Main-chain polymer radical

FIGURE 14.2 X-band TREPR spectra of main-chain polymer radical la produced from 248 nm laser flash photolysis of atactic, isotactic, and syndiotactic PMMA in propylene carbonate at 0.8 ps delay time. The temperature for each spectrum is shown in °C, and the magnetic held sweep width is 150 G for aU spectra, which exhibit net E CIDEP in aU cases. Simulations of each fast motion spectrum (highest temperature) are shown at the bottom of each data set. Hyperflne values for each simulation are 3 anCCHs) = 22.9 G, 2aH(CH2)= 16.4 G, 2aH(CH2) = 11.7G for isotactic PMMA 3ah(CH3) = 22.9G, 2aH(CH2>= 16.2G,... 

All these spectra were acquired at elevated temperatures ( 100°C), that is, where the observation of fast motion spectra is expected. In Fig. 14.4A, the TREPR spectrum of the main-chain polymer radical from photolysis of /-PMMA is repeated from the bottom left side of Fig. 14.2, as it is the starting point for comparisons of spectral features such as hnewidths and hyperfine coupling constants. The nomenclature used throughout this section is derived using the notations indicated in Scheme 14.1 and Chart 14.1. For example, a main-chain radical from PMMA will be denoted la, whereas the oxo-acyl radical from PFOMA will be designated as radical 6b, and so on. For all radicals simulated, the parameters used are listed in Table 14.1. 

SCHEME 14.1 The general stmcture of an acrylic polymer and the estahhshed photodegradation mechanism via Norrish I a-cleavage of the carhonyl side chain, leading to main-chain polymeric radical a and oxo-acyl radical b. The secondary P-scission rearrangement reaction leading to the propagating radical c is also shown. 

For main-chain acrylic radicals, created in solution at room temperature and above, the presence of a superposition of conformations or Gaussian distributions is unlikely. Polymers undergo conformational jumps on the submicrosecond timescale, even in bulk at room temperamre. ° The first two theories above require that the radicals be fairly rigid with little (Gaussian distribution) or no (superposition of static conformations) movement around the Cp bond. The main-chain radical is sterically hindered but still quite flexible, and a dramatic change in the hybridization at Ca is unlikely. We have approached our simulations with the hyperfine modulation model. 

The TREPR experiments and simulations described here have provided an enormous amount of structural and dynamic information about a class of free radicals that were not reported in the hterature prior to our first paper on this topic in 2000. Magnetic parameters for many main-chain acrylic radicals have been established, and interesting solvent effects have been observed such as spin relaxation rates and the novel pH dependence of the polyacid radical spectra. It is fair to conclude from these studies that the photodegradation mechanism of acrylic polymers is general, proceeding through Norrish 1 a-cleavage of the ester (or acid) side chain. Recently, model systems have... 

Combination of a living ionic polymerization and a metal-catalyzed radical polymerization also leads to comb polymers, where both the molecular weights of the arm and main-chain polymers are well controlled. PMMA with poly(vinyl ether) arm polymers of controlled molecular weights (C-l) were prepared by the copper-catalyzed radical polymerization of methacrylate-capped macromonomers carrying a poly-(isobutyl vinyl ether), which were obtained by living cationic polymerization with a methacryloxy-capped end-functionalized initiator.428 Comb polymers with... 

Another method is based on the metal-catalyzed polymerization from carbon—halogen bonds in the main-chain units, which was applied for the synthesis of C-3 and C-4.430 For C-3, the main chain polymers with controlled molecular weights were prepared via the copper-catalyzed radical polymerization of tri-methylsilyl-protected HEMA followed by the transformation of the silyloxyl group into 2-bromoisobu-tyrate. The pendant C—Br bonds were subsequently activated by the copper catalysts to polymerize styrene and nBA. A more direct way is employed for C-4 i.e., via conventional radical polymerization of 2-[(2-bromopropinonyl)oxy]ethyl acrylate followed by the copper-catalyzed graft polymerization of styrene and nBA from the C—Br substituent. 

As a companion to the biguanide main-chain polymers, acrylic polymers with biguanide pendant groups were also sjmthesized (Figure 19) (polymer Cll). Polymers were synthesized by free radical polymerization, both as homopolymers and as copolymers of the biguanide monomer with acrylamide. MIC values for these polymers were substantially higher than for the monomer. The authors hypothesized that these results may be due to the fact that the polymers complex with anionic components in the culture media. As a comparison to this... 

