Propagation Propiolactones

Ring-opening polymerization of racemic a-methyl-/J-propiolactone using lipase PC catalyst proceeded enantioselectively to produce an optically active (S)-enriched polymer [68]. The highest ee value of the polymer was 0.50. NMR analysis of the product showed that the stereoselectivity during the propagation resulted from the catalyst enantiomorphic-site control. 

Epoxides readily undergo anionic copolymerization with lactones and cyclic anhydrides because the propagating centers are similar—alkoxide and carboxylate [Aida et al., 1985 Cherdron and Ohse, 1966 Inoue and Aida, 1989 Luston and Vass, 1984]. Most of the polymerizations show alternating behavior, with the formation of polyester, but the mechanism for alternation is unclear. There are few reports of cationic copolymerizations of lactones and cyclic ethers other than the copolymerizations of [5-propiolactone with tetrahydrofuran and... 

More recently Teyssie determined the rate constants in the polymerization of e-caprolactone (eCL) initiated with aluminium alkoxides, believing that the covalent species are the only ones responsible for propagation [4]. For the same monomer Yamashita estimated tentatively rate coefficients of propagation using an anionic initiator [ ]. Lenz in his studies of substituted g-propiolactones (gPL) observed peculiar influence of structure on reactivity that can have its origin in the multiplicity of ionic structures involved [fi]. 

Photopolymerization of acrylamide by the uranyl ion is said to be induced by electron transfer or energy transfer of the excited uranyl ion with the monomer (37, 38). Uranyl nitrate can photosensitize the polymerization of /S-propiolactone (39) which is polymerized by cationic or anionic mechanism but not by radical. The initiation mechanism is probably electron transfer from /S-propiolactone to the uranyl ion, producing a cation radical which propagates as a cation. Complex formation of uranyl nitrate with the monomer was confirmed by electronic spectroscopy. Polymerization of /J-propiolactone is also photosensitized by sodium chloroaurate (30). Similar to photosensitization by uranyl nitrate, an election transfer process leading to cationic propagation has been suggested. 

The propagating species in the cationic polymerization can be examined from the copolymerization behavior (21). Cyclic ethers such as tetrahydrofuran (THF) or 3,3-bischloromethyloxetane (BCMO), and cyclic esters such as 0-propiolactone (/3-PL) or -caprolactone (c-CL) are classified as oxonium ion type monomers. Copolymerizations between these monomers are observed easily as in the case of BCMO-THF (12, 13), BCMO-/3-PL (14, 15), BCMO-c-CL (16), and THF- -CL (21). 

Propagation according to scheme (168a) is typical of carbocation initiators both modes were observed with acylium (RC+=0) initiators. During chain growth from y -propiolactone, the concentracion of acylium ions decreases until oxonium ions become the active centres. In e-caprolactone polymerization, both types of centre continue to operate. 

By comparison unsubstituted lactones such as propiolactone (G) polymerize more slowly and molecular weights are lower. The authors believe propagation is slower, the carboxylate anion formed from pivalolactone ring opening being more nucleophilic because of the inductive effect of the two methyl substituents. 

Table IV. Propagation Rate Constants and Activation Energies for Polymerization of a,a-Disubstituted-6-propiolactones in DMSO Initiated by TEAB...

These and related methods allowed us recently to reevaluate the structure of active centers in anionic polymerization of simple, unsubstituted lactones, 5-propiolac-tone. The rationale was put forward in terms of stereo -electronic factors to explain why g-propiolactone propagates on carboxylate and e-caprolactone on alcoholate anions. This is shown in scheme below ... 

The kinetics of these polymerizations is complex. Both complexed ion pairs and free ions are involved in the propagation reactions and the free ion rate constants depend on monomer concentration. The relative reactivity of complexed ion pairs and free ions is temperature dependent. Above the inversion temperature of —35°C, free ions are more reactive than ion pairs, but below this temperature the ion pairs are more reactive. At 30 °C in DMF, the observed (average) propagation rate constant is 0.13 l/(mol s) [146], The anionic polymerization of a,a-dialkyl-P-propiolactones such as pivalolactone (a,a,-dimethyl-P-propiolactone) initiated with carboxylate anions exhibits the main characteristics of living polymerizations. 

A similar proton migration takes place in copolymerizations of acrylamide with cyclic imino ethers. The proton migration is part of the propagation process. Other examples are copolymerizations of a nucleophilic monomer, 2-phenyl-1,2,3-dioxaphospholane with electrophilic monomers. Here, too, the electrophilic monomers can be either acrylic acid or propiolactone. Identical products are obtained from both reactions ... 

Ring chain equilibria in the polymerization of < -valerolactone by lithium butoxide in THF have been found to be in accord with Jacobson-Stockmayer theory. The relative reactivities in the propagation of /ff-propiolactone are markedly temperature sensitive k /k+ varies from 5.6 at —20 C to 150 at 35 °C, the counterion being potassium complexed with dibenzo-18-crown-6 in dichloromethane. Roda and co-workers have continued their study of 2-pyrrolidone. The reactions of di-isopropenylbenzene (DIB) continue to be of interest. A paper by Beinert et al. in 1978 described the synthesis of a reagent, claimed to be an efficient difunctional initiator, by the reaction of one mole of /m-DIB with two of s-butyl-lithium. Protonation of the reagent solution with methanol generated butane and the hydrocarbon (5) was recovered. This species... 

These investigations are continuing to more fully verify the solvent effects observed, and in addition, similar studies are in progress on the effect of substituent size as well as reaction solvent on the anionic polymerization of a series of a-methyl-a-alkyl-6-propiolactones. An equivalent series of B-propiolactam monomers was recently investigated in this laboratory with the surprising result that the rates of propagation within this series increased with increasing size of the a-alkyl substituent (6). 

Associated with this well-known propagating species equilibrium, P-lactones behave differently to other larger lactones, due to their high pwlarity and high internal strain of the four-membered ring (Table 9.1). Since 1948, when details of the P-propiolactone (PL) ionic polymerization were first pubhshed [20], much more informahon regarding the polymerization not only of P-lactones but also of substituted P-lactones, has become available [21-27]. 

When weak bases or ammonium carboxylates are used as initiators, a similar mechanism has been observed which involves the carboxylate anions responsible for propagation. Subsequently, it was confirmed that, in the case of a,a -dialkyl-P-propiolactone, no chain transfer could occur as a-proton abstraction could no longer take place [29]. Interestingly, a similar result was recently highlighted by Guerin et al. who showed that, compared to poly(malolactonate) (as obtained from a,a, P-trisubstituted P-lactones), polymers prepared by the ROP of P-lactones without substitution in the a-position were characterized by major discrepancies between the experimental molecular weights and the theoretical values expected for a controUed/ Uving polymerization (see below) [30]. 

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