Peroxide-amine initiation systems

Rate Equations of MMA Polymerization Using Peroxide-Amine Initiation Systems... 

Very recently, the feasibility of aliphatic tertiary amine like the triethylamine-benzoyl peroxide redox-initiating system in photopolymerization of methyl methacrylate [66] has been reported. In the dilatometric study of methyl methacrylate polymerization at 35°C with various solvents, the initiator exponent was 0.34. The monomer exponent depends on the solvents used. In acetonitrile, pyridine, and bromobenzene, the monomer exponent was 0.5, 0.67, and 1.1, respectively, within the concentration range studied. Benzene and chloroform give first-order dependence of rate on [monomer] and behave as normal (inert) diluents. The activation energy was 3.2 kcal mor. ... 

Wheieas the BPO—DMA ledox system works well for curing of unsaturated polyester blends, it is not a very effective system for initiating vinyl monomer polymerizations, and therefore it generally is not used in such appHcations (34). However, combinations of amines (eg, DMA) and acyl sulfonyl peroxides (eg, ACSP) are very effective initiator systems at 0°C for high conversion suspension polymerizations of vinyl chloride (35). BPO has also been used in combination with ferrous ammonium sulfate to initiate emulsion polymerizations of vinyl monomers via a redox reaction (36). 

Another common initiator system is based on chlorosulfonated polyethylene (trade-name HYPALON ). Adhesives based on this technology were referred to as RF (short for Reactive Fluid) or DH (DuPont HYPALON ) in the early literature. Chlorosulfonated polyethylene reacts with an amine (and optionally a peroxide) to form free radicals [55]. 

Organic peroxide-aromatic tertiary amine system is a well-known organic redox system 1]. The typical examples are benzoyl peroxide(BPO)-N,N-dimethylani-line(DMA) and BPO-DMT(N,N-dimethyl-p-toluidine) systems. The binary initiation system has been used in vinyl polymerization in dental acrylic resins and composite resins [2] and in bone cement [3]. Many papers have reported the initiation reaction of these systems for several decades, but the initiation mechanism is still not unified and in controversy [4,5]. Another kind of organic redox system consists of organic hydroperoxide and an aromatic tertiary amine system such as cumene hydroperoxide(CHP)-DMT is used in anaerobic adhesives [6]. Much less attention has been paid to this redox system and its initiation mechanism. A water-soluble peroxide such as persulfate and amine systems have been used in industrial aqueous solution and emulsion polymerization [7-10], yet the initiation mechanism has not been proposed in detail until recently [5]. In order to clarify the structural effect of peroxides and amines including functional monomers containing an amino group, a polymerizable amine, on the redox-initiated polymerization of vinyl monomers and its initiation mechanism, a series of studies have been carried out in our laboratory. 

The diacyl peroxide-amine system, especially BPO-DMT or BPO-DMA, has been used and studied for a long time but still no sound initiation mechanism was proposed. Some controversy existed in the first step, i.e., whether there is formation of a charge-transfer complex of a rate-controlling step of nucleophilic displacement as Walling 1] suggested ... 

The rate equations of MMA polymerization initiated by peroxide amine systems are listed in Table 4. It shows that No. 1, 2, 3, 8, 9 are in good agreement with the... 

With the present series of monomers, described in Table I, the polymerization methods studied were the same as those described in previous work (2). Again, polymerization of concentrated aqueous solutions of the amine hydrochlorides using the titaneous chloride/ hydrogen peroxide initiation system provided the best polymerization method. Where this technique did not yield polymers, other free-radical initiation systems were equally unsuccessful. Table II gives some examples of the results of these polymerization experiments. 

The most widely used approach to polymerization of modem composites is photoinitiation [8]. However, earlier formulations were supplied as two-paste systems, where mixing two components, one of which would be present in each of the pastes, formed the initiator. A typical initiator system for this approach is benzoyl peroxide, present in a base paste at about 1% by volume [11], with tertiary amine activator, present in the other catalyst paste at 0.5%. The most widely used activator in these two-component initiators is A,A-dihydroxyethyl-p-toluidine [12]. 

