Mid-chain radical

Typical levels of MAH in grafted PP of 0.5-2 wt % correspond to only one or two units per chain. If the MAII units arc grouped it follows that many chains may contain no MAH. It has also been suggested that for PP all MAH may appear at the chain ends. This is rationalized in terms of the reaction of mid chain radicals with MAH always being followed by intramolecular chain transfer and chain... 

As mentioned above, in the previous study examinig the pentameric oligomer of tBA a second radical migration step was not observed [15], At that time, the reason was not clear. When model radicals, exemplaiy of radicals present in the polymerizations of various kinds of alkyl acrylates, were observed by ESR some alkyl acrylates showed faster radical migration to form mid-chain radicals even at low temperatrrres. 

In the case of the radical prepared from the pentamer there are two chances of 1,5-hydrogen shift reaction from the initially generated radical. The initial radical center migrated to form mid-chain radical through six-membered ring stracture and then the mid-chain radical moved to form a radical located at the other end of the pentamer. Since expected spectroscopic features of radicals at... 

Figure 6 Expected ESR spectra for initially generated radical (a), mid-chain radical (b), and radical formed by two-step migration to locate at the other end...

Backbiting generates a tertiary (mid-chain) radical, R, by abstraction of a hydrogen atom from an acrylate unit on the backbone of the secondary (chain-end) radical R, most likely via the formation of a six-membered ring, as shown in Scheme 4.7 for BA. Subsequent addition of monomer to R creates a short-chain branch (SCB) in the polymer and leads to re-formation of a chain-end radical. The propagation rate coefficient for monomer addition to the MCR, kpt, is significantly lower than that for addition to the secondary chain end [10]. 

Shown in Fig. 4.8 is the conversion dependence of (kt) as determined via SP-PLP for MA and DA homopolymerizations at 40°C and 100 MPa both in bulk and in solution of 40 wt% CO2. The pressure and temperature conditions were chosen such as to yield excellent signal-to-noise quality of the monomer concentration vs time (after firing the laser pulse) traces. For in-depth mechanistic studies, so-called mid-chain radicals have to be accounted for [45]. However, the... 

In another study on conventional radical polymerizations of acrylates, large amounts of mid-chain radicals were detected by ESR spectroscopy and it was suggested that the... 

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. 

Here, k is the initiator decomposition rate, [I] is the initiator concentration and/is the initiator efficiency. Note that no assumption of a steady state radical concentration is made. Additionally, the non-classical relationship between Rp and the monomer concentration due to the formation of less reactive mid-chain radicals is accounted for using equation 3, which introduces a modified propagation rate coefficient, kp, and the possibility of accounting for virtual monomer reaction orders, co, other than nnP [3 ]... 

Tertiaiy Radical Formation (Backbiting) and Mid-Chain Radical Propagation and Termination. 

Accounting for mid-chain radical formation has been shown to lead to virtual monomer reaction orders (i.e., >) greater than one. Equation 10 depicts the... 

The impact of the value and chain length dependence of the that are introduced when backbiting and mid-chain radicals are accounted for (i.e., k, and cr ) on... 

Auto acceleration at relatively high conversion can also be caused by formation of fi"ee radicals from the polymer that has been produced (Woods and Pikaev 1994). The yield of radicals from the polymer, which is usually a saturated chain, maybe higher than the yield from the unsaturated monomer leading to an increase in the rate of polymerization due to the increased radical population. Termination reactions of these radicals with chain-end radicals give rise to branched chains (graft homopolymer), while self-termination of mid-chain radicals brings about cross-links. 

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. 

It is known that polyacrylates prepared by conventional radical polymerization contain many branches. The ESR study has now provided direct evidence for the origin of the branching. Note also that an estimate of kp for acrylates is difficult by ESR, because it would provide the sum of the concentration of the active growing terminal radicals and the less active mid-chain radicals. 

In the case of fBA, experimental evidence for the formation of a significant amount of mid-chain radicals from propagating radicals via 1,5-hydrogen shift was shown. Progress in controlled radical polymerization enables us to investigate conventional radical polymerization systems in detail. 

Fig. 17. Experimental (a) and simulated b) ESR spectra of mid-chain radical generated by hydrogen abstraction from polyfBA by peroxide radicals b). The spectrum was reasonably simulated based on the structure shown in the figure.

Intermolecular tranter to polymer The transfer of a radical center from a polymeric radical to another polymer chain is shown in Scheme 4.9. Addition of monomer to the mid-chain radical produces a polymer with a branch point, with the length controlled by the number of propagation events before chain transfer or radical-radical termination occurs. An additional subscript can be added to track the number of long-chain branches formed [Eq. (27)]. 

Scheme 4.12. Formation of a mid-chain radical by intramolecular chain transfer to polymer. Monomer addition to the new radical structure creates a short-chain branch in the polymer.

Scheme 4.13. -Scission of butyl acrylate mId-chaIn radical...

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]. 

Termination of the mid-chain radical (Q ) is also considered in the scheme. While the mechanism provides improved tmderstanding of this complex system, many questions still remain does the mid-chain radical terminate at the same rate as chain-end radicals what is the reactivity of the unsaturated chain end and does the raid-chain radical scission with equal probability in each direction The reaction engineering challenge is to consider the set of mechanisms required to represent the basic rate behavior and polymer architecture of the system without introducing unneeded complexity. 

Scheme 4.17. Fragmentation of a mid-chain radical with adjacent MA and MMA units (after Ref. 72).

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