Monofunctional monomer effect

Another method of achieving the desired molecular weight is by addition of a small amount of a monofunctional monomer. The monofunctional monomer effectively caps the chain end it reacts with and prevents further growth of the chain. 

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. 

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

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 ... 

Effect of End-Group Imbalance on Molecular Weight Effect of Monofunctional Monomers on Molecular Weight Kinetics... 

The presence of monofunctional monomer has a similar effect as unequal stoichiometry on the molecular weight distribution in a stepwise polymerization. There are two scenarios where monofunctional monomer must be considered. 

Polymerization occurs very quickly and the process is controlled via kinetic effects rather than thermodynamic ones. The net result is that the molecular weight distribution of the product does not match the thermodynamically stable one. If the chains were not capped with monofunctional phenols, the polymer chains would depolymerize, allowing the monomers to rearrange themselves at elevated temperature to approach the thermodynamically stable... 

Monomers are usually low-MW, monofunctional materials that chemically incorporate into the cured coating rather than volatilize into the atmosphere, as is common with solvent diluents. Monomer diluents are chosen on the basis of providing good solvency, effectively reducing the viscosity of the oligomer without excessively retarding the cure rate. Certain diluents will contribute to the physical properties of the adhesive. However, generally they provide soft, thermoplastic films because of their linear and uncrosslinked nature. 

One means of controlling the molecular weight obviously is to have one monomer in slight excess (r 1). A preferred method is usually the addition of a desired amount of a monofunctional reactant. The effect of a monofunctional reactant B on the degree of polymerization is indicated by the equation... 

Disulfonyl halides such as MI-22 to MI-26 are effective bifunctional initiators for various monomers including methacrylates, acrylates, and styrenes, because the sulfonyl halide part, as pointed out for their monofunctional versions, can induce fast initiation without a bimolecular termination reaction between the sulfonyl radicals.240-343... 

In view of the functionality thus created, it is interesting to consider possible applications for the epoxldlzed oils mentioned as epoxy monomers per se. Indeed, some epoxldlzed oils are commonly used as reactive diluents for other epoxy prepolymers in order to reduce cost or Improve processability (10,11) examples claimed in reference 11 Include epoxldlzed linseed, butylated linseed, soybean, and tall oils. However, although some fundfimental studies of the effects of monofunctional reactive diluents on the viscoelastic and other properties of epoxy resins have been published (see, for example, reference 12), little or no analogous Information on the effects of multifunctional reactive diluents appears to exist. At the same time, some reactive additives such as polyols (13), poly(ether esters) (14) and carboxy-terminated elastomers (15) have been used to provide an elastomeric toughening phase for epoxies. 

The use of lasers to initiate the polymerization of both monofunctional and multifunctional monomers has been reported in a number of papers during the past decade. Decker (1) was the first to demonstrate that pulsed lasers could be effectively used to obtain relatively high degrees of polymerization for trimethylolpropane triacrylate. He showed that even for pulsed lasers which deliver up to gigawatts of peak power, polymerization could be effectively carried out over a wide range of conditions (1). 

Example 3 (Effect of monofunctional chain stoppers or impurities) Let us suppose we have No moles of AE-type monomers and Nj moles of monofimctional impurity containing an A functional group, but no B group. Whenever this monofunctional impurity adds to the end of a molecule, that end can no longer participate in step-growth polymerization reactions. As in Equation 7.11, the number-average chain length is equal to the initial number of molecules divided by the final number of molecules, so that ... 

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