BPO-initiated polymerization

Many authors have studied radical polymerization in the solid state by using the ESR technique. An advantage of studying polymerizations in the solid state is the long life time of free radicals leading to much higher concentrations than that in the liquid state. As an example, the benzoyl peroxide (BPO)-initiated polymerization of methyl methacrylate (MMA) in frozen aromatic solvents with irradiation of light is shown in Fig. 7.4 [4]. 

Kamachi el al. making an ESR study of the BPO- initiated polymerization of methacrylates in frozen aromatic solvents with irradiation of light, obtained well-resolved 9-line spectra aced at about 11.5 G such as shown in Fig. 16. The intensity distributions of these spectra were found to depend on the temperature and the nature of the solvent. The radical polymerization of MMA-p-d2 was investigated by ESR... 

In an ESR study on the light-irradiated BPO-initiated polymerization of VAc as well as the diene compounds shown below in frozen benzonitrile at — 120 C, Kamachi et al. obtained well-resolved ESR spectra of the propagating radicals depicted in Fig. 26. 

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. 

Transfer to initiator can be a major complication in polymerizations initiated by diacyl peroxides. The importance of the process typically increases with monomer conversion and the consequent increase in the [initiator] [monomer] ratio.9 105160 162 In BPO initiated S polymerization, transfer to initiator may be lire major chain termination mechanism. For bulk S polymerization with 0.1 M BPO at 60 °C up to 75% of chains are terminated by transfer to initiator or primary radical termination (<75% conversion).7 A further consequence of the high incidence of chain transfer is that high conversion PS formed with BPO initiator tends to have a much narrower molecular weight distribution than that prepared with other initiators (e.g. AIBN) under similar conditions. 

The mechanism proposed for the production of radicals from the N,N-dimethylaniline/BPO couple179,1 involves reaction of the aniline with BPO by a Sn-2 mechanism to produce an intermediate (44). This thermally decomposes to benzoyloxy radicals and an amine radical cation (46) both of which might, in principle, initiate polymerization (Scheme 3.29). Pryor and Hendrikson181 were able to distinguish this mechanism from a process involving single electron transfer through a study of the kinetic isotope effect. 

The rate constants for benzoyloxy and phenyl radicals adding to monomer are high (> KF M-1 s for S at 60 CC - Table 3.7). In these circumstances primary radical termination should have little importance under normal polymerization conditions. Some kinetic studies indicating substantial primary radical termination during S polymerization may need to be re-evaluated in this light.161 Secondary benzoate end groups in PS with BPO initiator may arise by head addition or transfer to initiator (Section 8.2.1). 

The polymerization of St with 56 as the initiator is considered to proceed via a reaction mechanism in Eq. (56), being identical to the models in Eqs. (18) and (20). The structure of both chain ends of the resulting polymer was confirmed by NMR using the deuterated St as the monomer. The polymerization with BPO and TEMPO without isolation of the adduct would also proceed via a similar path. In the absence of BPO, it has been reported that the radicals produced by spontaneous initiation according to the Mayo mechanism react with TEMPO to yield the adducts, and then they initiate polymerization [206]. 

Fig. 19. The extent of aromatic substitution as a function of conversion during solution styrene polymerization using BPO initiator at 90 C...

The situation is less clear-cut for RAFT systems. For a dithiocarbonate-mediated styrene polymerization studied by Goto and co-workers, the steady-state kinetic analysis applied both in the presence and absence of a BPO initiator (Figure 3.10). Similarly, for the solution polymerization of methyl methacrylate, mediated by dithioesters containing a-cyanobenzyl groups in the presence of AIBN initiator, pseudo-first-order plots were obtained although a significant induction period was detected. 

There is eertainly strong experimental evidenee for the existenee of radieal-solvent eom-plexes. For instanee, Russelk and eo-workers eolleeted experimental evidenee for radi-eal-eomplex formation in studies of the photoehlorination of 2,3 -dimethylbutane in various solvents. In this work, different produets were obtained in aliphatie and aromatie solvents, and this was attributed to formation of a jr-eomplex between the Cl atom and the aromatie solvent. Complex formation was eonfirmed by flash photolysis. Complex formation was also proposed to explain experimental results for the addition of triehloromethane radieal to 3-phenylpropene and to 4-phenyl-1-butene and for hydrogen abstraetion of the t-butoxy radieal from 2,3-dimethylbutane. Furthermore, eomplexes between nitroxide radieals and a large number of aromatie solvents have been deteeted. Evidenee for eomplexes between polymer radieals and solvent moleeules was eolleeted by Flatada et al., in an analysis of initiator fragments from the polymerization of MMA-d with AIBN and BPO initiators. They diseovered that the ratio of disproportionation to eombination depended on the solvent, and interpreted this as evidenee for the formation of a polymer radieal-solvent eomplex that suppresses the disproportionation reaetion. 

