This article reviews the development of catalysts for ring-opening metathesis polymerization (ROMP), synthesis of polymers bearing amino acids and peptides by ROMP of functionalized norbornenes, formation of aggregates and micelles, and applications of the polymers to medical materials. It also describes the control of monomer unit sequences, that is, living polymerization to synthesize block copolymers, and alternating copolymerization that is achieved on the basis of acid-base interactions. Polymer Journal (2010) 42, 905-915; doi:10.1038/pj.2010.94; published online 13 October 2010Keywords: alternating copolymerization; amino acid; block copolymerization; living polymerization; metathesis catalyst; peptide; ROMP INTRODUCTIONOlefin metathesis reactions are metal-mediated carbon-carbon (C-C) double bond exchange processes, 1,2 which were discovered in the mid 1950s. Chauvin proposed the commonly accepted mechanism for metathesis involving a metallacyclobutane, as illustrated in Scheme 1. 3 Initially, olefin metathesis was regarded as an odd reaction, but now it has undoubtedly established the position as one of the most important C-C bond formation reactions applicable to synthesis of a wide variety of useful products. In the early stages, transition metal chlorides were used as catalysts for the reaction, but the transition metal carbene complex catalysts designed by Schrock and Grubbs have remarkably advanced mechanistic analysis and control of catalytic activity by the choice of ligands. In 2005, Chauvin, Grubbs and Schrock were awarded the Nobel Prize in chemistry for development of the metathesis method in organic synthesis.Olefin metathesis polymerization is an application of metathesis reactions to polymer synthesis and includes ring-opening metathesis polymerization (ROMP) and acyclic diene metathesis (ADMET) polycondensation (Scheme 2). ADMET has been extensively developed by Wagener since 1987 4 for the synthesis of polyolefins having regularly spaced functional group branches and high thermal stability and crystallinity. 5,6 ADMET is also useful for synthesizing polymeric materials containing in-chain functionality. Although the general structures of the polymers obtained by ROMP and ADMET are illustratable in the same fashion as shown in Scheme 2, a completely different treatment is necessary from the viewpoint of polymerization kinetics. The former involves chain polymerization, whereas the latter is a step-growth polymerization process.

This review summarizes the recent advances of substituted polyacetylene chemistry, including new polymerization catalysts and the properties and functions of the polymers.

N-Propargylamides having pendent groups with different lengths (HCtCCH2NHCOR, R ) (CH2)nH, n ) 1-8) were polymerized in the presence of a Rh catalyst [(nbd)Rh + B -(C6H5)4; nbd ) 2,5-norbornadiene] to obtain polymers with moderate molecular weight and high stereoregularity [poly(1)-poly(8)] in high yields. The conformational transition behavior of the resultant polymers was investigated by measuring UV-vis spectra in chloroform solution at different temperatures. Among the examined polymers, poly(5) and poly(6) took a stable helical conformation at relatively high temperatures, and their helical contents were the highest. Poly(2)-poly(4), which bear shorter alkyl pendent chains, did not exist in stable helical conformation owing to the lack of chain flexibility and the intermolecular hydrogen bonding. Poly(1) was not completely soluble in any solvents. Poly(7) and poly (8), which contain longer pendent chains, took helical conformation only at low temperatures because of the lower cis content of the polymer main chain. Both ∆H and ∆S for the conformational transition from random coil to helix assumed negative values, which also greatly depended on the length of the pendent groups. Whereas the helical conformation of poly(5) and poly(6) was readily generated in chloroform, neither THF nor toluene was favorable for helix formation.

N-Propargylamides with one, two, and three phenyl groups at the R-position of the carboxyl group [HC≡CCH2NHCOR; 1, R ) C(CH3)2C6H5; 2, R ) CCH3(C6H5)2; 3, R ) C(C6H5)3] were polymerized with a rhodium catalyst, (nbd)Rh + B -(C6H5)4 (nbd ) 2,5-norbornadiene), to obtain the corresponding polymers in 85-91% yields. Poly(1) possessed a moderate molecular weight (Mn ) 6300) and was thoroughly soluble in chloroform and dichloromethane. On the other hand, poly(2) and poly(3) were not totally soluble in the solvents. The Mn's of chloroform-soluble parts were less than 3000. The secondary structure of these three polymers in chloroform was examined by UV-vis spectroscopy with varying temperature. It was found that only poly(1) could adopt helical conformation even at 60 °C. By the copolymerization of either monomer 2 or 3 with HC≡CNH2CO(CH2)4H ( 4), the solubility of the polymers was effectively improved, and the Mn's were remarkably increased. When the content of unit 4 in poly-(2-co-4)s was 25% and above, the copolymers could form helical conformation with different degrees, among which poly(20.40-co-40.60) showed the largest helicity. When the content of unit 4 of poly(3-co-4)s exceeded 95%, the copolymer took helical structure partly.

