Polymerization of lipophilic monomers

The dispersion polymerization of lipophilic monomers in CO2 is initiated homogeneously with chain collapse into a discrete polymer particle at a critical molecular... 

A substantial number of studies on the dispersion polymerization of lipophilic monomers in CO2 have focused on methyl methacrylate (MMA). The first success in this area exploited the amphiphilic nature of the homopol5uner PFOA as a stabilizer (40) (eq. (2)). 

The performance of secondary alkanesulfonates in applications as emulsifiers in the widespread emulsion polymerization of vinyl monomers can be assessed by their hydrophilic-lipophilic balance (HLB) numbers. The HLB numbers can... 

Greth and Wilson [100] successfully applied, in the emulsion polymerization of unsaturated monomers, a method of classifying emulsifiers based on the HLB (hydrophilic/lipophilic balance) value. They plotted the most important properties of the emulsion polymerization system, i.e., the latex stability, particle size, emulsion viscosity, and rate of polymerization against the HLB value of emulsifiers used. The dependence is described by a curve with maximum or minimum at a certain value of HLB, as it is expected from the micellar model [6, 7]. 

In 1994, we reported the dispersion polymerization of MM A in supercritical C02 [103]. This work represents the first successful dispersion polymerization of a lipophilic monomer in a supercritical fluid continuous phase. In these experiments, we took advantage of the amphiphilic nature of the homopolymer PFOA to effect the polymerization of MMA to high conversions (>90%) and high degrees of polymerization (> 3000) in supercritical C02. These polymerizations were conducted in C02 at 65 °C and 207 bar, and AIBN or a fluorinated derivative of AIBN were employed as the initiators. The results from the AIBN initiated polymerizations are shown in Table 3. The spherical polymer particles which resulted from these dispersion polymerizations were isolated by simply venting the C02 from the reaction mixture. Scanning electron microscopy showed that the product consisted of spheres in the pm size range with a narrow particle size distribution (see Fig. 7). In contrast, reactions which were performed in the absence of PFOA resulted in relatively low conversion and molar masses. Moreover, the polymer which resulted from these precipitation... 

In inverse polymerization, water-soluble monomers are emulsified with low HLB (hydrophilic-lipophilic balance) surfactants in an organic medium and the reaction is initiated with water-soluble or oil-soluble initiators. A review of the subject can be found in a recent publication of Greenshields [46]. 

Besides this work with rather lipophilic catalyst precursors, two water-soluble analogues have been investigated [Rh(tos)(cod)(H2O)] (30 tos = tosylate) and the similar compound [Rh(tos)(nbd)(H20)] (31) [148]. The authors have verified that water is still the best reaction medium for polymerization of phenylacetylene compared to toluene, THF and neat monomer, thus indicating a possible involvement of water in the formation of the active catalyst (830 TO in water). With catalyst precursor 30, low polydispersities and a cis content of 90% are found, whereas catalyst precursor 31 affords a larger polydispersity but a higher cis content (100%). Interestingly, these catalysts proved to be tolerant to air since similar polymerization results are obtained with tap water without exclusion of air. 

A considerable amount of work has focused on the design and synthesis of macromolecules for use as emulsifiers for lipophilic materials and as polymeric stabilizers for the colloidal dispersion of lipophilic, hydrocarbon polymers in compressed CO2. It has been shown that fluorinated acrylate homopolymers, such as PFOA, are effective amphiphiles as they possess a lipophilic acrylate-like backbone and C02-philic, fluorinated side chains, as indicated in Figure 4.5-1 [100]. Furthermore, it has been demonstrated that a homopolymer which is physically adsorbed to the surface of a polymer colloid precludes coagulation due to the presence of loops and tails [110]. These fluorinated acrylate homopolymers can be synthesized homogeneously in CO2 as described in an earlier section. The solution properties [111,112] and phase behavior [45] of PFOA in SCCO2 have been thoroughly examined. Additionally, the backbone of these materials can be made more lipophilic in nature by incorporating other monomers to make random copolymers [34]. 

Qi and coworkers reported the first study combining RAFT and inverse miniemulsion. Their inverse miniemulsion system comprised cyclohexane as the continuous phase, B246SF as the surfactant, and aqneous solution containing a CTA, acrylamide, and costabilizer MgS04. They found that using a water-soluble initiator, 4,4 -azobis(4-cyanovaleric acid), afforded better eontrol of the polymerization of acrylamide than using a lipophilic one, 2,2 -azobis(2-methylpropionitrile) (AIBN). RAFT control was realized up to 50% monomer conversion and after that significant deviation from RAFT control was observed. More recently, they have extended their RAFT inverse miniemulsion polymerization approach to other hydrophilic (co)polymers. ... 

Polymerizations with SSC (2). Previously NaSS-styrene copolymer latexes have been made, but <3% of NaSS could be incorporated without simultaneous formation of a soluble polyelectrolyte or a bimodal distribution of particle sizes (15-17), Hence, a lipophilic monomer that is freely miscible with styrene and VBC must be employed in order to achieve a high charge density styrenesulfonate latex. 

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