Use of Multifunctional Monomers

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... 

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... 

As implied by the name, one of the fundamental distinctions between macromolecules and small molecules lies in their mass, or molecular weight, M. The lower limit for the molecular weight of a macromolecule is ill defined, but it is often considered to lie in the range M = 103-104. The upper limit for the molecular weight of a macromolecule is unbounded, and for all practical purposes it can be considered to be infinite. Of course, an infinite molecular weight is unattainable with a finite amount of material. However, molecular masses in covalently bound polymeric networks are conveniently expressed in units of kg molecule-1, rather than units of kg mol-1, and they are certainly an acceptable approximation to infinite in the context of this work. These polymeric networks contain numerous branch points, either as a consequence of the use of multifunctional monomers during the polymerization or as a consequence of crosslinking reactions after polymerization has occurred. In such systems, the molecular mass is of less fundamental interest than the mass of the network chains between crosslinks. 

The use of multifunctional monomers in anaerobics leads to a highly crosslinked thermoset polymer that is heat-resistant and has excellent resistance to oil and solvents. Anaerobics cure very quickly on clean surfaces made of iron, steel or brass where transition metal ions catalyse the initiation of polymerisation. However, they cure at a slower rate on plated surfaces, on oily surfaces, or in the presence of certain rust-inhibiting chemicals such as chromates. For very inactive surfaces or for fixing on plastics, surface primer solutions (usually amines or copper salts) can be used. 

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. 

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. 

The understanding of the macromolecular properties of lignins requires information on number- and weight-average molecular weights (Mn, Mw) and their distributions (MWD). These physico-chemical parameters are very useful in the study of the hydrodynamic behavior of macromolecules in solution, as well as of their conformation and size (1). They also help in the determination of some important structural properties such as functionality, average number of multifunctional monomer units per molecule (2, 3), branching coefficients and crosslink density (4,5). 

Our work on the photopolymerization of multifunctional monomers has been concentrated on the development of faster initiators to be used in the... 

Different architectures, such as block copolymers, crosslinked microparticles, hyperbranched polymers and dendrimers, have emerged (Fig. 7.11). Crosslinked microparticles ( microgels ) can be described as polymer particles with sizes in the submicrometer range and with particular characteristics, such as permanent shape, surface area, and solubility. The use of dispersion/emulsion aqueous or nonaqueous copolymerizations of formulations containing adequate concentrations of multifunctional monomers is the most practical and controllable way of manufacturing micro-gel-based systems (Funke et al., 1998). The sizes of CMP prepared in this way vary between 50 and 300 nm. Functional groups are either distributed in the whole CMP or are grafted onto the surface (core-shell, CS particles). 

Thus, the use of alkyllithium initiation offers the synthetic chemist a tool of enormous flexibility for "tailor-making" polymers of precise structure. Control of molecular weight, molecular-weight distribution, diene structure, branching, monomer-sequence distribution, and functionality can conveniently be achieved by such techniques as incremental or sequential addition of monomer, initiators, or modifier, programming of temperature, continuous polymerization, or the use of multifunctional reagents. 

Living polymerization processes pave the way to the macromolecular engineering, because the reactivity that persists at the chain ends allows (i) a variety of reactive groups to be attached at that position, thus (semi-)telechelic polymers to be synthesized, (ii) the polymerization of a second type of monomer to be resumed with formation of block copolymers, (iii) star-shaped (co)polymers to be prepared by addition of the living chains onto a multifunctional compound. A combination of these strategies with the use of multifunctional initiators andtor macromonomers can increase further the range of polymer architectures and properties. 

Nevertheless, the route is attractive, because many interesting ladder structures are not accessible using the polycondensation or polyaddition of multifunctional monomers [2, 5]. Besides the specific problems of ladder polymer synthesis discussed above, another problem is almost invariably associated with... 

