The Initiation Mechanisms

The initial mechanical system was cmde, but the advancement of engine and emission controls, in particular the use of electronics and computers, has brought about substantial refinement (36). 

The initiation mechanism is well defined because of the recent isolation and spectroscopic characterization of the initial zwitterion from ethyl cyanoacrylate (ECA) and a phosphine [8,9]. Specifically, zwitterion 4 was prepared from the reaction of equimolar amounts of dimethylphenyl phosphine, 5, and ECA, 6,... 

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. 

We have proposed an initiation mechanism for the ROOH-amine system in which some H-bond complex may be formed [36]. Then Sun et al. [37,38] thoroughly investigated the initiation mechanism of ROOH-Amine through IR spectra of TBH-triethylamine, TBH-DMT, and CHP-DMT. From the wideness of the shift of OH absorption bands at 3120, 3336, and 3257 cm were... 

The ethylenediamine derivative [31] possesses higher promoting activities than other diamines. This phenomenon may be ascribed to the copromoting effect of the two amino groups on the decomposition of persulfate through a CCT (contact charge transfer complex) formation. So we proposed the initiation mechanism via CCT as the intimate ion pair and deprotonation via CTS (cyclic transition state) as follows ... 

Moreover, the initiation mechanism of APS/ethylene-disecondary-amine and APS/ethylene-diprimary-amine systems has also been proposed [47]. 

Based on the ESR studies and the end group analysis, the initiation mechanism of Ce(IV) ion redox systems is proposed as ... 

Based on the ESR studies of Ce(IV) ion-BzyAcAc-MNP, Ce(IV) ion BzAc-MNP systems as mentioned before, the grafting reaction of P(St-CH2-AcAc) will take place on the methene carbon of 1,3-dikeone via the abstraction of hydrogen by the Ce(I V) ion to form radicals and then initiate monomer graft copolymerization. The initiation mechanism of graft copolymerization is proposed in Scheme (10). 

In eq. 8, the rate of polymerization is shown as being half order in initiator (T). This is only true for initiators that decompose to two radicals both of which begin chains. The form of this term depends on the particular initiator and the initiation mechanism. The equation takes a slightly different form in the case of thermal initiation (S), redox initiation, diradical initiation, etc. Side reactions also cause a departure from ideal behavior. 

As previously pointed out, this distribution is much sharper than the most probable one (see Fig. 56). Unless the initiation mechanism is of a sort which depends on the square of the monomer concentration (e.g., thermal initiation see Chap. IV), M- ]/[Af] will vary with conversion, causing the distribution for the aggregate of polymer formed to broaden with conversion. 

Scheme 6 Scheme of the initiation mechanism in ethylene polymerization according to a Ziegler-Natta-like behavior... 

As far as the initiation mechanism is concerned, the first complete characterization of the C2H4 TT-complexes formed on Cr(II) sites has been re-... 

TGA indicated that the initiation mechanism of degradation of POM was by random scission of the main chain. (Formaldehyde observed in the TGA-MS measurements is a direct consequence of random scission.)... 

The initiation mechanism comprises nucleophilic attack of the metal chloride at the least-hindered carbon. Thus hydrolysis of the 1 1 reaction product of (251) with PO yields l-chloropropan-2-ol.937... 

What is retained nowadays of the initial mechanism (Scheme 1) is the occurrence of a cationic intermediate. But bromine bridging is not general, and its magnitude depends mainly on the double bond substituents (Ruasse, 1990). For example, when these are strongly electron-donating, i.e. able to stabilize a positive charge better than bromine, / -bromocarbocations are the bromination intermediates. The flexibility of transition state and intermediate stabilization puts bromination between hydration via carbocations and sulfenylation via onium ions. 

Lewis et al.106 calculated four possible decomposition pathways of the ot-HMX polymorph N-N02 bond dissociation, HONO elimination, C-N bond scission, and concerted ring fission. Based on energetics, it was determined that N-N02 dissociation was the initial mechanism of decomposition in the gas phase, whereas they proposed HONO elimination and C-N bond scission to be favorable in the condensed phase. The more recent study of Chakraborty et al.42 using density functional theory (DFT), reported detailed decomposition pathways of p-HMX, which is the stable polymorph at room temperature. It was concluded that consecutive HONO elimination (4 HONO) and subsequent decomposition into HCN, OH, and NO are the most energetically favorable pathways in the gas phase. The results also showed that the formation of CH20 and N20 could occur preferably from secondary decomposition of methylenenitramine. 

In the field of cationic polymerisation a notoriously intractable problem is the mechanism of initiation by aluminium halides. Despite much work on the polymerisation itself, there are few studies of the initiation mechanism. Existing theories are shown to be inadequate to explain the most characteristic features of the reactions when a solution of an aluminium halide in an alkyl halide is introduced into a solution of isobutene, there ensues a fast polymerisation which generally stops at incomplete conversion, and the number of moles of polymer formed is much smaller than the number of moles of initiator these features are found over a very wide range of conditions. 

The work of the Brno group which for the first time included conductivity measurements on the same system, provided quite different insights and showed the way which we eventually followed but that too did not give any clear leads as to the initiation mechanism. Nor did any of the very few studies done with other monomers and aluminium halides provide any clear evidence on the mode of action of the initiators [1]. 

The initiation mechanism with PVPA was proposed as follows... 

The initiation mechanism for styrene has been established [Graham et al., 1979 Kaufman, 1979 Kothe and Fischer, 2001 Mayo, 1968 Olaj et al., 1976, 1977a,b]. It involves the formation of a Diels-Alder dimer (XII) of styrene followed by transfer of a hydrogen atom from the dimer to a styrene molecule (Eq. 3-63). Whether formation of the... 

The initiation mechanism for methyl methacrylate appears to involve the initial formation of a biradical by reaction of two monomer molecules followed by hydrogen transfer from... 

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