Industrially, ethylene is polymerized by the high-, medium-, or low-pressure process in bulk, solution, or in the gas phase (Table 25-1). The high-pressure-process polymerizes by a free radical mechanism addition of about 0.05% oxygen to ethylene presumably produces CH2=CH(OOH), which decomposes to provide the start free radicals. Correspondingly, hydroxyl groups have been found in high-pressure poly (ethylene). Inter-molecular transfer by polymer or initiator free radicals produce main-chain free radicals that initiate the polymerization of ethylene ... 

Table 3. Radiochemical Yields of Formation of Free Radicals in Aromatic Main-Chain Polymers... 

Needless to say, vinyl polymerization is one of the most important methods for polymer synthesis. A variety of carbon-carbon (C-C) main chain polymers have been prepared by the vinyl polymerization of monomers with diverse substituents, via radical, cationic, anionic, or coordination mechanism. Furthermore, with the technological achievement such as living and stereoselective (or stereospecific) polymerizations, fine-tuning of the polymer structure with respect to molecular weight and tacticity has been realized in a number of examples. In particular, polymers obtained with vinyl polymerization (vinyl polymer) as represented by polyethylene, polypropylene, polystyrene, and poly(methyl methacrylate) have contributed to the progress of modern society in various aspects as useful synthetic materials. 

The newly formed short-chain radical A then quickly reacts with a monomer molecule to create a primary radical. If subsequent initiation is not fast, AX is considered an inhibitor. Many have studied the influence of chain-transfer reactions on emulsion polymerisation because of the interesting complexities arising from enhanced radical desorption rates from the growing polymer particles (64,65). Chain-transfer reactions are not limited to chain-transfer agents. Chain-transfer to monomer is ia many cases the main chain termination event ia emulsion polymerisation. Chain transfer to polymer leads to branching which can greatiy impact final product properties (66). 

Other chain transfer processes may occur. For example, the radical may abstract an atom from along the backbone of a previously formed polymer molecule, and thus initiate the growth of a branch to the main chain. There can also be chain transfer to monomer, which in the nature of the polymerisation process must be a relatively rare phenomenon. However, it can occur infrequently and give rise to a restriction in the size of the polymer molecules without ceasing the overall radical chain reaction. 

Homolytic bond cleavage from excited states in irradiated polymers [30] can lead to a pair of free radicals via bond scission, involving main chain or side-chain substituents. 

In the case of radical formation by the main-chain scission of the polymer molecule, a high concentration of the radicals initiates the reactions involving the geminate pair [30]. 

Photoinduced free radical graft copolymerization onto a polymer surface can be accomplished by several different techniques. The simplest method is to expose the polymer surface (P-RH) to UV light in the presence of a vinyl monomer (M). Alkyl radicals formed, e.g. due to main chain scission or other reactions at the polymer surface can then initiate graft polymerization by addition of monomer (Scheme 1). Homopolymer is also initiated (HRM-). 

As for any chain reaction, radical-addition polymerization consists of three main types of steps initiation, propagation, and termination. Initiation may be achieved by various methods from the monomer thermally or photochemically, or by use of a free-radical initiator, a relatively unstable compound, such as a peroxide, that decomposes thermally to give free radicals (Example 7-4 below). The rate of initiation (rinit) can be determined experimentally by labeling the initiator radioactively or by use of a scavenger to react with the radicals produced by the initiator the rate is then the rate of consumption of the initiator. Propagation differs from previous consideration of linear chains in that there is no recycling of a chain carrier polymers may grow by addition of monomer units in successive steps. Like initiation, termination may occur in various ways combination of polymer radicals, disproportionation of polymer radicals, or radical transfer from polymer to monomer. 

A bulky methacrylate, triphenylmethyl methacrylate (TrMA), is a unique monomer which gives an almost 100% isotactic polymer in anionic polymerization with n-butyllithium both in nonpolar and polar solvents. Moreover, even free-radical polymerization affords a highly isotactic polymer from this monomer.23 The isotactic specificity of TrMA polymerization is ascribed to the helical formation of the main chain. When TrMA is polymerized in toluene at —78°C... 

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