Free radical initiator, PMMA

The PMMA bone cement is formed from a mixture of prepolymer PMMA powder, which contains a free-radical initiator, and liquid MMA monomer. In the operating theatre the powder and liquid are mixed, causing the initiator to dissolve and bring about polymerisation in the monomer component. The powder pre-polymer does not dissolve in the monomer but remains in the newly polymerised materials as a kind of reinforcing filler. 

A radical initiator based on the oxidation adduct of an alkyl-9-BBN (47) has been utilized to produce poly(methylmethacrylate) (48) (Fig. 31) from methylmethacrylate monomer by a living anionic polymerization route that does not require the mediation of a metal catalyst. The relatively broad molecular weight distribution (PDI = (MJM ) 2.5) compared with those in living anionic polymerization cases was attributed to the slow initiation of the polymerization.69 A similar radical polymerization route aided by 47 was utilized in the synthesis of functionalized syndiotactic polystyrene (PS) polymers by the copolymerization of styrene.70 The borane groups in the functionalized syndiotactic polystyrenes were transformed into free-radical initiators for the in situ free-radical graft polymerization to prepare s-PS-g-PMMA graft copolymers. 

PMMA can exist in two simple stereoregular forms, isotactic and syndiotactic, but commercially available samples—prepared via free-radical initiators—tend to have tacticities lying in the range 60-70% syndiotactic triad content, the exact content depending upon the reaction temperature.426 Several terminating side reactions have been identified, the most important of which is intramolecular cyclization leading to methoxide formation, as shown in Scheme 5.427... 

Park et al. (60) studied dispersion characteristics of MWCNT-PMMA composites synthesized by in-situ bulk polymerization using AIBN as free radical initiator. In their method, CNTs in varying amounts such as 0.001, 0.01 and 0.1 wt% with respect to MMA were first dispersed in MMA monomer by ultrasonication before polymerization. Experimental evidence such as molecular weight of free PMMA prepared via in-situ polymerization with and without CNTs, diameter of pristine MWCNT and diameter of MWCNT in composite, FTIR and SEM studies confirmed the role of AIBN and MWCNT in polymerization. The induced radicals on MWCNT by AIBN were found to trigger grafting of PMMA on to CNT surface. Solvent cast film of the composite was transparent and showed a better nanoscopic dispersion without aggregates compared to the cast film prepared from direct mixing of MWCNT and PMMA. 

The polymerization of vinyl monomers in liquid and supercritical CO2 has been studied extensively. Patents were issued in 1968 to the Sumitomo Chemical Company [81] and in 1970 to Fukui et al. [82] for the preparation of homopolymers of polystyrene, poly(vinyl chloride), poly(acrylonitrile) (PAN), poly-(acrylic acid) (PAA), and poly(vinyl acetate) (PVAc), as well as the random copolymers PS-co-PMMA and PVC-co-PVAc. Additionally, a patent was issued in 1995 to Bayer AG [83] for the preparation of styrene/acrylonitrile copolymers in SCCO2. In 1986, the BASF Corporation was issued a Canadian patent for the precipitation polymerization of 2-hydroxyethylacrylate and various N-vinylcarboxamides in compressed carbon dioxide [84]. In 1988, Terry et al. attempted to homopolymerize ethylene, 1-octene, and 1-decene in SCCO2 for the purpose of increasing the viscosity of CO2 for enhanced oil recovery [85]. These reactions utilized free-radical initiation with benzoyl peroxide and r-butylperoctoate at 71 °C and 100-130 bar for 24-48 h. Although the resulting polymers were not well characterized, they were found to be relatively... 

Using an original approach, Zhang and coworkers recently reported the synthesis of PMMA latex particles stabilized by MMT platelets tethered with poly[2-(dimethylamino)ethyl methacrylate] (PDMAEMA) brushes (Fig. 33) [290]. The PDMAEMA polyelectrolyte brush was synthesized by atom transfer radical polymerization using a cationic initiator previously introduced in the clay galleries. The PDMAEMA-functionaUzed clay platelets were further used to stabilize the emulsion polymerization of MMA initiated by the remaining free radical initiator present on the clay surface. 

The 500 MHz proton NMR spectra recorded with a superconducting magnet (11.7 T) for two samples of poly (methyl methacrylate) (PMMA) as 10% solutions in chlor-obenzene-ds at 100 °C are presented in Fig. 20.7 [3]. A free radical initiator was used in the polymerization of the syn-diotactic sample (s-PMMA) in (a), while the isotactic sample (i-PMMA) in (b) was obtained with an anionic initiator [4]. It is apparent from the methylene proton portions of both spectra that free radical and anionic initiated polymerization of methyl methacrylate results in PMMA samples with very different microstructures. 

Figure 1. The Uo MHz spectra of PMMA in chloroform. (a) Polymer prepared from free radical initiator (b) prepared from n-butyl-lithium in toluene, a typical anionic initiator.

The wide variation in the thermal degradation of PMMA can be explained in terms of the structure of the PMMA used and by the experimental conditions employed for preparing the polymer. A two-step degradation process results if the polymer has been prepared in the presence of air due to copolymerisation with oxygen but not to weak links formed by terminal combination since these would be present in all free-radical polymerisations. PMMA polymerised thermally is as stable as polymers initiated by free radicals in the absence of oxygen and peroxide impurities. It has a higher molecular weight and... 

