Methyl bifunctional initiation

Group transfer polymerization allows the synthesis of block copolymers of different methacrylate or acrylate monomers, such as methyl methacrylate and allyl methacrylate [Hertler, 1996 Webster and Sogah, 1989]. The synthesis of mixed methacrylate-acrylate block copolymers requires that the less reactive monomer (methacrylate) be polymerized first. The silyl dialkylketene acetal propagating center from methacrylate polymerization is more reactive for initiation of acrylate polymerization than the silyl monoalkylketene acetal propagating center from acrylate polymerization is for initiation of methacrylate polymerization. Bifunctional initiators such as l,4-bis(methoxytri methyl si loxymethylene)cyclohexane (XXXIII) are useful for synthesizing ABA block copolymers where the middle block is methacrylate [Steinbrecht and Bandermann, 1989 Yu et al., 1988]. 

The synthesis, starting from a bifunctional initiator followed by quenching the double-headed living ends, gives homotelechelic polymers (method B). Carboxylate-capped telechelic poly(isobutyl vinyl ether) has been obtained in this way [82], where the adduct of a bifunctional vinyl ether with trifluoroacetic acid is the initiator, and the quencher is the malonate anion. For method C, a bifunctional trimethylsilyl enol ether, CH2=C[OSi(CH3)3]C6H4OCH2CH20C6H4[(CH3)3SiO]C=CH2, is a useful terminator (chain coupler) for vinyl ethers [142,147] and a-methyl-styrene [159] (see also Section VI.B.4). 

Numerous examples of block copolymers formed in supercritical C02 via the bifunctional initiator approach have been reported [54], Perhaps the most common approach is to incorporate eROP with free-radical polymerization-the general scheme for this methodology is shown in Figure 13.3. Howdle et al. [55] was the first to report the synthesis of a block copolymer by the bifunctional initiator approach in supercritical C02 and showed the simultaneous eROP of e-caprolactone with controlled free radical polymerization of methyl methacrylate by atom transfer radical polymerization (ATRP)-at this time simultaneous eROP and ATRP had not been reported in any media. The bifunctional initiator incorporated both a primary hydroxyl group (as an initiation site for eROP of e-caprolactone) and a bromine moiety (for initiation of ATRP). Howdle showed that... 

The azoperester, t-butyl 4-methyl-4-(t-butylazo)peroxypentanoate, cleaves at the azo group on excitation to yield the y-perester radical (231). Cyclisation of (231) by attack on the peroxy linkage to yield the corresponding y-lactone occurs sufficiently slowly (1.5 x 10 s at 22 °C) that the azoperester precursor may be used as a photochemical bifunctional initiator. In contrast thermolysis occurs at the peroxy linkage. Decarboxylation and cyclisation of y-azo radical (230) occur so rapidly that an azo-containing polymer would not be produced.Diarylfur-oxans (232) yield 1,2-diarylacetylenes in low yield on irradiation. ... 

In a recent report, Chang et al. [201] proposed a simple strategy for the one-step synthesis of PSt-b-PCL by using a combination of conventional free radical or reverse ATRP and AROP. These strategies involved the use of a symmetric bifunctional initiator (2,2-azobis[2-methyl-N-(2-hydroxyethyl) propionamide]) that was able to combine two dissimilar polymerization systems simultaneously. 

Living NCA polymerizations of a wide variety of monomers have also been initiated with transition metal catalysts, allowing the preparation of proteins and block copolypeptide-sin high yield with low polydispersity. NCA catalysts have also been employed to produce low-polydispersity protein-synthetic polymer hybrid materials. Using a bifunctional initiator in nickel-mediated NCA polymerization followed by ATRP, poly(Y-benzyl-L-glutamate-l7-methyl methacrylate) (PBLG-b-PMMA) was prepared with polydispersity in the range of 1.2-1.4. ... 

By the sequential polymerization of TrMA to either DMA or MMA (or vice versa), the subsequent hydrolysis and methylation of the resulting block copolymer should yield stereoblock polymers of PMMA. Polymer architecture can be controlled with selection of either monofunctional or bifunctional initiators. Segment distribution can be controlled by initiator and monomer concentrations. 

