Polymer breaking reactions

It does not follow, as the authors affirm, that this chain-breaking reaction with free ions is a termination, nor that the entity at the growing end of the polymer chain, which reacts with the free ions, is itself a free ion. 

Acidic chloroaluminate ionic liquids are excellent media for polymer cracking reactions. With the huge quantities of polymers that need to be disposed of each year the ability to break them down into useful compounds for new synthesis or to use as liquid fuels is extremely important. While certain polymers such as poly(methyl methacrylate) are easily cracked into their constituent monomers that can be reused, the majority of polymers are extremely difficult to crack into useful organic compounds. However, merely dissolving polyethylene in acidic chloroaluminate ionic liquids containing a proton source results in the formation of a mixture of alkenes and cyclic alkenes [48], The key compounds produced are shown in Figure 10.10. 

Transfer of a P-proton from the propagating carbocation is the most important chain-breaking reaction. It occurs readily because much of the positive charge of the cationic propagating center resides not on carbon, but on the P-hydrogens because of hyperconjugation. Monomer, counterion or any other basic species in the reaction mixture can abstract a P-proton. Chain transfer to monomer involves transfer of a P-proton to monomer with the formation of terminal unsaturation in the polymer. 

Isobutylene is polymerized under conditions where chain transfer to monomer is the predominant chain-breaking reaction. A 4.0-g sample of the polymer was found to decolorize 6.0 mL of an 0.01 M solution of bromine in carbon tetrachloride. Calculate the number-average molecular weight of the polyisobutylene. 

Living polymerizations are useful for producing block copolymers and functionalized polymers. Facile chain-breaking reactions such as [3-hydride transfer greatly limit the possibility of living polymerization for most of the polymerizations described in this chapter, but there are significant differences between the different types of initiators ... 

Table III shows the increase of molecular weight of BCMO polymerization with conversion, although the polymer tends to precipitate. The monomer reactivity ratios of DOL-BCMO copolymerization were previously determined as rx (DOL) = 0.65 0.05, r2 (BCMO) = 1.5 0.1 at 0°C. by BF3 Et20 (8). Table IV shows a preparation of block copolymer of DOL, St, and BCMO. In the first step we polymerized DOL and St in the second step we added BCMO to this living system. The copolymer obtained showed an increase of molecular weight, and considerable BCMO was incorporated in the copolymer still remaining soluble in ethylene dichloride. The solubility behavior together with the increase of molecular weight with addition of BCMO shows that this polymer consists of block sequences of DOL-St and (St)-DOL-BCMO. This we call block and random copolymer of DOL-St—BCMO. We can deny the presence of BCMO, St, or DOL homopolymers in this system, but some chain-breaking reactions are unavoidable, leading to copolymer mixtures. Thus, the principle of formation of block copolymers by cationic system is partly substantiated.

Since the 1960s [20], much of the research in polymer synthesis has been directed at establishing living conditions for chain polymerizations. The only requirements for a polymerization to be considered living are that no chain-breaking reactions occur during the polymerization. That is, the rate constants of both chain transfer and termination should be equal to zero (Atr = 0, k, = 0). 

In practice, linear semilogarithmic kinetic plots and linear dependencies of molecular weight on monomer conversion require only that the rate constants of chain transfer and termination are much less than that of propagation klr kp, k,< kp). This is therefore the practical requirement for the synthesis of well-defined polymers, such that complete monomer conversion can be reached and the chain ends can be functionalized quantitatively. However, because chain-breaking reactions are actually present, we prefer to call such systems controlled polymerizations rather than living polymerizations. 

In addition to the absence of chain-breaking reactions and fast initiation, two additional constraints must be met to obtain polymers with nar-... 

Diblock, triblock, and multiblock copolymers are typically prepared by sequential monomer addition to polymerization systems in which the chain-breaking reactions are sufficiently suppressed. Polymer properties can thereby be varied by manipulating the constituent blocks compatibilities, hydrophilicities/hydrophobicities, and hardness/softness. New and/ or novel topologies can also be prepared by controlled processes, including cyclic polymers and/or copolymers, comb-like macromolecules, and star polymers. The synthetic range of cationic vinyl polymerizations will be discussed in detail in Chapter 5. 

Kinetics of radical chain process of polymer thermal destruction includes stages of initiation, growth of reaction chain, chain transmission, its break. Reaction of chain transmission occurs mainly at the expence of hydrogen break Irom polymer chain. 

Early studies of cellulose degradation revealed for the first time that hydrolytic agents selectively attacked the amorphous fraction (1) of the polymer, breaking and reordering accessible chain segments (2). Later work on both poly(ethylene terejhthalate) (3,4) and polyethylene (d) confirmed that localized reactions were characteristic of all polymers with impervious crystalline regions. 

Chain transfer to monomer is perhaps the most important chain breaking reaction in cationic polymerization. Transfer to monomer involves transfer of a /3-proton from the carbocation to a monomer molecule. This results in the formation of terminal unsaturation in the polymer molecule. Since in isobutylene there are two different types of j9-protons, two different unsaturated groups are possible ... 

In each system, the primary photo-event is dissociation of the cationic photoinitiator to produce an acid. This reaction proceeds with a quantum efficiency that is characteristic of the particular initiator. The photogenerated acid then interacts with a carefully chosen polymer matrix to initiate a chain reaction, or acts as a catalyst, such that a single molecule of photogenerated acid serves to initiate a cascade of bond making or breaking reactions. The effective quantum efficiency of the overall process is the product of the photolysis reaction efficiency times the length of the chain reaction (or the catalytic chain length). This multiplicative response constitutes... 

The lignin-degrading peroxidases operate by generating hydrogen peroxide, which dissociates into hydroxyl radicals that react with lignin by free radical substitution reactions to break down the network structure of the highly crosslinked, three-dimensional polymer. This reaction sequence is illustrated below for a representative section of the complex lignin macromolecule ... 

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