Summary: 2,2,6,6‐Tetramethylpiperidine 1‐oxyl (TEMPO)‐containing N‐propargylamide HCCCH2NHCO‐4‐TEMPO (1), propargyl ester HCCCH2OCO‐4‐TEMPO (2), phenylacetylene derivative HCCC6H3‐3,4‐(CO2‐4‐TEMPO)2 (3), and norbornene diester monomers, NB‐2,3‐exo,exo‐(CH2OCO‐4‐TEMPO)2 (4), NB‐2,3‐endo,exo‐(COO‐4‐TEMPO)2 (5a), NB‐2,3‐endo,endo‐(COO‐4‐TEMPO)2 (5b) (NB = norbornene, TEMPO = 2,2,6,6‐tetramethyl‐1‐piperidinyloxyl) were synthesized and polymerized with rhodium and ruthenium catalysts. Monomers 2, 5a, and 5b gave polymers with number‐average molecular weights of 47 000–185 000 in 59–100% yields, while 1, 3, and 4 gave polymers insoluble in common organic solvents in 88–100% yields. The capacities of cells fabricated with poly(1), poly(2), and poly(3) were 67, 82, and 23 Ah · kg−1 based on the weight, respectively. The capacity of poly(5a)‐based cell reached the theoretical value (109 Ah · kg−1) of the polymer.Charge–discharge curves of poly(5a) at a current density of 0.13 mA · cm−2 (100 mA · g−1‐cathode active material) in the voltage range of 2.5–4.2 V.magnified imageCharge–discharge curves of poly(5a) at a current density of 0.13 mA · cm−2 (100 mA · g−1‐cathode active material) in the voltage range of 2.5–4.2 V.

N-Propargylamides with bulky pendent groups [HCtCCH2NHCOR, 9: R ) CH2C(CH3)3, 10: R ) C(CH3)3, 11: R ) C(CH3)2CH2CH2CH3, 12: R ) CH(CH2CH3)2, 13: R ) CH(CH2CH2CH3)2] were polymerized with a rhodium catalyst, (nbd)Rh + B -(C6H5)4 (nbd ) 2,5-norbornadiene), to obtain the polymers in 80-92% yields. Poly(11) and poly(12) possessed moderate molecular weights (Mn g 10 000) and were totally soluble in a few solvents including chloroform. On the other hand, the Mn values of poly( 9), poly-(10), and poly(13) were no more than 5000, and these polymers were not completely soluble in any solvents. The conformational transition behavior of these polymers was examined by temperature-variable UVvis spectroscopy in chloroform solution, which revealed that poly(10)-poly(13) could form dynamically stable helical conformation even at 60 °C. By copolymerizations of monomers 9 and 10 with monomer 4, HCtCCH2NHCO(CH2)4H, the solubility of the polymers was effectively improved and almost all the copolymers totally dissolved in chloroform, while the molecular weights of the copolymers increased up to 18 600-45 000. Moreover, the helix contents of poly(4 0.63-co-90.37) and poly (40.40-co-100.60) were the highest among the two series of (co)polymers, respectively. It is concluded that the copolymerization of 9 and 10 with 4 effectively decreased the steric repulsion between the crowded side chains, which probably allowed the copolymers to take helical conformation efficiently.

The diimine platinum(II) ethylene hydride complex [(N/\N)Pt(H)(ethylene)][BAr'4] (1, N/\N = [(2,6-Me2C6H3)N=C(An)-C(An)=N(2,6-Me2C6H3)], An = 1,8-naphthalenediyl, Ar' = 3,5-(CF3)2C6H3) was prepared by protonation of the diethyl complex (N/\N)PtEt2 with [H(OEt2)2][BAr'4]. The energy barrier to interchange of the platinum hydride with the olefinic hydrogens in 1 was determined to be 19.2 kcal/mol by spin saturation transfer experiments. Complex 1 initiates ethylene dimerization; the ethyl ethylene complex (N/\N)Pt(Et)(ethylene)+ (2) has been identified as the catalyst resting state. Trapping of 1 by ethylene to yield 2 is a second-order process; kinetic studies suggest this occurs via trapping of a reversibly formed beta-agostic ethyl complex. Complex 2 has been isolated and characterized by X-ray crystallography. The barrier to migratory insertion of 2, the turnover-limiting step in catalysis, was determined to be 29.8 kcal/mol. The 1-butene hydride complex, (N/\N)Pt(H)(1-butene)+ (3), is a key intermediate in the dimerization cycle and has also been isolated and characterized. Surprisingly rapid rates of degenerate associative exchange of free ethylene with bound ethylene in complexes 1 and 2 as well as the rate of degenerate exchange of free nitrile with bound nitrile in (N/\N)Pt(Et)(CH3CN)+ are reported.

Acetylenic monomers containing indan and other groups (1a-m) were polymerized with TaCl 5n-Bu 4 Sn catalyst to give high molecular weight polymers. Most polymers were soluble in common organic solvents including toluene and chloroform, and they afforded free-standing membranes by the solution casting method. The onset temperature of weight loss of the polymers were over 400 °C, indicating high thermal stability. Despite the absence of bulky spherical groups, polymethylated indan-containing polymer membranes showed extremely high gas permeability. In particular, the oxygen permeability coefficients of polymers having 1,1,3,3tetramethylindan and either p-fluorophenyl or p,m-difluorophenyl groups (2b and 2e) reached 17 900 and 18 700 barrers, respectively, which are even larger than that of the most permeable polymer known, poly(1-trimethylsilyl-1-propyne).