Alkyl methacrylates, hydrolysis

Alkyl methacrylates, hydrolysis of polymeric ester functionality, 259 Aluminum-hydrogen bond, nucleophilic substitution, 264 Amines alkylation, 28 benzyl-group cleavage, 25 Aminomethylation chloromethylated polymers, 19 Deltfpine reaction, 19 Anionic polymerization advantages, 85... 

By employing anionic techniques, alkyl methacrylate containing block copolymer systems have been synthesized with controlled compositions, predictable molecular weights and narrow molecular weight distributions. Subsequent hydrolysis of the ester functionality to the metal carboxylate or carboxylic acid can be achieved either by potassium superoxide or the acid catalyzed hydrolysis of t-butyl methacrylate blocks. The presence of acid and ion groups has a profound effect on the solution and bulk mechanical behavior of the derived systems. The synthesis and characterization of various substituted styrene and all-acrylic block copolymer precursors with alkyl methacrylates will be discussed. 

Various substituted styrene-alkyl methacrylate block copolymers and all-acrylic block copolymers have been synthesized in a controlled fashion demonstrating predictable molecular weight and narrow molecular weight distributions. Table I depicts various poly (t-butylstyrene)-b-poly(t-butyl methacrylate) (PTBS-PTBMA) and poly(methyl methacrylate)-b-poly(t-butyl methacrylate) (PMMA-PTBMA) samples. In addition, all-acrylic block copolymers based on poly(2-ethylhexyl methacrylate)-b-poly(t-butyl methacrylate) have been recently synthesized and offer many unique possibilities due to the low glass transition temperature of PEHMA. In most cases, a range of 5-25 wt.% of alkyl methacrylate was incorporated into the block copolymer. This composition not only facilitated solubility during subsequent hydrolysis but also limited the maximum level of derived ionic functionality. 

Although the potassium superoxide route can be universally applied to various alkyl methacrylates, it is experimentally more difficult than simple acid hydrolysis. In addition, limited yields do not permit well-defined hydrophobic-hydrophilic blocks. On the other hand, acid catalyzed hydrolysis is limited to only a few esters such as TBMA, but yields of carboxylate are quantitative. Hydrolysis attempts of poly(methyl methacrylate) (PMMA) and poly(isopropyl methacrylate) (PIPMA) do not yield an observable amount of conversion to the carboxylic acid under the established conditions for poly(t-butyl methacrylate) (PTBMA). This allows for selective hydrolysis of all-acrylic block copolymers. 

Brown and White employed this approach to prepare block copolymers of styrene and mcthacrylic acid (6). They were able to hydrolyze poly(styrene-b-methyl methacrylate) (S-b-MM) with p-toluenesulfonic acid (TsOH). Allen, et al., have recently reported acidic hydrolysis of poly(styrene-b-t-butyl methacrylate) (S-b-tBM) (7-10). These same workers have also prepared potassium methacrylate blocks directly by treating blocks of alkyl methacrylates with potassium superoxide (7-10). 

We also explored the direct conversion of S-b-tBM to S-b-MA.K. Hydrolysis under basic conditions (KOH in refluxing aqueous THF) was again resulted in unchanged S-b-tBM. The reaction with potassium trimethylsilanolate for 1 hr in refluxing toluene gave very little reaction. Only 10% of the expected amount of potassium was found by ICP, and the NMR and IR spectra were little changed from those of the starting copolymer. This difference in reactivity between S-b-MM and S-b-tBM parallels that observed for the reaction of alkyl methacrylate blocks with potassium superoxide (7-10). 

Short-chain monomers, in particular ethyl acrylate and methyl methacrylate in a volume ratio of 5 1, are introduced in liquid form without solvents in a dosed system. Low-intensity y-radiation polymerizes the monomers under ambient conditions. Additionally, the liquid monomer mixture has been modified to indude an amine-substituted alkyl methacrylate to neutralize acidity and establish a buffer against future acid catalyzed hydrolysis [34]. The presence of a small amount of a diacrylate leads to cross-linking and hence strengthening of the paper. 

The results of this work are not limited to just S-b-MM and S-b-tBM, but may be extended to include styrene derivatives such as p-methylstyrene and p-t-butylstyrene 1). In addition to t-butyl methacrylate, other alkyl esters capable of stabilizing a carbonium ion, such as benzyl methacrylate and allyl methacrylate, should exhibit similar reactivity toward acidic hydrolysis and TMSI. In contrasting the hydrolysis of tBM blocks with TsOH and their reaction with TMSI, it should be noted that the hydrolysis is reportedly catalytic in nature (7-10), whereas the reaction with TMSI is stoichimetric. Therefore the latter approach may allow one to more easily "dial in" a desired level of methacrylic acid or metal methacrylate. 

Conversion of the phthalimide to the amine was confirmed by a peak at (5 3.2 ppm corresponding to the hydrogen adjacent to the amine group. The functionalization reaction was also monitored by MALDI-TOF MS. Characterization of the phthalimide-functionalized polymer confirmed the conversion of the bromide group. Characterization of the amine-functionalized polymer showed the presence of the desired product, but other side products were also observed. Upon hydrolysis of the phtha-limide-functionalized polymer, a transesterification reaction occurs converting the initiator moiety (ethyl-2-bromoisobuty-rate) from an ethyl ester to a t-butyl ester due to reaction with t-butyl alcohol. One drawback of this reaction is that the Gabriel reaction is only effective for primary alkyl halides and would not be useful for methyl methacrylates or methyl acrylates. 

Vinyl ethers copolymerize with methyl methacrylate to give alternating copolymer units as well as homopolymer segments of methyl methacrylate [35]. With other monomers this is also true, and either solution, emulsion, suspension, or bulk polymerization techniques may be used. The pH should be kept at about 8 or above in aqueous systems to prevent hydrolysis of the vinyl ether. The / 2 (alkyl vinyl ether) values are low and approach zero for bulk polymerization systems [36-38] utilizing such monomers... 

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