Real‐time FTIR monitoring of the mechanism of initiation of isobutylene polymerizations by epoxide/Lewis acid systems

Real‐time Fourier Transformation Infrared (FTIR) spectroscopy with a fiber optic transmission probe (TR) was used to monitor the polymerization of isobutylene (IB) initiated by α‐methylstyrene epoxide (MSE) and 1,2‐epoxi‐2,4,4‐trimethylpentane (TMPO‐1) in conjunction with TiCl₄ and BCl_3. In the presence of an equimolar amount of BCl₃, MSE and TMPO‐1 underwent ring opening via S_N1 mechanism. In contrast to TiCl₄‐coinitiated reactions, no oligoether formation via S_N2 mechanism was observed. TMPO‐1 with excess BCl₃ initiated IB polymerization, yielding a telechelic PIB carrying α‐primary OH and ω‐tertiary Cl functionalities with 70% initiator efficiency. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 3611–3618, 2008     

New epoxide initiators for the controlled synthesis of functionalized polyisobutylenes

The mechanism of initiation was investigated in isobutylene (IB) polymerizations initiated by epoxidized α‐methylstyrene (MSE) and 1,2‐epoxy‐2,4,4‐trimethylpentane (TMPO) in conjunction with TiCl₄. The proposed mechanism predicts primary OH head groups and tertiary Cl end groups in the PIB. Model studies conducted with MSE/TiCl₄ and diisobutylene lead to ring closure yielding a substituted furanyl structure. Real‐time fiber‐optic refractive index monitoring was used to follow the initiation with the TMPO/TiCl₄ system. It was found that the cleavage of TMPO proceeds simultaneously by S_N1 and S_N2 mechanisms as proposed. The carbocation forming by the S_N1 route is proposed to initiate the polymerization of IB, but it was shown that excess TiCl₄ relative to TMPO was necessary for propagation. Isomerization and polyether formation by the S_N2 pathway lead to side reactions, reducing the initiating efficiency.     

Investigation of the effect of epoxide structure on the initiation efficiency in isobutylene polymerizations initiated by epoxide/TiCl4 systems

The effect of the chemical structure of styrene-based epoxides, namely, styrene epoxide (SE), α-methylstyrene epoxide (MSE), p-methylstyrene epoxide (pM-SE) and α-methyl-p-methylstyrene epoxide (pM-MSE), in conjunction with TiCl₄, on the initiation efficiency (I_eff) in the carbocationic polymerization of isobutylene (IB) was investigated. SE yielded living polymerization, but the initiation efficiency was low when compared to MSE (I_eff=8% and 35%, respectively). pM-SE led to non-living IB polymerization, while pM-MSE revealed linear M_n-conversion plot and narrow MWD with a non-linear first order rate plot. Among the epoxides investigated, MSE was the best initiator to scale up the one-step synthesis of polyisobutylenes (PIBs) carrying one primary hydroxyl head group and one tertiary chloride end group. The hydroxyl functionality of these PIBs determined by ¹H-NMR was F_n=1.09±0.16 from 24 experiments.     

Direct functionalization of polyisobutylene by living initiation with α‐methylstyrene epoxide

This article describes the synthesis and characterization of polyisobutylene (PIB) carrying one primary hydroxyl head group and a tertiary chloride end group, [Ph? C(CH₃)(CH₂OH)–PIB–CH₂? C(CH₃)₂Cl] prepared with direct functionalization via initiation. The polymerization of isobutylene was initiated with the α‐methylstyrene epoxide/titanium tetrachloride system. Living conditions were obtained from ?75 to ?50 °C (198–223 K). Low molecular weight samples (number‐average molecular weight ～ 4000 g/mol) were prepared under suitable conditions and characterized by Fourier transform infrared and ¹H NMR spectroscopy. The presence of primary hydroxyl head groups in PIB was verified by both methods. Quantitative Fourier transform infrared with 2‐phenyl‐1‐propanol calibration and ¹H NMR performed on both the hydroxyl‐functionalized PIB and its reaction product with trimethylchlorosilane showed that each polymer chain carried one primary hydroxyl head group. The synthetic methodology presented here is an effective and simple route for the direct functionalization of PIB. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 1005–1015, 2002     

Sulfonation of tert-alkyl chlorides: Application to the tert-chloride-terminated polyisobutylene system

