Monomers monofunctional

Literature articles, which report the formation and evaluation of difunctional cyanoacrylate monomers, have been published. The preparation of the difunctional monomers required an alternative synthetic method than the standard Knoevenagel reaction for the monofunctional monomers, because the crosslinked polymer thermally decomposes before it can revert back to the free monomer. The earliest report for the preparation of a difunctional cyanoacrylate monomer involved a reverse Diels-Alder reaction of a dicyanoacrylate precursor [16,17]. Later reports described a transesterification with a dicyanoacrylic acid [18] or their formation from the oxidation of a diphenylselenide precursor, seen in Eq. 3 for the dicyanoacrylate ester of butanediol, 7 [6]. 

The molecular weight can also be controlled by adding a monofunctional monomer. The monofunctional endgroup, B, has the same functionality as monomer BB. In this case, the moles of A functional groups in the difunctional... 

Monomers that participate in step growth polymerization may contain more or fewer than two functional groups. Difunctional monomers create linear polymers. Trifiinctional or polyfunctional monomers introduce branches which may lead to crosslinking when they are present in sufficiently high concentrations. Monofunctional monomers terminate polymerization by capping off the reactive end of the chain. Figure 2.12 illustrates the effect of functionality on molecular structure. 

The type of copolymer formed during step growth polymerization depends on the reactivity of the functional groups and the time of introduction of the comonomer. A random copolymer forms when equal concentrations of equally reactive monomers polymerize. The composition of the copolymer, then, will be the same as the composition of the reactants prior to polymerization. When the reactivities of the monomers-differ, the more highly reactive monomer reacts first, creating a block consisting predominandy of one monomer in the chain the lower reactivity monomer is added later. This assumes that there is no chain transfer and no monofunctional monomer present. If either of these conditions were to exist,... 

Table X. Synergistic Effect of Monofunctional Monomers Including Methacrylates with Inorganic Salts(L) in Grafting Styrene to Polypropylene Initiated by UVa...

Another method of achieving the desired molecular weight is by addition of a small amount of a monofunctional monomer, a monomer with only one functional group. Acetic acid or lauric acid, for example, are often used to achieve molecular weight stabilization of polyamides. The monofunctional monomer, often referred to as a chain stopper, controls and limits the polymerization of bifunctional monomers because the growing polymer yields chain ends devoid of functional groups and therefore incapable of further reaction. Thus, the use of benzoic acid in the polyamide synthesis yields a polyamide (XI) with phenyl end groups that are unreactive toward polymerization. 

In order to properly control the polymer molecular weight, one must precisely adjust the stoichiometric imbalance of the bifunctional monomers or of the monofunctional monomer. If the nonstoichiometry is too large, the polymer molecular weight will be too low. It is therefore important to understand the quantitative effect of the stoichiometric imbalance of reactants on the molecular weight. This is also necessary in order to know the quantitative effect of any reactive impurities that may be present in the reaction mixture either initially or that are formed by undesirable side reactions. Impurities with A or B functional groups may drastically lower the polymer molecular weight unless one can quantitatively take their presence into account. Consider now the various different reactant systems which are employed in step polymerizations ... 

Monofunctional monomer (or monomers) used to dilute the formulation to the suitable application viscosity. 

It is reasonable to expect that in a viscous monomer such as trimethylol-propane triacrylate (>/ = 65 cp), bimolecular termination reactions proceed more slowly than in monofunctional monomers. However, considering the long lifetime observed for the polymer radicals in these monomers, caution must be exercised in the interpretation of the linear intensity dependence. Long-lived radicals are more likely to terminate by chain transfer and... 

As these reactive oligomers have a high viscosity, they are diluted with a multifunctional and if necessary a monofunctional acrylate monomer. The acrylate monomers reduce the viscosity of the mixture for easier application on substrates. The multifunctional acrylate monomers increase the crosslink density the monofunctional monomers decrease it. 

Homogeneous nonideal, e.g. open networks, obtained from the same chemistry as the previous ones. These networks contain dangling chains as a result of incomplete cure, nonstoichiometric composition, or presence of monofunctional monomers. 

More detailed studies were devoted to the mechanism of electrochemical polymerization applied to Fe(II), Ru(II) and Os(II) complexes containing 2,2 -dipyridyl and monofunctional monomer (L) such as 4-VP, bis(4-pyridyl)ethylene, trans-4-stilbazole or N-(4-pyridyl)acrylamide [86], The first stage of electrochemical polymerization is shown to be the formation of a radical-anion, e. g. by the following scheme... 

The selection of liquid monofunctional monomers for use in EB cure has been a key factor in the slow pace of development of acceptable formulations. These monomers play dominant roles in determining cure speed, viscosity, safety and cost, in addition to significant contributions to the physical properties of the cured films. 

Measurement of conversions of various formulations at various EB doses can be used to rank the reactivity of the formulation. A particularly useful procedure has been to prepare a standard mixture of an acrylate resin with various reactive diluent monomers in order to compare the volatility and reactivity of new monomers. For these studies, a mixture of 40 weight % of a Bis-phenol A epoxy dlacrylate resin with 60% of the test liquid monomer has proved convenient. A viscosity measurement of the mixture also provides information on the relative viscosity reducing ability of the test monomer. Illustrative examples of these measurements are shown in Table I and Figure 1. Mote from these examples that a monofunctional monomer, Monomer B, can be used to provide the low volatility and high reactivity typical of the multifunctional monomers, while also serving to reduce the crosslinking. Many other available monofunctional monomers are found to be either more volatile or less reactive than Monomer B. 

The relative reactivities of various other resins, monomers and combinations of monomers have been measured in a similar manner. From this information, it was learned that many commercially available monofunctional monomers were not suitable for use in radiation curable formulations. Mobil Chemical Company has now developed several proprietary monofunctional monomers which provide low volatility, high reactivity and safe handling properties to fulfill these needs. In addition, other material suppliers have begun to make suitable monomers available, so the overall availability of materials for formulating has improved greatly within the last few years. 

These monomers generally give better film properties than the linear monofunctional monomers. However in some cases they have quite distinct odours. 

Many chain-growth copolymerizations include dienes such as divinyl benzene or divinyl adipate that act as crosslinking agents and lead to gel formation. Polymerization kinetics in such cases are complex and are beyond the scope of a book on homogeneous reactions. Here, only binary copolymerization of monofunctional monomers will be examined. For an excellent and extensive treatment that includes copolymerization of more than two monomers as well as crosslinking by bifunctional monomers, the reader is refer to Odian s book [123]. 

The copolymerization of the main monofunctional monomer with a bifunctional monomer containing a luminescent group, e.g. with a dimethacrylic ester of... 

The most widely studied addition-polymerization reaction for crosslinking is the free-radical polymerization of difunctional monomers (such as divinyl benzene and ethylene glycol dimethacrylate) in the presence of the corresponding monofunctional monomer. This may be thermally or photochemically initiated, and the latter application is widely used for coatings and dental composites. This is shown in Figure 1.26. 

General difunctional monomer M-M General monofunctional monomer M ... 

Figure 1.26. A schematic diagram for the formation of a crosslinked network by the polymerization of a monofunctional monomer, e.g. methyl methacrylate (MMA), in the presence of a difimctional crosslinker, such as ethylene glycol dimethacrylate (EGDMA). Gelation occurs at low conversion (pc 12%-20%). Adapted from Pascualt et al. (2002).

Control of molecular weight is achieved by using a stoichiometric imbalance of the reactive functional group or a calculated amount of an appropriate monofunctional monomer. 

---