Epoxides polymerization kinetics

In order to optimize the structure and properties of composites, a knowledge of the polymerization kinetics and mechanism are required. Various approaches have been taken to a determination of the kinetics of the polymerization including infrared spectroscopy 30,31). Although the various epoxide/anhydride/amine systems are... 

Alkali metal naphthalene complexes have also been used to initiate epoxide polymerizations. Solov yanov and Kazanski [25] studied the polymerization of EO in tetrahydrofuran using sodium, potassium or cesium naphthalene as initiator. A living polymer was produced there is no chain rupture or transfer. The rate of polymerization depends on the concentration of active centres in a complex manner. The kinetic order varies from 0.23 for Na" (or 0.33 for K and Cs" ) up to full first order as initiator concentration decreases. The polymerization is first order in monomer, but deviations are observed at high concentrations. 

Wang and Pinnavaia [28] determined several kinetic and thermodynamic parameters for their clay -polyether nanocomposite systems. Heats of reaction for [H3N(CH2)11COOH]+-MMT and [H3N(CH2)5COOH]+-MMT as a function of clay concentration decreased linearly with increasing clay concentration, implying that the heat of reaction was primarily due to epoxide polymerization. 

Kinetics Most epoxide polymerizations have the characteristics of living polymerization, that is, initiation is fast relative to propagation and there is an absence of termination processes. The expressions for the rate and degree of polymerization used in living chain polymerizations (see Chapter 8) can thus be applied for epoxide polymerizations. The polymerization rate is given by... 

Since the network density is changed by the reaction between epoxide and alcohol or water, the mechanical properties of the resulting polymer are also influenced. This can be used, for example, in the flexibiHzation of dental materials with poly(l,4-butanediol) [11] or coatings with polyester polyols [12]. It is not only alcohols that influence the polymerization kinetics and the properties of the polymer, but also carboxylic acids. By the addition of a polymer with carboxylic acid groups instead of the polyol, a polyester is formed as a reaction product and not a polyether. This was examined in detail by Wu and Soucek [13]. 

Tsuruta found that the optically pure complex [(/ -salcy)Co] (15) was active for epoxide polymerization (Scheme 24.15) when activated with ALEt3. Although the system exhibited no enantioselectivity for the polymerization of propylene oxide, it was moderately selective (r = 1,5) for the kinetic resolutions of tert-butyl ethylene oxide and epichlorohydrin (Scheme 24.15). 

Epoxide reactivity and polymerization kinetics are also influenced by electronic and steric factors assodated with the... 

This means that the process is kinetically controlled and the growing centers are not blocked by the formation of partially crystalline polymer. A very important finding is that the effective activation energy is 7-8 kcal mol", which is much lower than the typical values 18-20 kcal mol" for epoxide polymerization processes. The low activation energy is apparently the main factor responsible for the efficiency of the Ca amide-modified catalysts. 

The corresponding reactions of transient Co(OEP)H with alkyl halides and epoxides in DMF has been proposed to proceed by an ionic rather than a radical mechanism, with loss of from Co(OEP)H to give [Co(TAP), and products arising from nucleophilic attack on the substrates. " " Overall, a general kinetic model for the reaction of cobalt porphyrins with alkenes under free radical conditions has been developed." Cobalt porphyrin hydride complexes are also important as intermediates in the cobalt porphyrin-catalyzed chain transfer polymerization of alkenes (see below). 

This is not the first time that the kinetics of bulk polymerizations has been analysed critically. Szwarc (1978) has made the same objection to the identification of the rate constant for the chemically initiated bulk polymerization of tetrahydrofuran as a second-order rate constant, k, and he related the correct, unimolecular, rate constant to the reported by an equation identical to (3.2). Strangely, this fundamental revaluation of kinetic data was dismissed in three lines in a major review (Penczek et al. 1980). Evidently, it is likely to be relevant to all rate constants for cationic bulk polymerizations, e.g., those of trioxan, lactams, epoxides, etc. Because of its general importance I will refer to this insight as Szwarc s correction and to (3.2) as Szwarc s equation . 

A very successful example for the use of dendritic polymeric supports in asymmetric synthesis was recently described by Breinbauer and Jacobsen [76]. PA-MAM-dendrimers with [Co(salen)]complexes were used for the hydrolytic kinetic resolution (HKR) of terminal epoxides. For such asymmetric ring opening reactions catalyzed by [Co(salen)]complexes, the proposed mechanism involves cooperative, bimetallic catalysis. For the study of this hypothesis, PAMAM dendrimers of different generation [G1-G3] were derivatized with a covalent salen Hgand through an amide bond (Fig. 7.22). The separation was achieved by precipitation and SEC. The catalytically active [Co "(salen)]dendrimer was subsequently obtained by quantitative oxidation with elemental iodine (Fig. 7.22). 

Kim et al. [67], used the self-polymerized heterometallic polymeric salen complexes 26-32 as efficient catalysts for kinetic resolution of terminal epoxides with phenols to give a-aryloxy alcohols in high yields (38-43%) and ee (92-99%) (Scheme 17). These catalysts were recycled up to three times without any loss in their performance. 

