Stereoselective Polymerization of an Aromatic Vinyl Monomer to Access Highly Syndiotactic Poly(vinyl alcohol)

Abstract

increase of crystallinity. [5] Radical or reversible addition-fragmentation chain transfer (RAFT) polymerization of vinyl esters (VA cannot exist as a stable chemical) followed by hydrolysis of resultant poly(vinyl ester) could only give syndio-enriched PVA (r = 78%). [6,7] Known that coordination polymerization is the most efficient manner to access highly stereoregular polymers, with respect of the polar vinyl esters; however, it suffers a serious problem that the catalyst is easily poisoned by the polar groups. To date copolymerizations of ethylene with a wide range of polar monomers have been achieved by protecting or separating the polar groups from contacting with the active species [8][9][10] or using late transition metal catalysts of tolerance to polar groups. [11][12][13][14][15][16] However, both the activity and the selectivity decrease obviously, and low molecular weight products containing a small amounts of polar monomer are always obtained. Regarding to homopolymerization of polar monomers, it is still a research topic far from success, although recent reports demonstrate that polar groups, in some cases, do not poison but activate the catalysts to afford high molecular weight and highly regular homo-or copolymers. [17,18] We wish to solve this problem in an alternative method by employing vinyl monomers containing nonpolar but "reactive" substituent to perform stereoselective polymerization, and then transferring the substituent into other functional groups through postpolymerization modification. [19,20] Vinyl compounds bearing boraza (BN) aromatic ring are such monomers where the amino nitrogen with high coordinative capacity is well protected by the single intramolecular boron atom instead of a bulky substituent; boron atom in the aromatic ring can be easily transformed into other group without racemizing the potential chirality of the adjacent carbon atom, thus the stereoregularity of resulting polymers is remained after postpolymerization modification. A series of BN containing monomers such as BN-styrene and BN-vinylbiphenyl are reported to polymerize via RAFT mechanism. [21,22] 2-Vinyl-2,1-borazanaphthalene (BN2VN) can be homo-and copolymerized with 2-vinylnaphthalene via radical catalysts to give high molecular weight polymers while it can only be homopolymerized in moderate syndioselectivity by titanium catalysts. Noted that, in all the above cases long reaction times are required. [23][24][25][26] Recently, rare-earth metal based catalytic systems have been reported to be extraordinary tolerance toward polar groups, which exhibit high activity and excellent stereo-selectivity for homo-and copolymerizations of polar styrenes. [17,18,[27][28][29][30][31][32][33][34] We Streoregular poly(vinyl alcohol) is hard to obtain because vinyl alcohol is unstable relative to its tautomer acetaldehyde, and the monomer precursor vinyl ester is poisonous to the coordination catalyst. Herein, the coordination poly merization of 2vinyl2,1borazanaphthalene (BN2VN) is reported by the linked or unlinked hal...

Azaborine benzylic ion stability and reactivity in ionic polymerization

“…Initial efforts focused on Lewis acid catalysts, given the observed stability of BN2VN to Lewis acidic coactivators in coordination polymerization. 16,17 However, a screen of several Al- and B-based Lewis acids known to promote styrene cationic polymerization failed to initiate BN2VN polymerization (Table 1). In contrast to styrene polymerizations conducted as control reactions, only unreacted starting material was observed by 1 H NMR spectroscopy (Fig.…”

“…16 Later work by Cui et al found that Sc-complexes were also effective for syndioselective polymerization of BN2VN. 17 The stability of BN2VN to coordination polymerization is notable, given the use of highly Lewis acidic complexes such as B(C 6 F 5 ) 3 or methylalumoxane (MAO), and suggested the potential viability of cationic polymerization.…”

Taisui TS-2007S, a Large Microbial Mat Discovered in Soil in China

“…This PVA solution was essentially made from polyvinyl acetate, not the polymerization of vinyl alcohol, because vinyl alcohol is a highly unstable monomer relative to its tautomer acetaldehyde. Furthermore, the vinyl ester monomer precursor is poisonous to the coordination catalyst, making PVA difficult to obtain (Halima, 2016;Huang et al, 2020). Taken together, these factors indicate that PVA should not be naturally formed.…”

Syndiospecific Polymerization of o-Methoxystyrene and Its Silyloxy or Fluorine-Substituted Derivatives by HNC-Ligated Scandium Catalysts: Synthesis of Ultrahigh-Molecular-Weight Functionalized Polymers

“…The coordination (co)­polymerization of polar olefins to prepare high functional polyolefins having superior surface properties, adhesion properties, and compatibility with other types of materials has always been a challenging and high-profile issue in academia and industry. − The most serious difficulty lies in the fact that the high oxyphilic early transition metal catalysts can be easily poisoned by polar olefins because the stronger Lewis basic heteroatom groups of polar olefins have more preferential coordination ability than the weaker Lewis basic double bond to the strong Lewis acidic metal centers. − Subsequently, the low oxyphilic late transition metal catalysts, which are more tolerant of heteroatom functionalities in the monomer and the polymer, have ascended on the history stage and gradually began to open up new areas for the coordination (co)­polymerization of polar olefins such as polar norbornene, acrylates, vinyl ethers, vinyl acetate, vinyl halides, acrylonitrile, CO, etc. − Recently, a significant breakthrough in the coordination (co)­polymerization of polar olefins has been made by the high oxyphilic rare-earth metal catalysts. − Activated by a catalytically equimolar amount of cocatalyst borate and/or an excess of AlR 3 , rare-earth metal alkyl cations [LLnR] + generated from a series of rare-earth metal dialkyl complexes LLnR 2 L' n (Ln is a rare earth metal center, L is a negative monovalent multi-dentate supporting ligand, R is a negative monovalent alkyl ligand, L' n is a neutral ligand like tetrahydrofuran (THF) solvent molecules with a number in the range of 0–2) exhibit high activities and stereoselectivities in the coordination (co)...…”