Multifunctional monomers, polymerizations

Attainment of a maximum double bond conversion is typical in multifunctional monomer polymerizations and results from the severe restriction on bulk mobility of reacting species in highly crosslinked networks [26]. In particular, radicals become trapped or shielded within densely crosslinked regions known as microgels, and the rate of polymerization becomes diffusion limited. Further double bond conversion is almost impossible at this point, and the polymerization stops prior to 100% functional group conversion. In polymeric dental composites, which use multifunctional methacrylate monomers, final double bond conversions have been reported ranging anywhere from 55-75% [22,27-29]. 

In multifunctional monomer polymerizations, the mobility of radicals through segmental diffusion falls well before their mobility through reaction diffusion at very low functional group conversions (as compared to linear polymerizations). From this point in the reaction, the termination and propagation kinetic constants are found to be related, and the termination kinetic constant as a function of conversion may actually exhibit a plateau region. Figure 6 illustrates the typical behavior of kp and k, vs conversion as predicted by a kinetic based model. 

Fig. 6. The characteristic behavior of the propagation kinetic constant, kp, and the termination kinetic constant, k as a function of double bond conversion for a multifunctional monomer polymerization...

Our interest from the outset has been in the possibility of crosslinking which accompanies inclusion of multifunctional monomers in a polymerizing system. Note that this does not occur when the groups enclosed in boxes in Table 5.6 react however, any reaction beyond this for the terminal A groups will result in a cascade of branches being formed. Therefore a critical (subscript c) value for the branching coefficient occurs at... 

For a fixed extent of reaction, the presence of multifunctional monomers in an equimolar mixture of reactive groups increases the degree of polymerization. Conversely, for the same mixture a lesser extent of reaction is needed to reach a specified with multifunctional reactants than without them. Remember that this entire approach is developed for the case of stoichiometric balance. If the numbers of functional groups are unequal, this effect works in opposition to the multifunctional groups. 

In resists of this class, the imaging layer contains a multifunctional monomer that can form an intercormected network upon polymerization, and a photosensitizer to generate a flux of initiating free radicals. Although not stricdy required for imaging, the composition usually includes a polymeric binder (typically an acryhc copolymer) to modify the layer s physical properties. Figure 7b shows the chemical stmctures of typical components. 

As mentioned previously, the use of multifunctional monomers results in branching. The introduction of branching and the formation of networks are typically accomplished using trifunctional monomers, and the average functionality of the polymerization process will exceed 2.0. As the average functionality increases, the extent of conversion for network formation decreases. In... 

Phenol, the simplest and industrially more important phenolic compound, is a multifunctional monomer when considered as a substrate for oxidative polymerizations, and hence conventional polymerization catalysts afford insoluble macromolecular products with non-controlled structure. Phenol was subjected to oxidative polymerization using HRP or soybean peroxidase (SBP) as catalyst in an aqueous-dioxane mixture, yielding a polymer consisting of phenylene and oxyphenylene units (Scheme 19). The polymer showed low solubility it was partly soluble in DMF and dimethyl sulfoxide (DMSO) and insoluble in other common organic solvents. 

The basic principle of the light-induced polymerization of multifunctional monomers can be represented schematically as follows ... 

Type 2 gels are essentially infinite molecular weight molecules Their three-dimensional macroscopic networks comprise structural components that are covalently linked through multifunctional units. This is a very broad class that includes linear polymers that have been chemically or radiochemically cross-linked into a permanent structure as well as networks that have been built up by the step or chain polymerization of difunctional and multifunctional monomers. 

Microgels may also be produced by dispersion polymerization of multifunctional monomers [276, 277]. Kim et al. synthesized microgels by copolymerization of acrylamide with acryloyl terminated polyethylene glycol macro-monomers in ethanol or in selective solvents [276]. The macromonomer acted... 