In the first way, the polyvinyl block is obtained by anionic polymerization as described before except that the monofunctional initiator (cumylpotassium) is replaced by a bifunctional initiator (dimer dianion of a-methyl-styrene/K). Then the amination of the two living ends and the synthesis of the polypeptide blocks are performed as described before for AB copolymers. 

Ozturk T, Yihnaz SS, Hazer B, Menceloglu YZ. ATRP of methyl methacrylate initiated with a bifunctional initiator bearing bromomethyl fimctional groups synthesis of the block and graft copolymers. J Polym Sci, Part A Polym Chem 2010 48 1364-73. 

Monofunctionalized PTHF is thus obtained by end capping living polymers with monofunctional initiators. Bifunctional initiators must be used to synthesize bifunctional telechelics. Yamashita has described the synthesis of bis(dioxolan-2-ylium) cations (60) and their use as initiators for the polymerization of THF. Bi- and tri-functional initiators (61)-(63) were synthesized by Penczek and co-workers. Chains reportedly grow independently at all sites, and copolymers can be formed if 10-20% of THF is replaced by methyl oxirane. ... 

Degradation of methyl terf-butyl ether by bifunctional aluminum in the presence of oxygen was investigated by Lien and Wilkin (2002). Bifunctional aluminum was synthesized by sulfating aluminum metal with sulfuric acid. When the initial methyl terf-butyl ether concentration was 14.4 mg/L, 90% of methyl ferf-butyl ether degraded within 24 h forming acetone, methyl acetate, tert-hniyX alcohol, and ferf-butyl formate. Carbon disulfide was tentatively identified as a reaction product by GC/MS. Product yields were 27.6% for acetone, 18.4% for methyl acetate, 21% for tert-hniyX alcohol, and 6.1% ferf-butyl formate. When the initial concentration of methyl tert-butyl ether was reduced to 1.4 mg/L, 99.5% of methyl terCbutyl ether reacted. Yields of acetone, methyl acetate, and /erf-butyl alcohol were 54.7,17.2, and 13.2, respectively. 

Dieb-Alder catalyst. The key step in a recent total synthesis of androstanes is a SnCVcatalyzed Diels-Alder reaction of 1 with the (Z)-dienophile 2. The geometry of the diene favors addition ami to the C,0-methyl group, and the catalyst promotes the desired enrfo-orientation. A1C1, and BF3 ctherate are less suitable for additions involving aliphatic bifunctional dienophiles. The initial adduct a can be isolated, but in only 15-20% yield. The synthesis of the androstane 4 is completed by ketalizatioh of 3 followed by a novel cyclization affected with dimsylsodium. ... 

The methyl-substituted BCR is useful for the construction of some iridoid cyclopentanoids. Keto alcohol (137), a crucial intermediate for a synthesis of ( )-chrysomelidial (138), can be prepared from the cycloadduct (108) of cyclopentenone and the methyl-TMM synthon (equation 145)7 This expedient approach to the keto cohol, four steps and 83% overall yield from the bifunctional reagent (107), is a considerable improvement over a previous 14-step sequence using conventional methodologies. Although the initial bicyclic ketone (108) is a 1 1 epimeric mixture, base-catalyzed equilibration of the products from ozonolysis results in only one epimer, having the required stereochemistry. This... 

Various other rhodium catalysts can initiate hydroacylation reactions. Thus, the indenyl complex [075-C9H7)Rh(J72-C2H4)2] is used in intermolecular hydroacylation44. Rhodium zeolites (RhNaX and RhNaY type zeolites) act as bifunctional catalysts for the synthesis of 2-methyl-3-hexanone and 4-heptanone (1 2 ratio) from propene, carbon monoxide and hydrogen53. In this case, the ketones may be formed via hydrocarbonylation (vide supra), however, according to control experiments, rhodium-free zeolites alone catalyze ketone formation from propene and butyraldehyde53. 

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