RING-OPENING POLYMERIZATION in .NET

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RING-OPENING POLYMERIZATION
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oxonium ion followed by propagation through acyl oxygen cleavage. However, polymer end-group analysis combined with trapping of propagating centers by reaction with triphenylphosphine indicates that this is not the mechanism. Initiation involves attack of a positive center on the exocyclic oxygen (the more basic oxygen) to form a dioxocarbocation (XLVIII). For example, for initiation by methyl carbocation derived from CH3 OSO2 CF3 or (CH3 2 I SbF 6
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R O CH3O C
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XLVIII
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Propagation follows in a similar manner with alkyl oxygen cleavage
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R CH3 O CO R
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Cationic polymerization is not nearly as useful as anionic polymerization for synthesizing high-molecular-weight polyesters. The cationic route appears to be limited by intramolecular transesteri cation (cyclization) as well as other chain transfer to polymer reactions (including hydride and proton transfers) although there are few details in the literature. However, molecular weights in the 100,000 range have been observed for the highly reactive monomer b-propiolactone. The highly strained b-propiolactone undergoes a mixture of alkyl oxygen and acyl oxygen cleavages under some reaction conditions. 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:
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AM polymerization offers the potential for suppressing side reactions and achieving living polymerization with the ability to control MW and achieve high molecular weights.
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Enzymatic Polymerization
Lipases catalyze the polymerization of lactones [Duda et al., 2002; Gross et al., 2001; Kobayashi, 1999; Kobayashi et al., 2001]. The reaction mechanism is similar to that for the enzymatic polymerization of hydroxyacids (Sec. 2-17a-2). Lipase reacts with lactone to produce enzyme-activated hydroxyacid and some of the latter reacts with water to produce hydroxyacid (Eqs. 7-81). Hydroxyacid and enzyme-activated hydroxyacid react to initiate polymerization (Eq. 7-82). Propagation proceeds by nucleophilic attack of
LACTONES
the hydroxyl end group of the propagating chain on the enzyme-activated hydroxyacid (Eq. 7-83a).
O R C O + lipase OH O HO R C O lipase
O HO R C OH
7-81
O HO R C O
O lipase + HO R C OH
lipase
7-82
HO R C O R C OH
O HO R C O lipase + HO
lipase
O HO R C O
7-83a
O H O R C O H
7-83b
7-5d
Other Cyclic Esters
Polymerization and copolymerization of the two 1,4-dioxane-2,5-diones (dilactones), glycolide and lactide (XLIX with R H and CH3 , respectively) proceeds using anionic initiators;
O R O O XLIX O O R CHR CO
7-84
cationic initiators are not as useful [Bero et al., 1990; Chamberlain et al., 2001; Chisholm et al., 2001; Kowalski et al., 1998; Kricheldorf and Kreiser-Saunders, 1990; Kricheldorf et al., 1987a,b, 2000; Leenslag and Pennings, 1987; Shibasaki et al., 2000; Stridsberg et al., 2000]. The polymerization rates are generally lower than those for lactones. Polylactide is of interest because it is both biocompatible and biodegradable. It has been used for absorbable sutures and has the potential for other biomedical applications such as drug delivery. Polymerization of a cyclic carbonate ester yields a linear polycarbonate [Kuhling et al., 1989; Rokicki, 2000]. For example, the cyclic oligomer (m 2 20 in Eq. 7-85) of the
C(CH3)2
O CO
C(CH3)2
O CO
7-85
RING-OPENING POLYMERIZATION
carbonate derived from bisphenol A, 2,20 -bis(4-hydroxyphenyl)propane, offers an alternate route to polycarbonates other than step polymerization of bisphenol A with phosgene or ester interchange with diphenyl carbonate (Sec. 2-8e) [Brunelle et al., 1989; Stewart, 1989]. The ROP route to polycarbonates offers potential advantages relative to the step polymerization process. One advantage is that higher MW can be more easily achieved. ROP, especially with anionic initiators, proceeds as a living system and MW is determined by conversion and the ratio of monomer to initiator. MW control in step polymerization is dependent on stoichiometric ratio and conversion. Molecular weights as high as 100,000 300,000 are reported for the ROP, while the highest MWs achieved by step polymerization are 40,000 60,000. It is more dif cult to achieve the very high conversions needed in step polymerization to reach the 105 -MW range than to control the monomer initiator ratio in ROP. Another advantage of the ROP process is the absence of by-products, which allows the use of reactive processing techniques in which cyclic monomer is directly polymerized into nal objects by extrusion or molding. ROP as an alternate to step polymerization is also being studied for other high-performance polymers such as polyesters, polyamides, and polyetherimides. The ROP route is viable when the cyclic oligomer can be synthesized and polymerized in high yield.