Polymers chain reaction/addition

Figure 5.13. Reactions involved in cationic addition polymerization. Shown are (a) generation of a carbo-cation intermediate from a Lewis acid initiator, (b) propagation of the polymer chain through the combination of the carbocationic polymer chain and additional monomers, and (c) termination of the polymer growth through either proton abstraction (i) or anionic attachment (ii) routes.

There have been attempts to remedy this situation. Chain transfer agents can be used to limit the molecular weight of the polymer chains. These additives react with the growing polymer chain, limiting the degree of polymerization without affecting the polymerization rate. Transfer agents can introduce functionality to polymer chain ends that will allow for post-polymerization reactions. 

We will discuss the various polymerization mechanisms in greater detail in Chapter 2. The original classification of polymers as either condensation or addition polymers as proposed by Carothers does not permit a complete differentiation between the two classes or polymers, particularly in view of the new polymerization processes that have been developed in recent years. Consequently, this classification has been replaced by the terms step-reaction (condensation) and chain-reaction (addition) polymerization. These terms focus more on the manner in which the monomers are linked together during polymerization. 

As disciissed in Chapter 1, under a scheme proposed by Carothers, polymers are classified as addition or condensation polymers depending on the type of polymerization reaction involved in their synthesis. This classification scheme, however, does not permit a complete difierentiation between the two classes of polymers. A more complete but still oversimplified scheme that is still based on the dilTerent polymerization processes places polymers into three classes condensation, addition, and ring-opening polymers. This scheme reflects the stractures of the starting monomers. Probably the most general classification scheme is based on the polymerization mechanism involved in polymer synthesis. Under this scheme, polymerization processes are classified as step-reaction (condensation) or chain-reaction (addition) polymerization. In this chapter, we will discuss the different types of polymers based on the different polymerization mechanisms. 

Basically, four different types of reactions are possible (Table 4) (I) recombination of radicals, (II) cross-linking of polymer chains, (111) addition of radicals from the reactive atmosphere, and (IV) addition of bi-functional molecules with... 

Free-radical polymerization n. A reaction initiated by a free radical derived from a polymerization catalyst. Polymerization proceeds by the chain-reaction addition of monomer molecules to the free-radical ends of growing chain molecules. Major polymerization methods such as bulk, suspension, emulsion, and solution polymerization involve free radicals. The free-radical mechanism is also useful in copolymerization, in which alternating monomeric units are promoted by the presence of free radicals. Lenz RW (1967) Organic chemistry of high polymers. Interscience Publishers, New York. Odian G (2004) Principles of polymerization, 4th edn. Wiley-Interscience, New York. 

It should be noted that reaction 6 does not lead to polymer chain scission ie, there is no molar mass reduction here, in contrast to reaction 5 where there is a molar mass reduction. It should also be noted that the formation and reaction of hydroperoxide groups attached to polymer molecules (reactions 3 and 4) are much slower, ie, rate determining, than the other processes shown. It will be apparent from the free-radical products formed in reactions 1 through 4 that this whole peroxidation procedure is a branching chain reaction. Additive chemistry is required to provide polyolefins with any sort of prolonged service fife, and such chemistry is well known and entirely effective. 

Autocatalysis, with n = 1, characterizes biological population growth, for example, since the number of offspring born is proportional to the number of individuals in the population. It leads to the Malthusian population explosion. In chemical systems, where autocatalysis is less common, it can also result in explosion, since the solution to Eq. (2.1) is an exponentially growing concentration. Of relevance to polymer systems is the fact that any exothermic reaction is inherently autocatalytic, since an increase in the product concentration corresponds to production of heat, which leads to an increase in the rate constant of the reaction via the Arrhenius factor. If the reaction in question is lengthening a polymer chain by addition of the monomer, the rate should increase as the chain grows if the heat produced is not rapidly removed from the system. 

The second category of polymerization reactions does not involve a chain reaction and is divided into two groups poly addition and poly condensation [4]. In botli reactions, tire growth of a polymer chains proceeds by reactions between molecules of all degrees of polymerization. In polycondensations a low-molecular-weight product L is eliminated, while polyadditions occur witliout elimination ... 

The addition polymerization of a vinyl monomer CH2=CHX involves three distinctly different steps. First, the reactive center must be initiated by a suitable reaction to produce a free radical or an anion or cation reaction site. Next, this reactive entity adds consecutive monomer units to propagate the polymer chain. Finally, the active site is capped off, terminating the polymer formation. If one assumes that the polymer produced is truly a high molecular weight substance, the lack of uniformity at the two ends of the chain—arising in one case from the initiation, and in the other from the termination-can be neglected. Accordingly, the overall reaction can be written... 

Chain-Growth Associative Thickeners. Preparation of hydrophobically modified, water-soluble polymer in aqueous media by a chain-growth mechanism presents a unique challenge in that the hydrophobically modified monomers are surface active and form micelles (50). Although the initiation and propagation occurs primarily in the aqueous phase, when the propagating radical enters the micelle the hydrophobically modified monomers then polymerize in blocks. In addition, the hydrophobically modified monomer possesses a different reactivity ratio (42) than the unmodified monomer, and the composition of the polymer chain therefore varies considerably with conversion (57). The most extensively studied monomer of this class has been acrylamide, but there have been others such as the modification of PVAlc. Pyridine (58) was one of the first chain-growth polymers to be hydrophobically modified. This modification is a post-polymerization alkylation reaction and produces a random distribution of hydrophobic units. 

If a linear mbber is used as a feedstock for the mass process (85), the mbber becomes insoluble in the mixture of monomers and SAN polymer which is formed in the reactors, and discrete mbber particles are formed. This is referred to as phase inversion since the continuous phase shifts from mbber to SAN. Grafting of some of the SAN onto the mbber particles occurs as in the emulsion process. Typically, the mass-produced mbber particles are larger (0.5 to 5 llm) than those of emulsion-based ABS (0.1 to 1 llm) and contain much larger internal occlusions of SAN polymer. The reaction recipe can include polymerization initiators, chain-transfer agents, and other additives. Diluents are sometimes used to reduce the viscosity of the monomer and polymer mixture to faciUtate processing at high conversion. The product from the reactor system is devolatilized to remove the unreacted monomers and is then pelletized. Equipment used for devolatilization includes single- and twin-screw extmders, and flash and thin film evaporators. Unreacted monomers are recovered for recycle to the reactors to improve the process yield. 

In order for a soHd to bum it must be volatilized, because combustion is almost exclusively a gas-phase phenomenon. In the case of a polymer, this means that decomposition must occur. The decomposition begins in the soHd phase and may continue in the Hquid (melt) and gas phases. Decomposition produces low molecular weight chemical compounds that eventually enter the gas phase. Heat from combustion causes further decomposition and volatilization and, therefore, further combustion. Thus the burning of a soHd is like a chain reaction. For a compound to function as a flame retardant it must intermpt this cycle in some way. There are several mechanistic descriptions by which flame retardants modify flammabiUty. Each flame retardant actually functions by a combination of mechanisms. For example, metal hydroxides such as Al(OH)2 decompose endothermically (thermal quenching) to give water (inert gas dilution). In addition, in cases where up to 60 wt % of Al(OH)2 may be used, such as in polyolefins, the physical dilution effect cannot be ignored. 

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