Chain end radical

The grafting reaction seems to be more efficient on the PE surface than on the PP (Figure 4). This is consistent with observations made by Tazuke and coworkers (11). One explanation is that the primary formed radicals on PP are more likely to undergo rearrangement to chain end radicals than radicals on PE. For the same reason it is more difficult to crosslink PP than PE. The chain end radicals can easily diffuse into the polymer matrix and there be out of reach for deposited monomer. 

The two polymer substrates investigated as part of the study of DBDPO mixtures were polypropylene (PP) and linear high density polyethylene (HDPE). while both PP and HDPE decompose by similar random chain scission, radical mechanisms, chain transfer occurs much more teadily during the pyrolysis of PP because of the presence of the tertiary hydrogens. In addition, only primary chain end radicals are formed when the HDPE chain cleaves homolytically. Therefore, a comparison of the PP/DBDPO and the HDPE/DBDPO mixtures volatile product distributions was undertaken. 

ATRP gives excellent control of polymer chain architecture. For industrial use, however, two problems need to be overcome residual halides and metals in the product would be a problem for electronic device uses. The rate of polymerization may be too slow. This is because the chain end concentrations must be low so that typical radical chain termination is kept to a minimum. Chain termination is a second order reaction and will be minimized by low concentrations of chain end radicals. The low rate of polymerization may increase the cost of the process since the optimum time for a polymerization run is about 6 h. 

Recently, the Research Group on NMR, SPSJ, assessed reliability of copolymer analysis by NMR using three samples of radically prepared copolymers of MMA and acrylonitrile with different compositions. 1H and 13C NMR spectra of the copolymers were collected from 46 NMR spectrometers (90 500 MHz) and the composition and sequence distribution were determined.232 Table 14 summarizes the monomer reactivity ratios determined by 13C NMR analysis. The large difference between rxx and r2X indicates the presence of a penultimate effect in this radical copolymerization, as previously reported.233 The values of riy, especially rxx, depended on the comonomer feed ratio, suggesting higher order of neighbouring unit effect on the reactivity of chain-end radicals. 

While long branches are formed by the normal chain transfer to polymer, as shown above [Eq. (6.124)], reactive radicals like those of polyethylene can also undergo self-branching by a backbiting intramolecular transfer reaction (see Fig. 6.9) in which the chain-end radical abstracts a hydrogen atom from a methylene unit of the same chain resulting in the formation of short branches (as many as 30-50 branches per 1000 carbon atoms in the main chain) that outnumber the long branches by a factor of 20-50. 

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]. 

One usually observes chain scission type radicals, —CpHp2 (Ca ) R(CHs) in y-irradiated PMMA. Is the chain end radical the primary species of y-irradiation Ichikawa et al. [12] studied the mechanism of radiation-induced degradation of PMMA by ESR and electron spin echo (ESE) methods. Figure 7.12 shows ESR spectra of y-irradiated PMMA observed at 77 K. They assigned spectrum (a) to three kinds of radicals, (C )HO, (C )H3, and —(C )(0 )(OCH3), which were a doublet... 

Fig. 7.24 ESR spectrum from fractured PTFE with ethylene monomer in vacuum at 77 K solid line) and the simulated spectmm of the chain end radicals of PE molecules based on free rotation around the C—C bond axis dashed line). The figure is adapted from [25] with permission from American Chemical Society...

A7.1 Two Site Model and Coordinate System of Polyethylene (PE) Chain End Radical, —Cp H 2 (Ca )Hot2 (Bond Scission Type Radical and Propagating Radical)... 

The irradiation of fluoropolymers at elevated temperatures has been explored for the development of materials with better mechanical properties [35]. This arises because of the radiation-induced crossUnking of chains and subsequent higher network density in the resultant polymer [36]. Here, the irradiation is accomplished at a temperature higher than the melting point of the polymer. In the molten state, the polymer behaves as an amorphous matrix and the mobility of molecular chains is considerably enhanced. This promotes the mutual recombination of radicals, i.e., crossHnking involving chain end radicals and chain alkyl radicals [37]. 

Not only in the mathematical description of copolymer composition, but also in that of monomer sequence distribution, is it convenient to use so-called conditional probabilities. These conditional probabilities are defined as the chance that a certain event takes place out of aU possibilities at a certain stage. For the purpose of the copolymerization equations, conditional probabilities related to propagation only are considered. In case of the TM, an example of such a conditional probability is the chance that monomer 2 will add to a monomer 1 chain-end radical (P12). In terms of eqns [l]-[4], this probability is the rate of reaction [2] divided by the sum of the rates of reaaions [1] and [2]. The two relevant conditional probabilities are defined as in eqns [6] and [7] ... 

At a later stage, it was discovered that a much larger sensitivity can be obtained if the ratio of propagating chain-end radicals can be determined as a function of comonomer feed ratio. EspedaUy, with the introduction of the IPUE (Section 6.12.2.2.), this propagating radical ratio is indispensable for modd discrimination. The problem is almost a chicken-and-egg situation, since now one knows what to look for, but... 

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. 

As might be expected, the emulsion polymerization system does not alter the basic mechanism of free radical polymerization as regards the chain unit structure. The latter is, of course, independent of the type of free radical initiator used, in view of the free nature of the growing chain end radical. The temperature of polymerization does exert some influence, as shown by the data in Table Vlll, but not to a very great extent. It can be seen that the 1,2 side-chain vinyl content is rather insensitive to the temperature, whereas the trans-, A content increases with decreasing temperature, at the expense of the m-1,4 content. The latter almost vanishes, in fact, at low temperatures and... 

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. 

The Montell process is based on electron-beam irradiation of PP powder taken directly from the synthesis process (without oxygen contact) at low temperatures (60-120°C). In this temperature range, the (3-scission of the tertiary C-radicals is very slow so that, due to recombination reactions between backbone radicals and chain-end radicals, the formation of long-chain branched PP is preferred. 

A growing chain with monomer 1 as the chain-end radical has a rate constant for self-addition of n the rate for addition of monomer 2 is ku- The self-addition rate for a terminal monomer 2 radical is given as 22 the rate for addition of monomer 1 is 21- The reactivity ratios can also he calculated from the Price-Alfrey measures (87) of resonance stabilization (Q) and polarity (e) which are shown for common acrylic esters in Table 8. NMR can also be used to determine the composition distribution characteristics of acrylic copolymers (88,89). 

The relative frequency of chain transfer compared to depropagation is determined by the stability of the chain-end radical and by the availability of... 

As mentioned above, the mechanism of degradation of PTFE is unclear, particularly the location of radiation-induced chemical reactions. However, ESR spectroscopy (290-293) has confirmed that the stable free-radical species at low temperatures are the chain-end radicals IX (shown below) and that at higher temperatures the secondary radicals X dominate the spectrum. Hedvig (291) has suggested that chain-end radicals terminate by abstraction to form the more stable secondary radical. These radicals persist for very long times, and this leads some authors (58) to suggest the suitability of PTFE as a radiation dosimeter. 

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