Syndiotactic radical PMMA

The reactions were carried out in dilute homogeneous solution in dipolar aprotic solvents ([ester]g=0.2-0.4 mole.l- ) using stereoregular (pure I or S) or predominantly syndiotactic radical (R) PMMA, polymethylacrylate (PMA) and radical azeotropic styrene-MMA copolymer (PSMMA, MMA mole.fraction = 0.47) as well as model monomeric (methylpivalate) and dimeric (dimethylglutarate) compounds. The overall reaction is outlined in the simplified scheme ... 

Puzin et al.h] reported that the tacticity of PMMA prepared in bulk is influenced (slight increase in syndiotacticity) by very small amounts of titanoecnc diehloride (1 O 3 M). Selective complexation of the propagating radical was postulated. 

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... 

FIGURE 14.2 X-band TREPR spectra of main-chain polymer radical la produced from 248 nm laser flash photolysis of atactic, isotactic, and syndiotactic PMMA in propylene carbonate at 0.8 ps delay time. The temperature for each spectrum is shown in °C, and the magnetic held sweep width is 150 G for aU spectra, which exhibit net E CIDEP in aU cases. Simulations of each fast motion spectrum (highest temperature) are shown at the bottom of each data set. Hyperflne values for each simulation are 3 anCCHs) = 22.9 G, 2aH(CH2)= 16.4 G, 2aH(CH2) = 11.7G for isotactic PMMA 3ah(CH3) = 22.9G, 2aH(CH2>= 16.2G,... 

Rh138 was almost the same (almost atactic, slightly syndiotactic) as the tacticity of those obtained with conventional radical initiators such as AIBN under similar conditions. The triad ratio of rr.mr.mm as determined by 13C NMR is usually 58 38 4 and does not change even with the use of chiral and/or bulky ligands.103116 These results may exclude a coordination mechanism and suggest a radical nature. However, the stereochemical structure alone is not strong evidence for the radical polymerization because, for example, group-transfer polymerization, basically via an anionic mechanism, results in a stereo structure of PMMA similar to those for free radical processes.263... 

The deuterium-labelled initiator method was also applied to the polymerization of macromonomer to determine the number of initiator fragments in a polymacromonomer chain (N) and initiator efficiency (/)-77 Isotactic and syndiotactic PMMA macromonomers having a styrene group as polymerizable function were polymerized with AIBN-<7 2 in toluene at 60°C and the resultant polymacromonomers were analysed by 2H NMR spectroscopy (Table 8). The N values were less than unity (0.50 0.72), indicating some kind of chain transfer reaction in the radical polymerization of macromonomer. The / values were 0.18 0.28 and much smaller than those for styrene polymerization (0.5 0.7). The isotactic macromonomer gave larger N and / values than the syndiotactic... 

The stereoregularity of the obtained PMMA samples was investigated via NMR analysis. The obtained data showed that polymers are predominantly syndiotactic (rr rm mm=58 37 6), as in case of free radical polymerization of MMA initiated by AIBN (i). 

Examples of these three types of structural arrangements are known in general, stereoregular polymers are synthesized by the use of coordination catalysts, whereas atactic polymers are formed by uncoordinated catalysts such as free radicals or free ions. Stereoregular polymers are often partially crystalline, and usually, even the isotactic and syndiotactic isomers have different properties. For example, isotactic poly(methyl methacrylate) (PMMA) has a glass-transition temperature of 35 °C, while that of the syndiotactic polymer is 105 °C. 

Chemical Properties. The chemical-resistance properties of methacrylic ester polymers are even higher than those of the acrylic esters. Methacrylic esters imdergo a lower degree of hydrolysis in either acidic or alkaline media than acrylics. Both acrylics and methacrylics easily outperform vinyl acetate-containing polymers which are well known to be susceptible to hydrolysis of the side-chain ester. There are marked differences in the chemical-resistance properties of different forms of PMMA. The syndiotactic (alternating) form of PMMA is the most chemically inert. The rate of hydrolysis for syndiotactic PMMA is lower than that for isotactic (26) radical polymerizations generate large portions of syndiotactic PMMA and benefit in terms of stability. 

Subsequent work by Collins and Ward confirmed this observation they showed that an equimolar mixture of the cationic alkyl complex [Cp2ZrMe(THF)]+[BPh4] (24, Scheme 23.8) and neutral Cp2ZrMe2 (25) generates low polydispersity PMMA (Mw/Mn = 1.2-1.4) with a syndiotactic bias comparable to free-radical MMA polymerizations (80% r dyads at 0 °C) and a microstructure... 

The next consideration is the assignment of the eight heterotactic triads. No purely heterotactic PMMA or MMA-MAA copolymer can be prepared at present. When using radical copolymerization of MMA and MAA a considerable amount of heterotactic triads may be formed, however. This is to be expected from the known triad statistics of the radical homopolymers PMMA and PMAA in which approximately one third of the triads are heterotactic and two thirds are syndio-tactic. The NMR spectra in pyridine of the radical copolymers, prepared in di-oxane at 60 °C with azoisobutyronitrile as initiator, are shown in Fig. 6. In the uppermost and lowermost traces I and IX the spectra of the radical homopolymers PMMA and PMAA are shown for comparison, while the copolymers are seen in traces II through VIII with decreasing MMA content from top to bottom. The chemical shifts of the six a-CH3 peaks in Fig. 6 are the same as in the syndio-tactic copolymers of Fig. 2. Therefore, the peaks of the heterotactic triads must coincide in some unknown way with these of the syndiotactic triads. 

Fig. 10. NMR spectra of a-CHa resonance lu DMF at 100 °C and 220 MHz from syndio-tactic copolymers with 67 and 34 mole % MMA, traces I and II, MMA-MAA ds and MMA d -MAA radical copolymers with 47 and 49 mole % MMA, traces III and IV, and isotactic copolymers with 68 and 32 mole% MMA, traces V and VI. Internal standard TMS. The small extraneous peaks on the low and high field slope of the third peak in traces I and II are due to a small amount of heterotactic copolymer triads which originate from the 8 % heterotactic triads in the syndiotactic PMMA used for hydrolysis...

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