Midchain radical

E) H-abstraction (metathesis) reaction on the polymer chain due to midchain radicals... 

Recent studies convincingly proved the existence of midchain radicals (MCRs) and the secondary reactions that are responsible for the formation of MCRs during the polymerization process [9]. For instance, the polymerization of butyl acrylate (BA), and in general of the acrylate monomer family, proceeds with the presence of two radicals a chain-end (secondary) radical and an MCR (tertiary and significantly... 

In Scheme 3.7(a), the transfer of a radical center from a polymeric radical to another polymer chain occurs via H-atom abstraction. Addition of monomer to the resulting midchain radical produces a polymer with a branch point, with the final length of the newly formed branch controlled by the kinetic chain length of the system. An additional subscript is added to track the number of LCBs formed. 

Scheme 3.9 Formation of a midchain radical by intramolecular chain transfer to polymer. Monomer addition to the new radical structure creates a SCB In the polymer.

Scheme 3.10 P-Scission of butyl acrylate midchain radical.

Chain scission. The midchain radical structure formed by intra- or intermolecular transfer to polymer is less reactive than a chain-end radical. Under higher temperature conditions, the radical may undergo -ffagmentation (chain scission) as shown in Scheme 3.10 for BA. As well as lowering polymer MW, sdssion produces an unsaturated chain end that can react further (Scheme 3.7b). Scission is important for acrylate polymerizations at temperatures > 140°C [18,21], is a dominant mechanism in styrene polymerizations at 260-340°C [15], and also occurs during LDPE production [14]. Kinetic treatment is difficult, as scission is coupled with LCB and/or SCB formation. 

One additional piece of evidence supporting the formation of a mid-chain radical was obtained from an examination of hydrogen abstraction from polyacrylates. This mid-chain radical can be formed by hydrogen abstraction from polyacrylates by oxygen centered radicals. PolyfBA and fert-butyl peroxide (tBPO) were dissolved in benzene, and the mixture exhibited the ESR spectra shown in Fig. 17a under irradiation. The spectrum was similar to both the spectra observed in the polymerization system (Fig. d) and that reported by Westtnoreland et al. Furthermore, it was reasonably simulated by considering two sets of methylene protons with restricted rotation at both sides of the mid-chain radical, as shown in Fig. lib. Consequently, the radical observed at high temperatures (Fig. lib) is due to the formation of midchain radicals. 

Scission events can occur in any system where mid-chain radicals are formed. However, scission is more temperature-activated than H-abstraction and thus becomes important only at elevated temperatures. The reaction is not believed to occur during butyl acrylate polymerization at 75 C [37], but is shown to be important at 140°C [29, 45], Scission is a dominant mechanism in styrene polymerizations at 260-340°C [26], and also occurs during LDPE production [30]. Scission of midchain radicals formed via intermolecular transfer to polymer can have a significant effect on the breadth and the shape of polymer MWD [46]. 

The primary radicals react readily with polyethylene chains forming midchain radicals by hydrogen abstraction. Hydrogen abstraction can lead to the following radicals ... 

Azukizawa M, Yamada B, Hill DJT, Pomery PJ. Radical polymerization of phenyl acrylate as studied by ESR spectroscopy concurrence of propagating and midchain radicals. Macromol Chem Phys 2000 201 774-781. 

Willemse RXE, van Herk AM, Panchenko E, Junkers T, Bubackm M. PLP-ESR monitoring of midchain radicals in n-butyl acrylate polymerization. Macromolecules 2005 38 5098-5103. 

Gaborieau M, Koo SPS, Castignolles P, Junkers T, Bamer-Kowollik C. Reducing the degree of branching in polyacrylates via midchain radical patching a quantitative melt-state NMR study. Macromolecules 2010 43 5492-5495. 

The EPR experiments of Yamada on methyl acrylate also support this reaction scheme. He was able to show the occurrence of the midchain radical and also the occurrence of j8-scission (Azukizawa et al., 2000 Tanaka et aL, 2000) in the temperature range 40-85°C. 

The transferred radical should have midchain type structure with methylene groups at both sides (H-EA-fBA( )-tBA-fBA-H). The spectrum of the radical shown in Fig. 8b is attributable to such a mid-chain radical. These findings provide clear experimental evidence of a 1.5-hydrogen shift at the propagating chain end of acrylate radical polymerizations. 

In air C=C bonds result in acids, ethers and aldehydes. Aldehydes can result from the isomerizing breakdown of midchain peroxide radicals as follows ... 

The most popular reagent for carboxylating polyolefins has been maleic anhydride (MAH), generally using a peroxide to strip an unstable hydrogen off of the polyolefin backbone, and thus creating a midchain free radical which can add to the C = C double bond of MAH quite efficiently [122]. Occasionally, in place of MAH, researchers may use acrylic or methacrylic... 

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