Radical reactive site, polymers

Free radicals are independently-existing species which possess an unpaired electron and normally are highly reactive with short lifetimes. Free-radical polymerizations are chain polymerizations in which each polymer molecule grows by addition of monomer to a terminal free-radical reactive site known as an active centre. Consequent upon every addition of monomer, the active centre is transferred to the newly-created chain end. 

First, in composites with high fiber concentrations, there is little matrix in the system that is not near a fiber surface. Inasmuch as polymerization processes are influenced by the diffusion of free radicals from initiators and from reactive sites, and because free radicals can be deactivated when they are intercepted at solid boundaries, the high interfacial area of a prepolymerized composite represents a radically different environment from a conventional bulk polymerization reactor, where solid boundaries are few and very distant from the regions in which most of the polymerization takes place. The polymer molecular weight distribution and cross-link density produced under such diffusion-controlled conditions will differ appreciably from those in bulk polymerizations. 

The time-dependent nature of migration and chemical reaction of free radicals [30] in irradiated polymers can play an important role in altering the polymer structure and properties, e.g., cross-link formation via reactive sites or chain scission, or postirradiation oxidative influences (irradiation in presence of air or oxygen). 

When a PVC film is exposed to the UV-visible radiation of an incandescent lamp in the presence of pure chlorine, at room temperature, the chlorine content of the polymer increases from 56.8 % initially to over 70 I after a few hours of irradiation (8). As the reaction proceeds, the rate of chlorination decreases steadily as shown by the kinetic curves of figure 2, most probably because of the decreasing number of reactive sites on the polymer chain that remain available for the attack by chlorine radicals. 

Under UV irradiation, the photoinitiator cleaves into radical fragments that react with the vinyl double bond and thus initiate the polymerization of the monomer. If the latter molecule contains at least two reactive sites, the polymerization will develop in three dimensions to yield a highly crosslinked polymer network. 

Reactive macroalkyl radicals are formed during stress-initiated scission of the polymer backbone occassioned by the application of mechanical shear during industrial processing of thermoplastic polymers. These radicals undergo further reactions with other species or reactive sites, most important of which is molecular oxygen (dissolved or trapped in the polymer feed), with deleterious consequences. 

Hawker et al. 2001 Hawker and Wooley 2005). Recent developments in living radical polymerization allow the preparation of structurally well-defined block copolymers with low polydispersity. These polymerization methods include atom transfer free radical polymerization (Coessens et al. 2001), nitroxide-mediated polymerization (Hawker et al. 2001), and reversible addition fragmentation chain transfer polymerization (Chiefari et al. 1998). In addition to their ease of use, these approaches are generally more tolerant of various functionalities than anionic polymerization. However, direct polymerization of functional monomers is still problematic because of changes in the polymerization parameters upon monomer modification. As an alternative, functionalities can be incorporated into well-defined polymer backbones after polymerization by coupling a side chain modifier with tethered reactive sites (Shenhar et al. 2004 Carroll et al. 2005 Malkoch et al. 2005). The modification step requires a clean (i.e., free from side products) and quantitative reaction so that each site has the desired chemical structures. Otherwise it affords poor reproducibility of performance between different batches. 

During the plasma surface reaction, the plasma and the solid are in physical contact, but electrically isolated. Surfaces in contact with the plasma are bombarded by free radicals, electrons, ions, and photons, as generated by the reactions listed above. The energy transferred to the solid is dissipated within the solid by a variety of chemical and physical processes, as illustrated in Figure 7.95. These processes can change surface wettability (cf. Sections 1.4.6 and 2.2.2.3), alter molecular weight of polymer surfaces or create reactive sites on polymers. These effects are summarized in Table 7.21. 

Tcrminadon is commonly diffusion-controlled, i.c., it is governed by the rate at which the reactive sites in growing radicals can come together rather than by chemical factors. In viscous media, termination may be so seriously impeded that both the overall rate of polymerization and the degree of polymerizadon increase markedly. In systems where the polymer is insoluble in the reacdon medium, polymer radicals may be trapped in the precipitated material and be able to grow but unable to participate in temunation processes. 

In this process, some initiator molecule adds to one carbon of the CC double bond of the monomer to generate a reactive site, such as a radical or a caibocation, at the other carbon. This reactive carbon species then adds to another monomer to produce another reactive carbon species, and the process continues until a laige number of monomers have been connected. Another way to represent this reaction shows the repeating unit that is formed when the monomers react to give the polymer ... 

Growth is terminated by coupling of radicals from two growing polymer molecules or by the reactive sites becoming buried in the polymer matrix. Polymers formed by the di-p-xylylene process have been shown (6, 7,8, 9,10, II, 13) to be living polymers and exhibit radical concentrations of 5-10 X 10 4 mole of free electrons per mole of p-xylylene. 

Cellulose can also be modified by introducing long-chain polymer(s) onto its main chain. The preparation of a graft copolymer requires the formation of a reactive site on cellulose in the presence of a polymerizable monomer. The principal techniques frequently used are (1) grafting initiated by free radical polymerization, (2) grafting initiated by ionic polymerization (3)... 

Although the telechelic functional polymers are very attractive from a fundamental point of view, their synthesis is often impossible. Much more commonly, the active groups are incorporated in the chain either by a free-radical copolymerization with a small amount of functionalized comonomer or by functionalization of the chain after polymerization in the presence of free radicals (typical of the functionalization of the polyolefins). Either method generally produces several reactive sites per chain. 

Crosslinked copolymer formation may also occur via direct reaction of mechano-chemically-generated free radicals of each polymer in the absence of added radical initiator, or through radical trapping by a reactive site, such as olefin or acetylene groups in the second polymer (Process 4a). Alternatively, a radical initiator may be added to generate radical sites on one or both polymers (Process 4c). A crosslinked copolymer will be formed if the involved polymers are not degraded. 

---