Oligomers emulsion polymerization

Figure C2.3.11 Key surfactant stmctures (not to scale) in emulsion polymerization micelles containing monomer and oligomer, growing polymer particle stabilized by surfactant and an emulsion droplet of monomer (reservoir) also coated with surfactant. Adapted from figure 4-1 in [67],...

The progression of an ideal emulsion polymerization is considered in three different intervals after forming primary radicals and low-molecular weight oligomers within the water phase. In the first stage (Interval I), the polymerization progresses within the micelle structure. The oligomeric radicals react with the individual monomer molecules within the micelles to form short polymer chains with an ion radical on one end. This leads to the formation of a new phase (i.e., polymer latex particles swollen with the monomer) in the polymerization medium. 

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. 

In the case of more water-soluble monomers and (amphiphilic) macromonomers, the Smith-Ewart [16] expression does not satisfactorily describe the particle nucleation. The HUFT [9,10] theory, however, satisfactorily describes the polymerization behavior or the particle nucleation of such unsaturated hydrophilic and amphiphilic monomers. The HUFT approach implies that primary particles are formed in the aqueous phase by precipitation of oligomer radicals above a critical chain length. The basic principals of the HUFT theory is that formation of primary particles will take place up to a point where the rate of formation of radicals in the aqueous phase is equal to the rate of disappearance of radicals by capture of radicals by particles already formed. Stabilization of primary particles in emulsifier-free emulsion polymerization may be achieved if the monomer (or macromonomer) contains surface active groups. Besides, the charged radical fragments of initiator increases the colloidal stability of the polymer particles. 

Thus in the emulsifier-free emulsion copolymerization the emulsifier (graft copolymer, etc.) is formed by copolymerization of hydrophobic with hydrophilic monomers in the aqueous phase. The ffee-emulsifier emulsion polymerization and copolymerization of hydrophilic (amphiphilic) macromonomer and hydro-phobic comonomer (such as styrene) proceeds by the homogeneous nucleation mechanism (see Scheme 1). Here the primary particles are formed by precipitation of oligomer radicals above a certain critical chain length. Such primary particles are colloidally unstable, undergoing coagulation with other primary polymer particles or, later, with premature polymer particles and polymerize very slowly. 

The maleic Surfmers were tested in core-shell emulsion polymerization of styrene/butyl acrylate in comparison with a standard nonreactive surfactant (nonyl phenol reacted with 30 mol of EO - NP30). While the methacrylic-derived Surfmer was completely incorporated during the polymerization (although about one-third of it was buried inside the particles) the NP30, the maleic Surfmer and the allylic and vinyl Surfmers were not incorporated and could be extracted with acetone (for the last two probably because of the formation of acetone-extractable oligomers due to a chain transfer behavior) [31]. 

Emulsion polymerization has become an important process for the production of a large number of industrial polymers in the form of polymer colloids or latexes. They are the base of adhesives, paints and especially of waterborne coatings. An interest has been developed in recent years in emulsion polymerization systems in which the classical low molecular weight surfactaints are replaced by polymeric surfactants, either hydrophilic-hydrophobic block and graft copolymers (1-4) or functionalized oligomers (5). 

Starved-feed emulsion polymerization can be conducted without emulsifiers if suitable comonomers and procedures are utilized.341 Polymerization of a water-soluble methacrylate like HEMA in the presence of a CCT agent is carried out initially. The resulting HEMA oligomer is further copolymerized with hydrophobic monomers so that the resulting diblock copolymer serves as a surfactant (see, for instance, sections 5.3 and 5.4). During the cross-linking process, all of this surfactant is incorporated into the polymer backbone and is thus immobilized, overcoming the problem of residual surfactant in the final product. 

There are few reports on the influence of the initiator on the precipitation polymerization but as expected, the polymerization rate (Figure 12.7) increases together with the initiator concentration, as is often the case in emulsion polymerization. This behavior is attributed to an increase in the polymerization loci. It is interesting to note that an increase in the initiator concentration leads to an increase in the water-soluble polymer formation (oligomers bearing low molecular weight) and... 

Initiation. Water-soluble initiators are normally used in emulsion polymerization, and droplet initiation can only take place when a waterborne oligomer diffuses into the monomer droplet. Although such diffusion does take place, in most emulsion polymerization systems the bulk of the Initiation and propagation occurs in the particles. Oil-soluble initiators... 

A second possible problem with batch reactors is composition drift of copolymer systems. As with bulk, solution, and suspension systems, the more reactive monomer polymerizes first, and the least reactive polymerizes last. Two additional factors must be considered in emulsion polymerization. First, the water solubilities of the monomers can influence the course of the polymerization because of reaction in the water phase to produce copolymer oligomers or even water-soluble polymer. These molecules can be rich in the water-soluble monomer even if its reactivity is relatively low. Second, the high degree of subdivision achieved by producing small polymer particles can lead to phase domains that are smaller than those in copolymer produced by other processes. 

PolywettB. [Uniroyal] Polyfiinctional oligomer salts emulsifier for emulsion polymerization dispersant for miner-... 

I. Emulsion polymerization in the presence of ionic emulsifiers in which radical formation and polymerization initiation occur in aqueous phase and the resulting oligomer radicals precipitate from solution, followed either by homogeneous self-nucleation and formation of Initial polymer particles, or by capture of the already existing polymer particles. 

It must be noted that the process of seeded emulsion polymerization does not lead to an equilibrium structure. Hence, the sharp interface between PS and PMMA observed in the above core-shell particles cannot be explained by thermodynamic arguments. A possible mechanism maybe sought in the adsorption of oligo(methylmethacrylate) radicals from the water phase onto the PS-seed particles [45]. The temperature of the seeded emulsion polymerization (80 °C [45]) is well below the glass transition temperature of polystyrene and the adsorbed chains bear a sulfate endgroup. The adsorbed oligomers will therefore remain at the surface of the core particles and in consequence there is no extended interface between PS and PMMA in these. particles. 

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