The authors feel, that the classification of the synthetic principles applied here is somewhat arbitrary. Multifunctional polycondensations which are conducted in a two-step manner (generation of single-strand intermediates, followed by cyclization), could be classified with the same justification as stepwise processes. On the other hand some of the stepwise syntheses of ladder structures constitute condensations of multifunctional monomers (e.g. the use of butadiynes as starting compounds, see Sect. 4.1.). 

The present high level of industrial activity in the field of light initiated polymerizations has stimulated research In basic polymerization phenomena including polymerization klnetics.(1-4) Unfortunately, previously used methods of determining monomer conversion such as dilatometry or measurements of unreacted monomers are not easily adapted to thin coating films. In addition, the presence of multifunctional monomers yielding networks at low conversion in photopolymerizable formulations also complicates analyses. 

Since TEMPO is only a regulator, not an initiator, radicals must be generated from another source the required amount of TEMPO depends on the initiator efficiency. Application of alkoxyamines (i.e., unimolecular initiators) allows for stoichiometric amounts of the initiating and mediating species to be incorporated and enables the use of multifunctional initiators, growing chains in several directions [61]. Numerous advances have been made in both the synthesis of different types of unimolecular initiators (alkoxyamines) that can be used not only for the polymerization of St-based monomers, but other monomers as well [62-69]. Most recently, the use of more reactive alkoxyamines and less reactive nitroxides has expanded the range of polymerizable monomers to acrylates, dienes, and acrylamides [70-73]. An important issue is the stability of nitroxides and other stable radicals. Apparently, slow self-destruction of the PRE helps control the polymerization [39]. Specific details about use of stable free radicals for the synthesis of copolymers can be found in later sections. 

Enzyme regioselectivity also enables the conversion of multifunctional monomers (functionality >3) to linear or nearly linear homo- and copolymers. In 1991, Dordick and co-workers [60] reported that, by using the protease Proleather, condensation polymerizations (45 °C, 5 days) performed in pyridine between sucrose and bis(2,2,2-trifluoroethyl) sebacate proceed with high regioselectivity giving sucrose oligoesters (DP 11) in 20% yield (see also Chapter 1). This inspired subsequent work by others that demonstrated such copolymerizations with polar multifunctional polyols could be performed under bulk reaction conditions without activation of carboxylic acids (see below). 

In interfacial polymerization, the two reactants in a polycondensation meet at an interface and react rapidly. The substances used are multifunctional monomers. Generally used monomers include multifunctional isocyanates and multifunctional acid chlorides. The basis of this method is the classical Schottenn-Baumann reaction between an acid chloride and a compound containing an active hydrogen atom, such as an amine or alcohol, polyesters, polyurea, and polyurethane. Under the right conditions, thin, flexible waUs/sheU will be formed at or on the surface of the droplet or particle by polymerization of the reactive monomers. 

Use of multifunctional chain-growth, addition-type monomers. Use of such monomers allows simultaneous polymerization and development of a crosslinked network. For example, the copolymerization of small amounts of divinyl benzene with styrene yields a crosslinked polymer ... 

Dipentaerythrityl acrylate Synonyms Acrylated dipentaerythritol Dipentaerythritol acrylate DPHPA Uses Reactive multifunctional monomer which increases cure speed and abrasion resist. Dipentaerythrityl hexacaprylate/hexacaprate CAS 68130-24-5 EINECS/ELINCS 268-581-5 Synonyms Decanoic acid, ester with 2,2 -[oxybis (methylene)] bis [2-(hydroxymethyl)-1,3-propanediol] octanoate pentanoate Definition Hexaester of a mixture of caprylic and capric acids and a dimer of pentaerythritol Uses Emulsifier, smoothing agent, emollient, thickener, pigment dispersant in cosmetics Trade Name Synonyms Liponate DPC-6 [Lipo http //www.iipochemicais. com] Dipentaerythrityl hexahydroxystearate Synonyms Octanoic acid, 12-hydroxy, hexaester with 2,2 -[oxybismethylene] bis [2-(hydroxymethyl)-l,3-propanediol]... 

---