Hydrophobic monomers

Early efforts to produce synthetic mbber coupled bulk polymerization with subsequent emulsification (9). Problems controlling the heat generated during bulk polymerization led to the first attempts at emulsion polymerization. In emulsion polymerization hydrophobic monomers are added to water, emulsified by a surfactant into small particles, and polymerized using a water-soluble initiator. The result is a coUoidal suspension of fine particles,... 

The Smith-Ewart expression (eq. 1) accurately predicts the particle number for hydrophobic monomers like styrene and butadiene (21), but fails to predict the particle number (22) for more hydrophilic monomers like methyl methacrylate and vinyl acetate. A new theory based on homogeneous particle... 

Emulsification is the process by which a hydrophobic monomer, such as styrene, is dispersed into micelles and monomer droplets. A measure of a surfactant s abiUty to solubilize a monomer is its critical micelle concentration (CMC). Below the CMC the surfactant is dissolved ia the aqueous phase and does not serve to solubilize monomer. At and above the CMC the surfactant forms spherical micelles, usually 50 to 200 soap molecules per micelle. Many... 

Mechanical properties of a hydrogel lens also are affected by the use of a hydrophobic monomer, such as a low alkyl methacrylate. This is particularly important when the water content of the hydrogel lens is very high. The use of these methacrylates helps preserve the required mechanical strength. Methyl methacrylate [80-62-6] (MMA) (I2I), isobutyl methacrylate [97-86-9] (122), and / -pentyl methacrylate [2849-98-1] (123) all have been used for this purpose. 

Because of the many choices of hydrophilic monomers, cross-linkers, and hydrophobic monomers, a large number of formulations have been developed and manufactured into hydrogel lenses. The water content of these hydrogel lenses ranges from about 38%, for HEMA-based lenses, to 80%, for poly(vinyl alcohol) and partially hydrolysed acrylonitrile lenses. Table 2 gives a representative Hst of FDA approved hydrogel materials available to the consumer in the early 1990s. 

Following is a typical procedure for the emulsion polymerization of hydrophobic monomers in batch fashion. The first step in this process is the preparation of the continuous phase solution. As mentioned before, water is usually selected as the base material of the continuous phase for the emulsion polymerization of hydrophobic... 

The uniform polymeric microspheres in submicron-or micron-size range can also be prepared as seed particles by the soapless emulsion or dispersion polymerization of a hydrophobic monomer like styrene. The uniform seed particles are swollen with the organic phase including functional comonomer, monomer, and oil-soluble initiator at a low temperature in an aqueous... 

The charged segments of the AMPS units in these amphiphilic copolymers effectively solubilize the sequences of hydrophobic monomer units to water. In fact, the copolymers ASt-72 (7 with x = 72), APh-50 (8 with x = 50), APy-50 (9 with x = 50), and ALa-44 (10 with x = 44) were all soluble in water. The copolymer ACh-x was a little less water soluble ACh-23 (11 with x = 23) was almost soluble, whereas ACh-60 was insoluble. All these copolymers were soluble in methanol, N, AT-dimethylformamide, and dimethylsulfoxide, but insoluble in most of other common organic solvents. 

Radicals typically are generated in the aqueous phase and it is now generally believed that formation of an oligomer of average chain length z (z-mer P/) occurs in the aqueous phase prior to particle entry.44 The steps involved in forming a radical in the particle phase from an aqueous phase initiator are summarized in Scheme 3.17. The length of the z-mcr depends on the particular monomer and is shorter for more hydrophobic monomers. 

A novel approach to RAFT emulsion polymerization has recently been reported.461529 In a first step, a water-soluble monomer (AA) was polymerized in the aqueous phase to a low degree of polymerization to form a macro RAFT agent. A hydrophobic monomer (BA) was then added under controlled feed to give amphiphilic oligomers that form micelles. These constitute a RAFT-containing seed. Continued controlled feed of hydrophobic monomer may be used to continue the emulsion polymerization. The process appears directly analogous to the self-stabilizing lattices approach previously used in macromonomer RAFT polymerization (Section 9.5.2). Both processes allow emulsion polymerization without added surfactant. 

A water-soluble copolymer of hydrophilic and hydrophobic monomers, acrylamide-acrylate of silane or siloxane... 

A terpolymer can be obtained from a water-soluble nonionic monomer, such as acrylamide a cationic monomer, such as 3-acrylamidopropyltrimethyl ammonium chloride and a hydrophobic monomer, such as an alkyl-acrylamide... 

Table 2 Nonionic, Hydrophobic Monomers Used for Hydrogel Synthesis... 

To strengthen mechanical properties and vary the transition temperature, hydrophobic monomers such as BMA were introduced into poly(IPAAm). The unique swelling properties with varying BMA composition are shown in Figure 11. 

Hard contact lenses are composed of a polymer that repels water because the constituent repeating units (the monomers that link together to form the polymer) are nonpolar, hydrophobic segments. The first hard contact lens was constructed in 1948 from the monomer known as methyl methacrylate (MMA), yielding the polymer poly(methyl methacrylate) or PMMA. This material offers durability, optical transparency, and acceptable wettability for optimal comfort. Today the rigid lens material of hard contact lenses is often constructed by combining MMA with one or more additional hydrophobic monomers to provide better gas permeability. 

Emulsifiable oil metal-working fluids, 1 22 Emulsification, 10 126 16 211 cosmetics, 7 837-841 hydrophobic-monomer, 14 717 Emulsified wax, in paper manufacture,... 

As an extension of the perspective of micelle formation by amphiphihc block copolymers the following part will focus on two other types of polymers. The micellar structures that will discussed are (i) micelles and inverse micelles based on a hyperbranched polymers and (ii) polysoaps, that are copolymers composed of hy-drophihc and amphiphihc or hydrophobic monomers. Whereas the first class of polymers is stiU very new and only few examples exist of the synthesis and appH-cation of such stracture in catalysis, the synthesis and aggregation characteristics of polysoaps has already been intensively discussed in the hterature. 

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