Monomer continued radicals

According to the first method, each micelle in an emulsion behaves like a separate micro-continuous reactor which contains all the components, i.e. monomers and radicals from the aqueous phase. Thus, analogous to the latex particles in emulsion polymerization, microgels formed by emulsion polymerization are distributed in the whole available volume. 

One proposed mechanism for the electrochemical polymerization of aniline is shown in Fig. 66 [287]. Aniline is oxidized to cation radical 445 which dimerizes to form dication dimer 446. Deprotonation (- 2H +) of 446 gives 443. The oxidation of dimer 443 gives cation radical 447 which is further oxidized to diiminium dication 448. The coupling of 448 with 445 followed by the loss of two protons gives 449. The addition of aniline units to the polymer chain continues in a similar manner by the coupling of the terminal diiminium dication group with monomer cation radical 445. 

Polymerization of unsaturated organic compounds (monomers) is a special type of chain reaction. In a process of breaking of multiple bonds, new monomers continuously attach to the radical or ionic chain carriers. An example of this is the cationic polymerization of vinylchloride into polyvinylchloride (PVC) ... 

Emulsion polymerization Emulsion polymerization starts with a dispersion of monomer micelles in a nonsolvent medium stabilized by appropriate surfactant molecules. Polymerization proceeds within each spherical micelle once a radical enters the micelle from the continuous phase, which finally results in a dispersion of latex. Dispersed phase (monomer), continuous phase (nonsolvent and initiator), surfactant < 1 pm, uniform in size... 

A propagating polymeric radical with a PAVE active-radical center can have one of two possible reaction pathways. First, and most obvious, it can cross-pro-pagate to monomer, continuing the polymerization reaction, or it can undergo yS-scission, resulting in an acid fluoride-terminated polymer and a perfluoroalkyl radical capable of initiating further polymerization. Essentially this is a chain-transfer-to-monomer step, the details of which are outlined in Scheme 9.1 [6, 9]. 

Figure 3.33 lists a recipe for emulsion polymerization of polystyrene in a water dispersion of monomer droplets and soap micelles [20]. The reaction is started by light-sensitive, water-soluble initiators, such as benzoyl peroxide. If one compares the sizes of the dispersed droplets, one notices that the small soap micelles that contain also styrene in their interior are most likely to occasionally initiate a polymerization of the monomer on absorption of a free radical. Once initiated, the reaction continues until a second free radical molecule enters the micelle. Then the reaction is terminated, until a third radical starts another molecule. Monomers continuously add to the micelles, so that the polymerization continues. Keeping the free radical generation constant, a relatively narrow molar mass distribution can be obtained. 

The monomer continues to react with the end of the growing polymer chain throughout an addition polymerization reaction until the reactive intermediate is destroyed in a termination reaction. Disproportionation and dimerization are two possible termination reactions. In disproportionation, a hydrogen atom at a carbon atom a to the radical center is abstracted by a radical in another chain. This produces a double bond in one polymer molecule, and the other polymer molecule becomes saturated. Because no new radical intermediates are formed, the propagation steps are terminated. 

A chain transfer reagent must be reactive enough to transfer a hydrogen atom, but the resulting radical must be reactive enough to add to a double bond. The polymerization continues, and monomer continues to be consumed. However, the average molecular weight of the product is smaller because more chains are formed by the chain transfer process. 

The monomer droplets and the micelles swollen with monomer compete for the free radicals generated in the aqueous phase, but since there are many more micelles than droplets in the system most of the free radicals enter micelles. Polymerization is initiated within individual micelles. The monomer consumed during the resulting polymerization is replenished by diffusion of new monomer molecules from the aqueous phase, which in turn, is kept saturated with monomer from the droplets of monomer. Polymerization continues within a given micelle until a second free radical enters the micelle, in which case termination quickly occurs because of the small volume of the reaction locus. The micelle then remains inactive until a third free radical enters, and so on. As reaction proceeds the micelles become larger and are disrupted to form particles of polymer swollen with monomer which are stabilized by soap molecules around the periphery. Monomer continues to diffuse into these particles and polymerization is maintained therein until the monomer supply is exhausted. The final product is a stable dispersion (latex)... 