The effects of increasing the concentration of initiator (i.e., increased conversion, decreased M , and broader PDi) and of reducing the reaction temperature (i.e., decreased conversion, increased M , and narrower PDi) for the polymerizations in ambient-temperature ionic liquids are the same as observed in conventional solvents. May et al. have reported similar results and in addition used NMR to investigate the stereochemistry of the PMMA produced in [BMIM][PFgj. They found that the stereochemistry was almost identical to that for PMMA produced by free radical polymerization in conventional solvents [43]. The homopolymerization and copolymerization of several other monomers were also reported. Similarly to the findings of Noda and Watanabe, the polymer was in many cases not soluble in the ionic liquid and thus phase-separated [43, 44]. 

By contrast, much of the work performed using ruthenium-based catalysts has employed well-defined complexes. These have mostly been studied in the ATRP of MMA, and include complexes (158)-(165).400-405 Recent studies with (158) have shown the importance of amine additives which afford faster, more controlled polymerization.406 A fast polymerization has also been reported with a dimethylaminoindenyl analog of (161).407 The Grubbs-type metathesis initiator (165) polymerizes MMA without the need for an organic initiator, and may therefore be used to prepare block copolymers of MMA and 1,5-cyclooctadiene.405 Hydrogenation of this product yields PE-b-PMMA. N-heterocyclic carbene analogs of (164) have also been used to catalyze the free radical polymerization of both MMA and styrene.408... 

Controlled free-radical polymerization methods, like atom-transfer radical polymerization (ATRP), can yield polymer chains that have a very narrow molecular-weight distribution and allow the synthesis of block copolymers. In a collaboration between Matyjaszewski and DeSimone (Xia et al., 1999), ATRP was performed in C02 for the first time. PFOMA-/)-PMMA, PFOMA-fr-PDMAEMA [DMAEMA = 2-(dimethylamino)ethyl methacrylate], and PMMA-/)-PFOA-/)-PM M A copolymers were synthesized in C02 using Cu(0), CuCl, a functionalized bipyridine ligand, and an alkyl halide initiator. The ATRP method was also conducted as a dispersion polymerization of MMA in C02 with PFOA as the stabilizer, generating a kine-... 

The homopolymer and block copolymer macromonomers were copolymerized with MMA by free-radical polymerization in benzene at 60 °C using AIBN as an initiator typical concentration were [MMA]=1.2 mol 1 1 and [macromonomer] =0.020 mol l"1. MMA was completely converted in 18 h and the macromonomers conversion reached more than 70% as determined by lH NMR. Incomplete conversion was explained by steric hindrance. Free-radical copolymerization resulted in high MW graft copolymers with PMMA backbone and relatively rigid, nonpolar poly(P-pinene) branches. 

However, literature sources very often claim that the presence of CNT in the reaction mixture significantly changes reaction kinetics of radical polymerization. This can be seen as consumption of free radicals, for example isobutyronitrile radicals from thermally decomposed initiator AIBN (43). It leads to increase of molecular weight of the resulting polymer, as shown in Table 8.1 for PMMA... 

The steady-state conversion data for VA and MMA, shown in Figures 1, 2 and 9, are qualitatively consistent with proposed mechanisms if one considers the transfer of free radicals out of particles and the gel effect. If radicals are completely free to move into and out of polymer particles, one would expect, in the absence of a gel effect, that Rp would depend on the squcure root of initiation rate and would not depend at all on the emulsifier concentration. Ley et al (17) demonstrated that free radicals do transfer out of particles in PVA and PMMA emulsions, and that the transfer rate is considerably higher for vinyl acetate than for MMA. 

Specihcally with regard to the pyrolysis of plastics, new patents have been filed recently containing variable degrees of process description and equipment detail. For example, a process is described for the microwave pyrolysis of polymers to their constituent monomers with particular emphasis on the decomposition of poly (methylmethacrylate) (PMMA). A comprehensive list is presented of possible microwave-absorbents, including carbon black, silicon carbide, ferrites, barium titanate and sodium oxide. Furthermore, detailed descriptions of apparatus to perform the process at different scales are presented [120]. Similarly, Patent US 6,184,427 presents a process for the microwave cracking of plastics with detailed descriptions of equipment. However, as with some earlier patents, this document claims that the process is initiated by the direct action of microwaves initiating free-radical reactions on the surface of catalysts or sensitizers (i.e. microwave-absorbents) [121]. Even though the catalytic pyrolysis of plastics does involve free-radical chain reaction on the surface of catalysts, it is unlikely that the microwaves on their own are responsible for their initiation. 

The term free radical is often used in the context of a reactive intermediate, as in the case of polymerization of vinyl monomers, but the same structure (unpaired electron) can and does exist in a kind of immobilized environment. For example, a bulk-polymerized (monomer and initiator only in the polymerization system) poly(methyl methacrylate) (PMMA) contains an appreciable number of free radicals that can be detected by electron spin resonance (ESR) [1]. When the polymerization system becomes highly viscous toward the end of the bulk polymerization, gel formation occurs and immobilizes the growing end of free radical chain growth polymerization, preventing recombination of two free radical ends of growing chains. 

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