Unzipping depolymerization

For PMMA, a polymer that undergoes unzipping depolymerization... 

It has been reported that the thermal degradation of PLA predominantly consists of random main-chain scission and unzipping depolymerization reactions. The random degradation reaction involves hydrolysis, oxidative degradation, c/s-elimination, and intramolecular and intermolecular transesterification. Almost all the active chain-end groups, residual catalysts, residual monomers, and other impurities enhance the thermal degradation of PLA. As a consequence... 

The thermal degradation of PLA has been claimed to mainly occur via random scission based on a linear relationship between inverse of the number-average degree of polymerization P and time as shown in Equation 23.2 [28]. Recently, Aoyagi et al. [9] and Abe et al. [29] suggested that the isothermal degradation of PLLA at 220, 290, and 330°C proceeded not only via simple random scission, but also via an unzipping depolymerization of the polymer chain based on the nonlinear relationships of l/P and P with time. 

To clarify the effects of chain-end structures of PLA, Lee et al. [34] synthesized C1-, NH2-, and COOH-terminated PLLAs from OH-terminated PLLAs. The thermal stability of OH-terminated PLLAs was poor, whereas NH2- and Cl-terminated PLLAs were more resistant to thermal degradation. The main mechanisms of PLLA thermal degradation are transesterification and backbiting reactions that cause random degradation and unzipping depolymerization, respectively, starting from the carboxyl and/or hydroxyl chain ends [7, 8, 10, 29, 49]. 

MgO even in the lower temperature range. This characteristic antiracemization effect of MgO is due to the lower basicity of Mg compared to Ca. At temperatures lower than 270°C, the pyrolysis of PLLA/MgO (5 wt%) composite occurred causing unzipping depolymerization, resulting in selective L,L-lactide production. 

A two-step degradation mechanism for pofycaprolactone has been proposed by Persenaire et al. [42], They studied thermal degradation of PCL by high resolution thermogravimetric analysis (TGA) simultaneously coupled with mass spectrometry (MS) arrd Forrrier transform infrared spectrometry (FTIR). Based on evolved gas analysis by both MS and FTIR it was concluded that the first step was a random rupture of polyester chains via cw-elimination reaction which produced H2O, CO2, and 5-hexanoic acid. The second step is an unzipping depolymerization process at the chain ends with hydroxyl end groups to form e-caprolactone (see Fig. 4.2). 

The TGA and DTA studies reported by Aoyagi et al. [5] suggested a single-step degradation of PCL. However, they do not exclude the possibihty of a random rupture of the pofyester chain via a c/5-elimination reaction, because it has been pointed out by Persenaire et al. [42] that the ci5-elimination reaction and the unzipping depolymerization proceed consecutivefy at very close temperatures, so that these two steps may not be resolved by a conventional DTA technique. 

Figure 4-2 Unzipping depolymerization process during thermal degradation of PCL.

This unzipping depolymerization occurs during polymerization, but it may also occur under thermal stress with the neutralized polymer, unzipping then starts from neutral but unstable end groups such as -OH or -CHO or from statistical chain sdssion. Unzipping will stop at comonomer units such as those from ethylene oxide, dioxepane, and similar monomers, which are not able to depolymerize, and a then stable copolymer will result. Homopolymers, which are usually polymerized anionically from formaldehyde (see Section 7.2.3) will not be stable unless unstable end groups are transformed to stable ones. 

Abe et al. [21] also showed the same effect for the hydroxyl-end acetylation. Additionally, they investigated effects of ce-carboxylic acid chain end protection. From changes in activation energy and molecular weight of residual polymers together with Py-GC/MS analysis results, they deduced that the residual Zn compounds catalyzed the selective unzipping depolymerization of PLA with... 

Fan et al. [32,47] found that Ca-ion-catalyzed depol5mierization of PLLA with PLLA-Ca caused considerable racemization at temperatures lower than 250 C, forming a large amount of meso-lactide as a by-product They proposed a novel racemization mechanism based on a 5 2 reaction at an asymmetrical methine carbon, which occurred as a back-biting reaction from an active chain end structure R-COO" Ca of PLLA (Scheme 9.4) [8, 47]. At temperatures over 320 C, side reactions such as the ester-semiacetal tautomerization, caused the formation of meso-lactide, but not dominantly. At 250-320°C, L,L-lactide is produced exclusively, because unzipping depolymerization proceeds as the main reaction. 

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