In this paper, we report efforts to find donor/acceptor systems, comprised of at least one multifunctional monomer, capable of sustaining rapid free-radical polymerization without the need for external photoinitiators. Although we will include in this report comonomer systems which form ground state CT complexes, we stress that the primary mechanism for generating free-radical in each case may not be via excitation of ground state CT complexes. 

Trifluorostyrene-based monomers and fheir derivatives are known to exhibit dimerization preferentially over polymerization in confrasf to fhe hydrocarbon analogue slyrene. Eord, DesMarfeau, and Smifh, Smifh and Babb,i i and Smith et al. have advantageously used this behavior to produce 6 (where E can be a large number of differenf spacer groups buf also typically be sulfonamide-based) via cyclopolymerization of multifunctional monomers bearing at least two trifluorovinyl ether units. The polymers themselves have perfluorocyclobutane (PFCB) rings as part of the main chain. 

DSC studies have shown that multifunctional monomers react quickly to form densely crosslinked networks from liquid monomer solutions. However, even a small amount of unreacted monomer can effectively plasticize a crosslinked network, rendering it more pliable. Eor this reason, mechanical analysis was combined with DSC studies to characterize the physical changes occurring in the proposed dimethacrylate system as polymerization proceeds. Static compression tests (Perkin-Elmer, DMA7e) were completed on disks (d = 11.5 mm, t = 1.7) immediately after they were irradiated for varied times. 

Covalent bonding of acrylic or methacrylic monomer to the template leads to multifunctional monomers (multimonomers).If monomer units are connected by covalent bonds within the frame of the template and polymerization proceeds according to the zip mechanism , a product with ladder-type structure can he expected. The structure of products obtained depends on the competition between the reactions proceeding on the template and the reaction between groups belonging to different macromolecules (templates). Template homopolymerization in this case can he represented by the scheme given in Figure 9.1. 

As described above, the majority of materials used in dental applications contain multifunctional monomers that polymerize to form highly crosslinked polymer networks. In addition, many of the applications, such as tooth restorations, require that the crosslinked polymer is polymerized intraorally. This restriction can often complicate the cure of the monomers since the material is exposed to oxygen and moisture in the oral environment. Also, depending on the thickness of the restoration, the material might not be uniformly cured because of variations in light intensity with depth in the sample. These problems... 

Fig. 1. Simulated profile of conversion of double bonds in a multifunctional monomer vs polymerization time at different depths in the polymer (-, surface), (----, 1,4 mm), (---, 2.8 mm), and...

Polymerization reactions of multifunctional monomers such as those used in dental restorations occur in the high crosslinking regime where anomalous behavior is often observed, especially with respect to reaction kinetics. This behavior includes auto acceleration and autodeceleration [108-112], incomplete functional group conversion [108,109,113-116], a delay in volume shrinkage with respect to equilibrium [108, 117,118], and unequal functional group reactivity [119-121]. Figures 3 and 4 show a typical rate of polymerization for a multifunctional monomer as a function of time and conversion, respectively. Several distinctive features of the polymerization are apparent in the rate profiles. 

Fig. 4 A typical rate of polymerization, for a multifunctional monomer vs conversion of double...

Another unique attribute of polymerizations of multifunctional monomers is the dominance of reaction diffusion as a termination mechanism [134,136, 143-146]. Reaction diffusion involves the mobility of radicals by propagation through unreacted functional groups. This termination mechanism is physically different from translation and segmental diffusion termination mechanisms which involve the diffusion of polymer macroradicals and chain segments to bring radicals within a reaction zone before terminating. Whereas normal termination mechanisms are related to the diffusion coefficient of the polymer, reaction diffusion must be considered differently. In essence, reaction diffusion is... 

Fig. 10. Kinetic gelation model prediction of the relative fraction of crosslinks, primary cycles, and secondary cycles as a function of double bond conversion for the polymerization of a multifunctional monomer...

In this section the occurrence of LCB in several other of the more important polymers during free-radical polymerization will be discussed branching due to irradiation or the use of multifunctional monomers (including dienes) will not be... 

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