Monomers difunctional

General. Successive reactions between difunctional monomer A-A and difunctional monomer B-B ... 

The elastomers consist of very high moleculcU weight (-0.5 X 10 ) linecu gums cross-linked after fabrication. In order to achieve such polymers it is necessary that very pure difunctional monomers be employed since the presence of monofunctional material will limit the molecular weight while trifunctional material will lead to cross-linking. Where dimethylsilicone rubbers are being prepared, the cyclic tetramer, octamethylcyclotetrasiloxane, which may be obtained free from mono- and trifunctional impurities, is often used. This tetramer occurs to the extent of about 25% during the hydrolysis of dichlorosilanes into polymers. 

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]. 

Crosslinking has been claimed to improve thermal resistance of the cyanoacrylate adhesive [18]. However, in other reports [6], little or no improvement in thermal resistance of the adhesive was demonstrated by the addition of a difunctional monomer. As seen in Fig. 2, the addition of varying amounts of crosslinker 7 provided no improvement in the tensile adhesive strength of ethyl cyanoacrylate on steel lapshears after thermal exposure at 121 °C for up to 48 h. 

A difunctional monomer commonly employed to cross-link ion-exchange polymers. 

The molecular weight of a polymer will be reduced if either die extent of conversion or the average functionality is decreased. At 95% conversion of difunctional monomers, for example, Xn is only 20.25 The molecular weight is also related to a stoichiometric imbalance, r, which is normally defined to be less than 1.0 ... 

The number-average molecular weight of a polymer may be controlled by offsetting the stoichiometry of two dissimilar mutually reactive difunctional monomers. The polymer will have the same endgroup functionality as that of the monomer used in excess. For a generic polymer made from a difunctional monomer AA with A functional groups and an excess of difunctional monomer BB widt B functional groups, r is defined as... 

Difluoro monomers, 341 Difunctional monomers, 12 Dihalide monomers, modified with functional groups, 357 Dihalides, activated, 346, 356-359 Dihydroxybenzophenone (DHBP), 342-343, 420... 

Stage 1 Difunctional monomers A, with functional groups called c, react by an alternating polyaddition reaction with an excess mixture of difunctional D and trifunctional T monomers, which all have the same functional groups, called h (and thus are equally reactive), to (mainly) h-terminated prepolymer PI. In some calculations tetra-functional Q monomers with equally reactive h functional groups were present as well. 

In all calculations the molar masses given in the top of Table I were used. First of all, the effects of variations in the concentration of trifunctional monomers were determined, as exemplified by the nine formulations of Table I and the resulting prepolymer characteristics after full conversion given in Table II. Formulations FIO to F40 result in branched prepolymers, which are cured in the third stage by difunctional monomers. On the other hand, formulations FOO to F04 result in the same linear prepolymer, which is subsequently cured with various mixtures of di- and trifunctional monomers. The number average functionalities of PI (and P2) and of the mixtures of E and F monomers are varied systematically between 2.0 and 2.4. Therefore, the only difference between formulations FjO and FOj is the stage in which the branching units are added. 

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. 

Bisphenol A, whose official chemical name is 2,2-bis(4-hydroxyphenyl)propane, is a difunctional monomer with two reactive hydroxyl groups, as shown in Fig. 20,2. It polymerizes svith dicarbonyl organic monomers, such as phosgene or diphenyl carbonate, which are illustrated in Fig. 20.3. During polymerization, shown in Fig. 20.4, the hydroxyl groups of the bisphenol A deprotonate in the presence of a base. After deprotonation, the oxygen atoms on the bisphenol A residue form ester bonds with the dicarbonyl compounds. The polymerization process terminates when a monohydric phenol reacts with the growing chain end. 

Controlling the stoichiometry by adding either difunctional monomers or a monofunctional reactant. 

Fatty acids have also been converted to difunctional monomers for polyanhydride synthesis by dimerizing the unsaturated erucic or oleic acid to form branched monomers. These monomers are collectively referred to as fatty acid dimers and the polymers are referred to as poly(fatty acid dimer) (PFAD). PFAD (erucic acid dimer) was synthesized by Domb and Maniar (1993) via melt polycondensation and was a liquid at room temperature. Desiring to increase the hydrophobicity of aliphatic polyanhydrides such as PSA without adding aromaticity to the monomers (and thereby increasing the melting point), Teomim and Domb (1999) and Krasko et al. (2002) have synthesized fatty acid terminated PSA. Octanoic, lauric, myristic, stearic, ricinoleic, oleic, linoleic, and lithocholic acid acetate anhydrides were added to the melt polycondensation reactions to obtain the desired terminations. As desired, a dramatic reduction in the erosion rate was obtained (Krasko et al., 2002 Teomim and Domb, 1999). 

Postpolymerization of difunctional monomers to effect star branching has been successfully applied in cationic polymerization, e.g. in the case of polyisobutylene initiated with 2-chloro-2,4,4,-trimethylpentane/TiCl4. Addition of divinylbenzene leads to star polymers [104], Vinyl ethers, when polymerized with HI/ZnI2 in toluene at — 40°C, can be copolymerized with divinylether... 

Extended application of the procedure by the Russian group has led to the synthesis of difunctional monomers having an orthoester and a potential alcoholic group within the same molecule, and their polymerisation to synthetic polysaccharides of regular structure (e.g. Scheme 10)... 

Note 4 Ethene and ethylene glycol are examples of difunctional monomers, glycerof is an example of a trifunctional monomer, and divinylbenzene and pentaerythritol are examples of tetrafunctional monomers. 

Mecerreyes D, Humes J, Miller RD, Hedrick JL, Lecomte Ph, Detrembleur C, Jerome R (2000) First example of an unsymmetrical difunctional monomer polymerizable by two living/controlled methods. Macromol Rapid Commun 21 779-784... 

Stars with high arm numbers are commonly prepared by the arm-first method. This procedure involves the synthesis of living precursor arms which are then used to initiate the polymerization of a small amount of a difunctional monomer, i.e., for linking. The difunctional monomer produces a crosslinked microgel (nodule), the core for the arms. The number of arms is a complex function of reaction variables. The arm-first method has been widely used in anionic [3-6,32-34], cationic [35-40], and group transfer polymerizations [41] to prepare star polymers having varying arm numbers and compositions. 

Allyl methacrylate and allyl acrylate are difunctional monomers, triallyl phosphate is a trifunctional monomer, and polyethylene glycol dimethacrylate is a polyfunctional monomer. All these lead to cross-linked graft copolymers. 

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