Solomon et al. [75] first used nitroxyl radicals and alkoxyamines in a radical polymerization, but their work was limited to production of low-molecular weight polymers. In 1993, Georges et al. [76] used a mixture of benzoyl peroxide (BPO) initiator and 2,2,6,6-tetramethylpiperidinyloxy (TEMPO) to produce low-polydispersity and high molecular weight polystyrene. Since then many papers about SFRP (also known as nitroxide mediated polymerization or NMP), mainly focused on styrene polymerization in the presence of TEMPO, have been published. Other nitroxide mediators are being developed that are better suited to polymerization of more polar monomers such as meth(acrylates) [77]. 

Figures 5.27 and 5.28. Benzoylperoxide BPO/amine (N,N-methyl, ethylformate-tert-butyl-qniline) initiates polymerization by heat. Camphorquinone/amine (N,N-methyl, ethyl-p-methylaniline) initiates polymerization by visible light.

Figure 11.7 shows the temperature history at a fixed point in the reaction tube as a front passes. The temperature at this point is ambient when the front is far away and rises rapidly as the front approaches. Hence, a polymerization front has a very sharp temperature profile (Pojman et al., 1995b). Figure 11.7 shows five temperature profiles measured during frontal free-radical polymerization of methacrylic acid with various concentrations of BPO initiator. Temperature maxima increase with increasing initiator concentration. For an adiabatic system, the conversion is directly proportional to the difference between the initial temperature of the unreacted medium and the maximum temperature attained by the front. The conversion depends not only on the type of initiator and its concentration but also on the thermodynamic characteristics of the polymer (Pojman et al., 1996b). 

The key to success in synthesizing polymers with narrow polydispersity and well-de ned chain end structure by carrying out free-radical polymerization in the presence of nitroxide SFRs such as TEMPO, is the essentially simultaneous initiation and reversible termination of the polymer radical with the SFR (Georges et al., 1994). However, the dissociation such as depicted in Fig. 11.4 for the polystyrene (PSt)-TEMPO adduct is known to occur in a limited number of systems (at high temperatures). A versatile use of the simple TEMPO-based SFRP is therefore not possible. For example, attempts to perform SFRP of monomers such as acrylonitrile (AN), methyl and ethyl acrylates (MA and EA), and 9-vinylcarbazole (VCz) with benzoyl peroxide (BPO) and TEMPO have not been successful. Interestingly, however, styrene has been successfully copolymerized (see Section 11.2.4) with these monomers using BPO initiator and TEiMPO under a living fashion (Fukuda etal., 1996). 

When the stationary concentration in the radical polymerization of vinyl compounds is larger than 10 M, ESR measurement of growing radicals does not require use of the special cavities described in Sect. 7.2. Infact, Kamachi etal. were able to observe, even at room temperature, a well-resolved 5-line spectrum in BPO-initiated, UV-irradiated polymerization of TPMA by use of the commercially available TE j... 

Others have also looked at the kinetic characteristics of the solution homopolymerization of MA, using benzene, dioxane, and acetic anhydride solvents with BPO initiator. In dioxane, polymerization rates are proportional to initiator concentration to the power, supporting a recombination mechanism for termination of the kinetic chains. Reminiscent of the y-radiation polymerization systems, an increased rate of polymerization and greater molecular weight is observed in acetic anhydride solvent. All evidence supports the assumption that acetic anhydride is not a pure solvent for the... 

Attempts to polymerize MA with 1 wt. % BPO in toluene at 60 C and 90°C and under 4900 atm gave low yields (20-70%) of dimer, depending on reaction temperature. " Using toluene solvent, others observed good results with dropwise addition of BPO initiator and running the polymerization for ca. Shat 90-95°C. " ... 

Mixtures of A-vinyl- -caprolactam with MA undergo spontaneous copolymerization to give alternating copolymer. Using BPO initiator, DMF solvent, and a polymerization temperature of 70°C, maximum rates were observed at a 1 1 monomer feed. The activation energy for the system is 13.0 kcal/mol, compared to 16.0 kcal/mol reported for alternative copolymerization of styrene.Intrinsic viscosities [77] of the copolymer could be calculated by the relationship... 

With acrylic acid, alternating copolymers are reported to be obtained only when MA is present in excess, otherwise some acrylic acid homopolymer is produced along with the 1 1 copolymer.These conclusions were reached after various molar ratios of the monomer pair were polymerized both in bulk and solution with BPO initiator. 

---