When initiator is first added the reaction medium remains clear while particles 10 to 20 nm in diameter are formed. As the reaction proceeds the particle size increases, giving the reaction medium a white milky appearance. When a thermal initiator, such as AIBN or benzoyl peroxide, is used the reaction is autocatalytic. This contrasts sharply with normal homogeneous polymerizations in which the rate of polymerization decreases monotonicaHy with time. Studies show that three propagation reactions occur simultaneously to account for the anomalous auto acceleration (17). These are chain growth in the continuous monomer phase chain growth of radicals that have precipitated from solution onto the particle surface and chain growth of radicals within the polymer particles (13,18). 

The following conditions are stipulated the catalyst decomposition rate constant must be one hour or greater the residence time of the continuous reactor must be sufficient to decompose the catalyst to at least 50% of the feed level the catalyst concentration must be greater than or equal to 0.002 x Q, where the residence time, is expressed in hours. An upper limit on the rate of radical formation was also noted that is, when the rate of radical formation is greater than the addition rate of the primary radicals to the monomers, initiation efficiency is reduced by the recombination of primary radicals. 

Bulk Polymerization. This is the method of choice for the manufacture of poly(methyl methacrylate) sheets, rods, and tubes, and molding and extmsion compounds. In methyl methacrylate bulk polymerization, an auto acceleration is observed beginning at 20—50% conversion. At this point, there is also a corresponding increase in the molecular weight of the polymer formed. This acceleration, which continues up to high conversion, is known as the Trommsdorff effect, and is attributed to the increase in viscosity of the mixture to such an extent that the diffusion rate, and therefore the termination reaction of the growing radicals, is reduced. This reduced termination rate ultimately results in a polymerization rate that is limited only by the diffusion rate of the monomer. Detailed kinetic data on the bulk polymerization of methyl methacrylate can be found in Reference 42. 

Figure 4c illustrates interfacial polymerisation encapsulation processes in which the reactant(s) that polymerise to form the capsule shell is transported exclusively from the continuous phase of the system to the dispersed phase—continuous phase interface where polymerisation occurs and a capsule shell is produced. This type of encapsulation process has been carried out at Hquid—Hquid and soHd—Hquid interfaces. An example of the Hquid—Hquid case is the spontaneous polymerisation reaction of cyanoacrylate monomers at the water—solvent interface formed by dispersing water in a continuous solvent phase (14). The poly(alkyl cyanoacrylate) produced by this spontaneous reaction encapsulates the dispersed water droplets. An example of the soHd—Hquid process is where a core material is dispersed in aqueous media that contains a water-immiscible surfactant along with a controUed amount of surfactant. A water-immiscible monomer that polymerises by free-radical polymerisation is added to the system and free-radical polymerisation localised at the core material—aqueous phase interface is initiated thereby generating a capsule sheU (15). 

One of the key benefits of anionic PS is that it contains much lower levels of residual styrene monomer than free-radical PS (167). This is because free-radical polymerization processes only operate at 60—80% styrene conversion, whereas anionic processes operate at >99% styrene conversion. Removal of unreacted styrene monomer from free-radical PS is accompHshed using continuous devolatilization at high temperature (220—260°C) and vacuum. This process leaves about 200—800 ppm of styrene monomer in the product. Taking the styrene to a lower level requires special devolatilization procedures such as steam stripping (168). 

Monomer molecules, which have a low but finite solubility in water, diffuse through the water and drift into the soap micelles and swell them. The initiator decomposes into free radicals which also find their way into the micelles and activate polymerisation of a chain within the micelle. Chain growth proceeds until a second radical enters the micelle and starts the growth of a second chain. From kinetic considerations it can be shown that two growing radicals can survive in the same micelle for a few thousandths of a second only before mutual termination occurs. The micelles then remain inactive until a third radical enters the micelle, initiating growth of another chain which continues until a fourth radical comes into the micelle. It is thus seen that statistically the micelle is active for half the time, and as a corollary, at any one time half the micelles contain growing chains. 

---