationic polymerization of cyclic ethers, acetals, esters, amides, siloxanes, and so forth. 

Penczek and Kubisa developed a new cationic polymerization technique for cyclic monomers in which the chain propagation involves the reaction of a protonated (activated) monomer molecule with a nucleophilic site in the neutral growing macromolecule. This so-called activated monomer (AM) polymerization is depicted mechanistically in Scheme 58. According to this mechanism, when the polymerization of an oxirane (a cyclic ether) is carried out in the presence of... 

Alternatively, polymerization of 3,3-bis(chloromethyl)oxacyclobutane may be effected by aluminium compounds such as alkoxides, amalgam and hydride at elevated temperatures (150—200°C). The mode of operation of these initiators is unknown they are usually associated with anionic reactions whereas the polymerization of cyclic ethers (other than epoxides) generally involves homogeneous cationic mechanisms. It may be that at high temperatures either the monomer is activated and anionic polymerization can occur or there is reaction between the initiator and monomer to form cationic species. 

The synthesis of block copolymers of controlled structures is most conventionally accomplished through the use of living anionic polymerization. One can easily imagine, however, desirable block copolymers derived from monomers which are inert to anionic polymerization conditions, or which do not share any common mode of polymerization. In a recent series of papers (24-34), Richards and coworkers have addressed this problem in a general way, and have developed methods which convert one kind of active center into another. Within the context of cyclic ether polymerizations, Richards has focused on the preparation of block copolymers of styrene and tetrahydrofuran (THF) several methods of accomplishing this copolymerization are described in the following paragraphs. 

In addition to ethylene and propylene oxide, a variety of other cyclic ethers have also been copolymerized with MA. Monomers such as cyclohexene oxide,piperylene dimer mono and diepoxide, epichlorohydrin, " " 3,3,3-trichloropropylene oxide, " tetrahydrofuran, " " and ethylene carbonate or ethylene sulfite " have received attention. Condensation reactions between allyl glycidyl ether and MA are reported to be highly useful for preparing plastics with remarkable hardness, high heat distortion, and brilliant clarity.The cyclohexene oxide copolymerizations were second order in MA, with an activation energy of 13.8 kcal/mol. For the epichlorohydrin system the rate was dependent on the temperature and proportional to the catalyst concentration, with an activation energy of 14.5 kcal/mol. 

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