The direct sulfonation of tert-chloride-terminated polyisobutylene was carried out using acetyl sulfate in methylene chloride/hexanes diluent to produce low molecular weight model ionomer systems with narrow molecular weight distribution. The room temperature sulfonation of 2-chloro-2,4,4-trimethylpentane, which serves as a model for the polyisobutylene chain end obtained in the controlled cationic polymerization of isobutylene, was found to be quantitative after 3 h reaction. The two major products, isomeric β,γ-unsaturated sulfonic acids differing only in the location of the double bond, were the same products obtained from the sulfonation of 2,4,4-trimethyl-1-pentene, which serves as a model for the polyisobutylene chain end after quantitative dehydrochlorination. Near-monodisperse, tert-chloride-terminated, three-arm star polyisobutylene was synthesized under living cationic polymerization conditions using the tricumychloride/TiCl₄/pyridine initiation system in hexanes/methyl chloride consolvents. Sulfonation was carried out directly upon the obtained polymer, and the sulfonated product was purified using ion-exchange chromatography. Structural elucidation was performed using NMR. Titration and molecular weight characterization revealed that sulfonation produced exactly one sulfonic acid group per polyisobutylene chain end.     

The role of solvent‐ligated metal(II) complexes incorporating (fluoroalkoxy)aluminates as weakly coordinating anions in isobutylene polymerization

Nitrile‐ligated copper(II) and zinc(II) complexes comprising (fluoroalkoxy)aluminates as weakly coordinating anions (WCAs) have been synthesized and applied for the polymerization of isobutylene at room temperature (30°C). The polymers obtained are in the low and moderate molecular weight range and show characteristics of the highly reactive polyisobutylene. Results indicate that the fluoroalkoxy aluminate WCAs have even a higher tolerance toward water in IB polymerization than the earlier tested perfluoroborate WCAs. Studies showed that water plays an important role in the polymerization process, which indicates a polymerization mechanism similar to a proton‐initiated carbocation polymerization. The role of the WCAs and their importance for the room‐temperature polymerization process was re‐examined, and the effect of the addition of proton and electron donors including proton traps (2,6‐di‐tert‐butyl‐4‐methylpyridine or DTBP) was studied in detail. The polymerization reaction seems to be dominated by transfer reactions that lead to the high content of exo double bonds while propagation proceeds via conventional cationic polymerization. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

Cationic polymerization with boron halides. IV. Elucidation and control of initiation and termination by the help of model experiments

The proposition that BCl₃-coinitiated olefin (isobutylene, styrene) polymerizations terminate by chlorination has been corroborated by model experiments. Key experiments showed that under simulated polymerization conditions neither tert-butyl chloride nor 2-chloro-2,4,4-trimethylpentane reacts with BCl₃; that H₂O/BCl₃ + 2,4,4-trimethyl-1-pentene (TMP) produce 2-chloro-2,4,4-trimethylpentane; and that H₂O/BCl₃ + isobutylene gives rise to tert-butyl chloride. Extended model studies demonstrated that certain alkyl and benzyl chlorides produce carbenium ions in the presence of BCl₃ and that TMP can readily be alkenylated by using 1-substituted allyl chlorides in conjunction with BCl₃. These experiments led to the discovery that olefin polymerizations may be initiated by suitable allyl or benzyl chlorides and BCl₃. Accordingly, polymerizations of isobutylene have been carried out with RCl/BCl₃, where R is allyl or benzyl. These experiments suggest that both controlled initiation and termination, i.e., initiation by alkenylation and termination by chlorination, can be achieved with the allyl chloride/BCl₃ initiator system opening new avenues toward the synthesis of asymmetric telechelic polymers.     

Construction of PIB‐b‐PDEAEMA well‐defined amphiphilic diblock copolymers via sequential living carbocationic and RAFT polymerization

A series of well‐defined amphiphilic diblock copolymers consisting of hydrophobic polyisobutylene (PIB) and hydrophilic poly(2‐(diethylamino)ethyl methacrylate) (PDEAEMA) segments was synthesized via the combination of living carbocationic polymerization and reversible addition fragmentation chain transfer (RAFT) polymerization. Living carbocationic polymerization of isobutylene followed by end‐capping with 1,3‐butadiene was first performed at ?70 °C to give a well‐defined allyl‐Cl‐terminated PIB with a low polydispersity (M_w/M_n =1.29). This end‐functionalized PIB was further converted to a macromolecular chain transfer agent for mediating RAFT block copolymerization of 2‐(diethylamino)ethyl methacrylate at 60 °C in tetrahydrofuran to afford the target well‐defined PIB‐b‐PDEAEMA diblock copolymers with narrow molecular weight distributions (M_w/M_n ≤1.22). The self‐assembly behavior of these amphiphilic diblock copolymers in aqueous media was investigated by fluorescence spectroscopy and transmission electron microscope, and furthermore, their pH‐responsive behavior was studied by UV‐vis and dynamic light scattering. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 1478–1486     