Nevertheless, cobaltous ions form efficient redox initiating systems with peroxydisulfate ions. " Tertiary aromatic amines also participate in bimolecular reaction with organic peroxides. One of the unpaired electrons on the nitrogen atom transfers to the peroxide link, inducing decomposition. No nitrogen, however, is found in the polymer. It is therefore not a true redox type initiation and the amine acts more like a promoter of the decomposition. Two mechanisms were proposed to explain this reaction. The first one was offered by Homer et al... 

It was evident from some initial results that the compounds made using the sulfuric acid catalyst would not cure due to the inhibiting effect of the acid. Attempts to use organic amines to neutralize the acid as the nut/bolt combinations were made did not prove very successful. The addition of the filler, particularly alumina or calcium carbonate was the preferred method of catalyst neutralization. In addition, the filler could be used to control the viscosity of the final mixture which was of concern in the peroxide-cured adhesive systems. 

It includes the stages of interaction to be typical for the radical initiating systems peroxides organic amines such as the formation of donor-acceptor complex Kj (charge-transfer complex) which may subsequently transform by radical (formation of Kj) or ion-radical (formation of K ) ways, and each of them is capable of the initiation of polymerization in the presence of the monomer. In addition, the formation of the Kj complex takes into account the catalytic fimction of triazine. The carrying out of several consecutive and parallel processes may be the reason of changing the role of additives which is observed when the polymerization temperature varies. 

No attempt will be made here to discuss the mechanism of all the redox decompositions involved for the various initiators reported in the literature. However, a likely mechanism for the most common initiator system used, the hydroperoxide/saccharin/aromatic amine system, will be described. As previously mentioned, both hydroperoxides and the saccharin/amine complex can serve as initiators of free radical polymerization. Lai reported in early work on the ability of saccharin/amine salts to effect the free radical polymerization of acrylates. He further described the catalytic affect of peroxides on this system. Krieble described the hydroperoxide/ saccharin/ amine combination and emphasized the greater speed of cure achieved on a variety of substrates. This phenomenon can be explained by the affect of soluble metal ions on both the hydroperoxide and the saccharin/ amine complex. 

Unsaturated polyester resins are usually crosslinked at an elevated temperature or at room temperature in combination with suitable initiating systems. An organic peroxide initiator and a cobalt salt or aromatic amine as accelerators are predominantly used for curing at room temperature. Some typical aromatic amine accelerators, e.g. aniline and its derivatives, are classified as toxic substances, therefore the accelerating effects of other compounds were tested. Different organic peroxides with ferrocene and its derivatives were used to initiate the crossUnking process of UPs [ 167] and a mechanism of the formation of active radicals was proposed (Fig. 19). 

In our own laboratory, we have carried out formulation studies on the use of polyurethane chains with pendant or alpha-omega acrylates as fastsetting impression materials using peroxide initiation systems accelerated with aromatic amines. These compositions have a working time between 3 and 5 minutes. They set in less than 10 minutes at room temperature. The temperature rise which is always a problem with polymerization of large masses of acrylate monomers was as little as 5.5°C. 

Eor apphcation temperatures below 10°C or for acceleration of cure rates at room temperature, nonredox systems such as ben2oyl peroxide initiated by tertiary amines such as dimethylaruline (DMA) have been appHed widely. Even more efficient cures can be achieved using dimethyl- -toluidine (DMPT), whereas moderated cures can be achieved with diethylaruline (DEA). 

Composite resins can be cured using a variety of methods. Intraoral curing can be done by chemical means, where amine—peroxide initiators are blended in the material to start the free-radical reaction. Visible light in the blue (470—490 nm) spectmm is used to intraoraHy cure systems containing amine—quin one initiators (247). Ultraviolet systems were used in some early materials but are no longer available (248). Laboratory curing of indirect restorations can be done by the above methods as well as the additional appHcation of heat and pressure (249,250). 

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