Kureshy, R. I. Singh, S. Khan, N. H. Abdi, S. H. R . Agrawal S. Jasra R. V. (2006) Enantioselective aminolytic kinetic resolution (AKR) of epoxides catalyzed by recyclable polymeric Cr(III) salen complexes Tetrahedron Asymmetry 17 1638-1643. 

Enantiomer-differentiating co-polymerization of terminal epoxides is achieved by chiral chromium and cobalt complexes. Jacobsen etal. reported the co-polymerization of 1-hexene oxide with GO2 by using complex 35a. The reaction proceeds with kinetic resolution at 90% conversion, the unreacted epoxide is found to be enriched in the (i )-enantiomer of 90% ee. Detailed information about the resultant polymer, however, is not described. As discussed in the previous section, chiral cobalt-salen complex 34c co-polymerizes PO and GO2 (Table 3). When 34c with /r<3 / j--(li ,2i )-diaminocyclohexane backbone is applied to the co-polymerization, (A)-PO is consumed preferentially over (i )-enantiomer with a of 2.8 to give optically active PPG (Equation (8)). In a similar manner, a binary catalyst system, 34d/Bu4NGl, preferentially consumes (A)-PO over R)-PO with = 2.8-3.5. ... 

Thus kp for lithium counterion is 1/300 of kp for potassium counterion. The low reactivity and association of lithium alkoxide was reported in the anionic polymerization of epoxides.We have found that two fold increase of the lithium initiator concentration has led to a decrease of the kp nearly to one half. This indicates that the kinetic order with respect to the initiator would be near to zero, suggesting a very high degree of association of the active species. Thus the propagation reaction appears to proceed in practice through a very minor fraction of monomeric active species in case of lithium catalyst. 

The solution thus consists of different particles denoted as contact ion pairs, solvent-separated ion pairs and free ions. The fraction of the individual particles depends on the type of salt, type of solvent, polymerization system, temperature, and salt concentration. The catalytic effect of these particles may be very different as is evident in anionic polymerization of vinyl monomers. For instance, free polystyryl anion is 800times more reactive than its ion pair with the sodium counterion 60 . From this fact it follows that, although the portion of free ions is small in the reaction system, they may play an important role. On the other hand, anionic polymerization and copolymerization of heterocycles proceeds mostly via ion pairs. This is due to a strong localization of the negative charge on the chain-end heteroatom which strongly stabilizes the ion pair itself62. Ionic dissociation constants and ion contributions to the reaction kinetics are usually low. This means that for heterocycles the difference between the catalytic effect of ion pairs and free ions is much weaker than for the polymerization of unsaturated compounds. This is well documented by the copolymerization of anhydrides with epoxides where the substi-... 

A number of studies of the kinetics and mechanism of the base catalysed reaction of epoxides with phenolic alcohols have served as background for the polymerization studies. These studies [14] showed that both the alcohol and the alkoxide participate in the rate determining step and subsequently a termolecular mechanism was proposed. 

In a companion paper Price and Akkapeddi [22] report the kinetics of base initiated polymerization of epoxides in DMSO and hexamethyl-phosphoramide (HMPT). The initiator is potassium t-butoxide. Second order rate coefficients for (R,S)—PO were about double those for (+)—(R) or (—)—(S) monomer. They conclude that the steric factor favouring alternation of isotactic and syndiotactic placement of the t-BuEO also influences PO. Chain transfer to solvent (DMSO) was also studied. For PO polymerization in DMSO they obtain k = 1.5 x 10 exp(—17,200/RT). However, due to some erratic results they are not very confident about the accuracy. In HMPT rates are about three fold faster than in DMSO k = 7.3 X 10 exp(—16,300/RT). Three other epoxides were also studied in HMPT EO, k = 2.75 x 10 exp(-13,300/RT) t-BuEO, fe = 2.0 x 10 exp(-17,100/RT) phenylglycidyl ether (PGE), fc = 5.4 x 10" ... 

The coordinate type catalysts are also effective for thiirane polymerizations. The types of systems used are also similar. Thus diethylzinc and in particular diethylzinc/water mixtures have been studied [44]. Other studies made using triethylaluminium and diethylcadmium indicated that these metal alkyls all behave similarly. The reactions seem to be rather complex, and, as also was the case with the epoxides, no well defined kinetic studies have appeared. The polymers produced are of high molecular weight and are often crystalline. Thus stereospecific polysulphides have been reported. Again the bulk of the studies involve PS. Stereoselective polymerization of racemic monomer has been accomplished [45, 46] using a catalyst prepared from diethylzinc and (+) borneol. The marked difference between PO and PS in their polymer-... 

A large number of substituted 1,3-epoxides as well as the parent oxetane (oxacyclobutane, trimethylene oxide, 1,3-epoxypropane) have been prepared and polymerized. Likewise a wide variety of substances have been used to initiate their polymerizations. Much of this work has been extensively reviewed previously [1, 3, 7] and the interested reader is referred to these earlier reviews. Here we confine ourselves to reporting representative major kinetic studies. In this section the organization is by monomer. 

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