Precision synthesis and characterization of thymine‐functionalized polyisobutylene

A new two‐step synthesis of polyisobutylene (PIB) with precisely one thymine functionality per chain (PIB‐T) is reported. The primary hydroxyl‐functionalized PIB (PIB‐OH) precursor was prepared by direct functionalization via living carbocationic polymerization of isobutylene initiated by the α‐methylstyrene epoxide/TiCl₄ system. Matrix assisted laser desorption/ionization time‐of‐flight mass spectrometry (MALDI‐ToF MS) of a low molecular weight PIB‐OH precursor demonstrated the effectiveness of direct functionalization by this method. A PIB‐acrylate precursor (PIB‐Ac) was obtained from such a PIB‐OH, and the PIB‐T was subsequently prepared by Michael addition of thymine across the acrylate double bond. MALDI‐ToF MS of the products verified that all polymer chains carried precisely one thymine group. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 3501–3506, 2010     

Highly reactive polyisobutylenes via AlCl3OBu2‐coinitiated cationic polymerization of isobutylene: Effect of solvent polarity,temperature, and initiator

The cationic polymerization of isobutylene using 2‐phenyl‐2‐propanol (CumOH)/AlCl₃OBu₂ and H₂O/AlCl₃OBu₂ initiating systems in nonpolar solvents (toluene, n‐hexane) at elevated temperatures (?20 to 30 °C) is reported. With CumOH/AlCl₃OBu₂ initiating system, the reaction proceeded by controlled initiation via CumOH, followed by β‐H abstraction and then irreversible termination, thus, affording polymers (M_n = 1000–2000 g mol^?1) with high content of vinylidene end groups (85–91%), although the monomer conversion was low (≤35%) and polymers exhibited relatively broad molecular weight distribution (MWD; M_w/M_n = 2.3–3.5). H₂O/AlCl₃OBu₂ initiating system induced chain‐transfer dominated cationic polymerization of isobutylene via a selective β‐H abstraction by free base (Bu₂O). Under these conditions, polymers with very high content of desired exo‐olefin terminal groups (89–94%) in high yield (>85%) were obtained in 10 min. It was shown that the molecular weight of polyisobutylenes obtained with H₂O/AlCl₃OBu₂ initiating system could be easily controlled in a range 1000–10,000 g mol^?1 by changing the reaction temperature from ?40 to 30 °C. The MWD was rather broad (M_w/M_n = 2.5–3.5) at low reaction temperatures (from ?40 to 10 °C), but became narrower (M_w/M_n ≤ 2.1) at temperatures higher than 10 °C. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Photoinduced and thermally induced cationic polymerizations using S,S‐dialkyl‐S‐(3,5‐dimethylhydroxyphenyl)sulfonium salts

A study of the photoinitiated and thermally initiated cationic polymerizations of several monomer systems with S,S‐dialkyl‐S‐(3,5‐dimethylhydroxyphenyl)sulfonium salt (HPS) photoinitiators bearing different lengths of alkyl chains on the positively charged sulfur atom has been conducted. HPS photoinitiators are capable of photoinitiating the cationic polymerization of a wide variety of epoxy and vinyl ether monomers directly on irradiation with short‐wavelength UV light. Aryl ketone photosensitizers are effective in extending the spectral response of these photoinitiators into the long‐wavelength UV region. Kinetic studies with real‐time infrared spectroscopy show that HPS photoinitiators exhibit good efficiency in the polymerization of epoxide and vinyl ether monomers. Comparative studies also demonstrate that S,S‐dimethyl‐S‐(3,5‐dimethyl‐2‐hydroxyphenyl)sulfonium salts are more active photoinitiators than their isomeric S,S‐dimethyl‐S‐(3,5‐dimethyl‐4‐hydroxyphenyl)sulfonium salt counterparts. Both types of HPS photoinitiators display reversible photolysis as a result of facile termination reactions that take place between the growing chains ends with the photogenerated sulfur ylides. Preliminary studies have shown that HPS photoinitiators can also be employed as thermal initiators for the cationic ring‐opening polymerization of epoxides at moderate temperatures. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 2570–2587, 2003     

Olefin Polymerization and Copolymerization with Alkylaluminum-lnitiator Systems. VII. Initiation by Electrophilic Halogens

