Styrene oxide polymerization formation

Oligomer formation by a mechanism different from the polymerization mechanism has been proposed by Kern (7) for the tetramer formation from a number of epoxides and by Pasika (10) for styrene oxide dimer formation, although in the last case the initiation reactions are probably the same. However, in both cases degradation of polymer would be a possible alternative mechanism. 

The styrene oxide polymerization by the ZnEt2/H20 (1/0.8) catalyst was found to result in the formation of two polymer fractions, a partly crystalline fraction, I, and an amorphous fraction, II. The syntheses of poly(styrene oxide) starting from the R monomer showed both polymers were formed by almost exclusively... 

These results provide yet another example of an anionic ring-opening polymerization, in addition to that of styrene oxide polymerization, which can occur with formation of different active centers under the same reaction conditions. These results are in contradiction to those known until now, and to the general opinion that the anionic polymerization of 6-lactones proceeds by either the opening of alkyl-oxygen bond or an acyl-oxygen bond. 

Lu and coworkers have synthesized a related bifunctional cobalt(lll) salen catalyst similar to that seen in Fig. 11 that contains an attached quaternary ammonium salt (Fig. 13) [36]. This catalyst was found to be very effective at copolymerizing propylene oxide and CO2. For example, in a reaction carried out at 90°C and 2.5 MPa pressure, a high molecular weight poly(propylene carbonate) = 59,000 and PDI = 1.22) was obtained with only 6% propylene carbonate byproduct. For a polymerization process performed under these reaction conditions for 0.5 h, a TOF (turnover frequency) of 5,160 h was reported. For comparative purposes, the best TOF observed for a binary catalyst system of (salen)CoX (where X is 2,4-dinitrophenolate) onium salt or base for the copolymerization of propylene oxide and CO2 at 25°C was 400-500 h for a process performed at 1.5 MPa pressure [21, 37]. On the other hand, employing catalysts of the type shown in Fig. 12, TOFs as high as 13,000 h with >99% selectivity for copolymers withMn 170,000 were obtained at 75°C and 2.0 MPa pressure [35]. The cobalt catalyst in Fig. 13 has also been shown to be effective for selective copolymer formation from styrene oxide and carbon dioxide [38]. 

According to these experimental results, the proposed reaction mechanism for the formation of poly(styrene oxide) with a regular chain structure by anionic polymerization involves the oxirane ring opening exclusively at the 3 position. However, two kinds of active centers, A and B in the reactions above, occur in the initiation step. The active center A, formed by a-ring opening, adds to a monomer molecule in the next step, but in the second step the oxirane ring is opened at the 3 position. 

From the Ah-NMR analysis of poly(3,8-d2-styrene oxide) obtained using ZnEt2/H20 as initiator Figure 2), the formation of the partly crystalline fraction can be described by first order Markov statistics, while that for the amorphous fraction follows Bemoulllan statistics. Different chain propagation mechanisms are, therefore, responsible for the formation of the two different polymer fractions obtained from this particular catalyst. Consequently, the existence of two different active centers, responsible for the two polymerization mechanisms and for formation of fractions I and II, are clearly indicated. 

The formation of cyclic dimer (2-10 % yield) in the cationic polymerization of styrene oxide was reported by different authors 25 27). More recently, Yamashita studied the catonic oligomerization of styrene oxide in the presence of various Lewis and protic acids as initiators (SnCl4, BF3 0(C2H5)2, HOS02CF,) and found a 100 o conversion to cyclic oligomers with n = 2-5 28). 

This chapter describes our recent advances on the utilization of polymer-modified laccase complexes in aqueous systems towards the oxidation/polymerization of naturally hydro-phobic steroidal compounds, Equilin (EQ) and 17-P-estradiol (P-EST). We elucidate the kinetic and synthetic aspects of the process with the model compoimd 5,6,7,8-tetrahydro-2-naphthol (THN). The nano-reactor system is composed of linear poly(ethylene oxide)-dendritic poly(benzyl ether) diblock copolymer (G3-PE013k) and laccase isolated from Trametes versicolor. Other advantages of the complex in comparison to the native enzyme are its recyclability, enhanced stability, activity, and overall simplicity in product harvesting and isolation. A principle of action of the complex is suggested based on these findings and is further supported by the biphasic solid-liquid nature of the reaction medium, which exhibits continuous influx of starting material and steady solid product expulsion. Comparative experiments with linear-linear poly(styrene)-Woc -poly(ethylene oxide) copolymer under identical conditions do not evince formation of a... 

Watanebe et al. [163] noticed the formation of styrene oxide during the polymerization of styrene using gem-bis(tert-alkyldioxy)alkanes. They suggested radical expoxidation of styrene by expeQed t rt-butylperoxy radicals as shown below. 

Another mechanism of chain transfer to the monomer has been highlighted by Stolarzewicz in the case of phenyl glycidyl ethers (PGEs). As shown in Scheme 13, it involves abstraction of a proton from the methine carbon of the ring and the formation of new carbonylated initiating species. The author argues that this reaction could also occur during the polymerization of propylene and styrene oxides. 

As noted above, when Al-porphyrin complexes [97] or Zn compounds [98] are used as catalysts for the carboxylation of epoxides, the formation of polymers is observed. A1 catalysts are now used in a plant in China. The mechanism of the polymerization reaction has been studied and the most credited mechanism when Zn compounds are used is shown in Scheme 1.12. The molecular mass of the polymers varies with the catalyst. Primarily propene oxide and styrene oxide have been used so far, with some interesting applications of cyclohexene oxide. It is wished to enlarge the use of substrates in order to discover new properties of the polymers. 

A similar mechanism of chain oxidation of olefinic hydrocarbons was observed experimentally by Bolland and Gee [53] in 1946 after a detailed study of the kinetics of the oxidation of nonsaturated compounds. Miller and Mayo [54] studied the oxidation of styrene and found that this reaction is in essence the chain copolymerization of styrene and dioxygen with production of polymeric peroxide. Rust [55] observed dihydroperoxide formation in his study of the oxidation of branched aliphatic hydrocarbons and treated this fact as the result of intramolecular isomerization of peroxyl radicals. 

SCBs play an important role in the formation of other block copolymers. For example, the relatively less nucleophilic poly(ethylene oxide) oxyanion cannot initiate the polymerization of styrene, which needs a more nucleophilic alkyllithium initiator. To enable the synthesis of multi-block copolymers from various combinations of monomers by anionic mechanisms, it is important to modify the reactivity of the growing anionic chain end of each polymer so as to attack the co-monomer. There have only been a few reports on the polymerization of styrene initiated by an oxyanion (see <2001MM4384> and references cited). Thus, there exists a need for a transitional species that is capable of converting oxyanions into carbanions. In 2000, Kawakami and co-workers came up with the concept of the carbanion pump , in which the ring-strain energy of the SCB is harnessed to convert an oxyanion into a carbanion (Scheme 13) <2000MI527>. 

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