Polymerization mechanisms chain growth

Table 2.1 Advantages and Disadvantages of Chain Growth Polymerization Mechanisms ...

An oligomer is a very low molecular weight polymer. It consists of only a anall number of mers. The definition of a telomer is that of a chain-growth polymer that is composed of molecules with end groups consisting of different species from the monomer units. Telomers can form by either free-radical or by ionic chain-growth polymerization mechanism. 

As two non-petroleum chemicals readily accessible from renewable resources, both furfural and HMF are suitable starting materials for the preparation of versatile fine chemicals [14, 102-106] and can also serve as renewable monomers for preparation of sustainable polymer products [107]. Schemes 3, 4, and 5 depict the stmctures of the selected furan-based monomers [107-113]. As a typical precursor, furfural can be converted to a vast array of furan-based monomers bearing a moiety which can normally be polymerized by chain-growth polymerization mechanisms [108-113]. As shown in Scheme 3, these monomers are all readily polymerizable by chain-growth reactions. However, depending on their specific structure, the nature of the polymerization mechanism is different, ranging from free radical, cationic, anionic, to stereospecific initiation [108-113]. On the other hand, furfuryl... 

The practical value of the Fischer-H opsch reaction is limited by the unfavorable Schulz-Flory distribution of hydrocarbon products that is indicative of a chain growth polymerization mechanism. In attempts to increase the yields of lower hydrocarbons such as ethylene and propylene (potentially valuable as feedstocks to replace petrochemicals), researchers have used zeolites as supports for the metals in attempts to impose a shape selectivity on the catalysis [114] or to control the performance through particle size effects. [IIS] These attempts have been partially successful, giving unusual distributions of products, such as high yields of C3 [114] or C4 hydrocarbons. [116] However, the catalysts are often unstable because the metal is oxidized or because it migrates out of the zeolite cages to form crystallites, which then give the Schulz-Flory product distribution. 

Thus, the monomer 26a was polymerized with LiHMDS as a base in the presence of a core initiator and LiQ at 30°C to yield hyperbranched polyamide (HBPA) with narrow molecular weight distribution (MJM < 1.14) and a degree of branching of about 0.5. The matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectra showed that all HBPAs with different molecular weights contained the initiator unit. The value of HBPA increased linearly in proportion to the ratio of [26a](/[initiator]0 up to 40,000, while the M /M ratio remained at 1.14 or less. Therefore, the polymerization of 26a proceeds through a chain-growth polymerization mechanism from the initiator without side reaction... 

Step-growth polymerizations can be schematically represented by one of the individual reaction steps VA + B V —> Vab V with the realization that the species so connected can be any molecules containing A and B groups. Chain-growth polymerization, by contrast, requires at least three distinctly different kinds of reactions to describe the mechanism. These three types of reactions will be discussed in the following sections in considerable detail. For now our purpose is to introduce some vocabulary rather than develop any of these beyond mere definitions. The principal steps in the chain growth mechanism are the following ... 

Photoinitiation is not as important as thermal initiation in the overall picture of free-radical chain-growth polymerization. The foregoing discussion reveals, however, that the contrast between the two modes of initiation does provide insight into and confirmation of various aspects of addition polymerization. The most important application of photoinitiated polymerization is in providing a third experimental relationship among the kinetic parameters of the chain mechanism. We shall consider this in the next section. 

Both modes of ionic polymerization are described by the same vocabulary as the corresponding steps in the free-radical mechanism for chain-growth polymerization. However, initiation, propagation, transfer, and termination are quite different than in the free-radical case and, in fact, different in many ways between anionic and cationic mechanisms. Our comments on the ionic mechanisms will touch many of the same points as the free-radical discussion, although in a far more abbreviated form. 

Fig. I. Mechanism for free radical chain growth polymerization.

Chain gro tvth polymerization begins when a reactive species and a monomer react to form an active site. There are four principal mechanisms of chain growth polymerization free radical, anionic, cationic, and coordination polymerization. The names of the first three refer to the chemical nature of the active group at the growing end of the monomer. The last type, coordination polymerization, encompasses reactions in which polymers are manufactured in the presence of a catalyst. Coordination polymerization may occur via a free radical, anionic, or cationic reaction. The catalyst acts to increase the speed of the reaction and to provide improved control of the process. 

Chain-growth polymerizations are diffusion controlled in bulk polymerizations. This is expected to occur rapidly, even prior to network development in step-growth mechanisms. Traditionally, rate constants are expressed in terms of viscosity. In dilute solutions, viscosity is proportional to molecular weight to a power that lies between 0.6 and 0.8 (22). Melt viscosity is more complex (23) Below a critical value for the number of atoms per chain, viscosity correlates to the 1.75 power. Above this critical value, the power is nearly 3 4 for a number of thermoplastics at low shear rates. In thermosets, as the extent of conversion reaches gellation, the viscosity asymptotically increases. However, if network formation is restricted to tightly crosslinked, localized regions, viscosity may not be appreciably affected. In the current study, an exponential function of degree of polymerization was selected as a first estimate of the rate dependency on viscosity. 

PHA is produced in Alcaligenes eutrophus from acetyl CoA in three steps and the last step is the chain growth polymerization of hydroxyalkanoate CoA esters catalyzed by PHA polymerase (synthase), yielding PHA of high molecular weight. Kinetics and mechanism of the polymerization of hydroxyalkanoyl CoA monomers with this bacterial polymerase have been investigated. 

On the basis of polymerization mechanism, the processes of polymerization can be classified in two groups (i) step growth polymerization and (ii) chain growth polymerization. 

Chain-growth polymerizations are characterized by chains that propagate by adding one monomer molecule at a time, ie, rr-mer + monomer — x + l)-mer. There are, however, several mechanisms by which this occurs. 

Some 50 years ago, Paul Flory chose the terms step-growth and chain-growth polymerization to describe the processes by which many monomers are converted to polymer (Flory 1953). Although not perfect, the terms are still commonly used and can help us understand the major mechanisms of polymerizations. A mechanism for a reaction describes the processes and pathways by which that reaction proceeds. Mechanisms are important because they help us understand the details of a chemical reaction as well as help us predict the outcome of new reactions. 

The polymerization of some monomers does not fall neatly into either of the mechanisms discussed above. We will take up a few of them (e.g., anionic and coordination polymerizations) after we further develop step-growth and chain-growth polymerizations. Some polymerizations can proceed by either mechanism, depending upon the specific monomer or the reaction conditions. The most notable examples, ring-opening polymerization and some of the newer chemistries, are presented as separate categories toward the end of the chapter. 

Use mechanisms to show how monomers polymerize under acidic, basic, or free-radical conditions. For chain-growth polymerization, determine whether the reactive end is more stable as a cation (acidic conditions), anion (basic conditions), or free radical (radical initiator). For step-growth polymerization, consider the mechanism of the condensation. 

Lewis acids initiate cationic chain-growth polymerizations. There are several possible chain propagation reactions, and the mechanism of cationic chain growth is still open to... 

The most common type of chain-growth polymerization is free-radical polymerization. An initiator or a photochemical reaction produces a free radical that attaches itself to a monomer molecule, creating a group with odd-electron configuration (reactive center) at which monomer molecules are added until two such centers react with one another or, more rarely, a center is deactivated by some other process. This is a mechanism much like that of ordinary chain reactions (see Chapter 9 the term "chain" in chain growth refers to that kind of mechanisms, not to the growing molecular chain of repeating units in the polymer.)... 

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