Abstract

The discovery of initiation of cat ionic polymerization of isobutylene and styrene by electrophilic chlorine generated by the reaction of chlorine and trimethylaluminum in the temperature range -40 to -100° is reported. Bromine and trimethylaluminum is a very poor initiating system, and iodine and trimethylaluminum does not initiate the polymerization of either isobutylene or styrene. Polymerization of isobutylene initiated by chlorine and trimethylaluminum shows a linear plot of log M_v vs 1/T with an overall E_DPof ～1.9 kcal, mole. The molecular weights (M_w) of polystyrene obtained with the Cl₂/Me₃Al system appear to be the highest ever reported for cationic polymerization of this monomer under comparable conditions. The mechanism of initiation has been investigated by model experiments: The reaction between Cl₂/ Me₃Al and 2,2,4-trimethyl-l-pentene gave three chlorinated products (2-chloromechyl-4,4-dimethyl-l-pentene, 1-chloro-2,4,4-trimethyl-1-pentene, and 2-chioromethyl-2,4,4-trimethyl-pentane). The position of chlorine in these compounds indicate initiation by electrophilic chlorine, Cl. Some preliminary results obtained using diethylaluminum chloride-halogens as coinitiator-initiator systems are also described.     

Reversible addition‐fragmentation chain transfer polymerization of diisopropyl fumarate using various dithiobenzoates as chain transfer agents

Dialkyl fumarates as 1,2‐disubstituted ethylenes exhibit unique features of radical polymerization kinetics due to their significant steric hindrance in propagation and termination processes and provide polymers with a rigid chain structure different from conventional vinyl polymers. In this study, we carried out reversible addition‐fragmentation chain transfer polymerization of diisopropyl fumarate (DiPF) in bulk at 80 °C using various dithiobenzoates with different leaving R groups as chain transfer agents to reveal their performance for control of molecular weight, molecular weight distribution, and chain end functionality of the resulting poly(DiPF) (PDiPF). 2‐(Ethoxycarbonyl)‐2‐propyl dithiobenzoate ( DB1 ) and 2,4,4‐trimethyl‐2‐pentyl dithiobenzoate ( DB2 ) underwent fragmentation and reinitiation at a moderate rate and consequently led to the formation of PDiPF with well‐controlled chain structures. It was confirmed that molecular weight of PDiPF produced by controlled polymerization with DB1 and DB2 agreed with theoretical one and molecular weight distribution was narrow. Dithiobenzoate and R fragments were introduced into the polymer chain ends with high functionality as 95% by the use of DB1 . In contrast, polymerizations using 1‐(ethoxycarbonyl)benzyl dithiobenzoate ( DB3 ), 1‐phenylethyl dithiobenzoate ( DB4 ), and 2‐phenyl‐2‐propyl dithiobenzoate ( DB5 ) resulted in poor control of molecular weight, molecular weight distribution, and chain end structures of PDiPF. Fragmentation and reinitiation rates of the used benzoates as chain transfer agents significantly varied depending on the R structures in an opposite fashion; that is, introduction of bulky and conjugating substituents accelerated fragmentation, but it retarded initiation of DiPF polymerization. It was revealed that balance of fragmentation and reinitiation was important for controlled polymerization of DiPF. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 3266–3275     

Functionalization of polyisobutylene and polyisobutylene oligomers via the ritter reaction

The Ritter reaction, that is, reaction of a carbocation with a nitrile, was carried out on polyisobutylene (PIB) using a variety of reaction conditions. End quenching of PIB carbocations with acrylonitrile under living polymerization conditions (methyl chloride (MeCl)/hexane 60/40 (v/v) solvent mixtures at −70 °C) resulted in either tert‐chloride end groups or loss of chain‐end fidelity via carbocation rearrangement, as evidenced by NMR spectroscopy. Exo‐olefin functionalized PIB substrates were also reacted with nitriles under a variety of reaction conditions including various acid and solvent medium combinations. In all cases, the result was either no reaction or PIB that had undergone severe backbone degradation, as determined via NMR spectroscopy and gel permeation chromatography. Finally, the Ritter reaction was performed on a series of exo‐olefin functionalized oligoisobutylenes using acrylonitrile as the nitrile and either 60/40 dichloromethane/hexane or excess acrylonitrile as the solvent. In 60/40 dichloromethane/hexane, significant carbocation rearrangement and/or degradation resulted in a variety of isomeric, acrylamide‐functionalized oligomers. In excess acrylonitrile, the desired Ritter reaction was the only reaction observed, resulting in the smooth formation of the terminal acrylamide. The various N‐oligoisobutylacrylamides thus obtained represent new hydrophobic monomers useful for the introduction of hydrophobic moieties into acrylamide‐based water‐soluble polymers. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 840–852     

Synthesis of highly reactive polyisobutylenes using solvent‐ligated manganese(II) complexes as catalysts

Manganese complexes with benzonitrile ligands were synthesized, characterized, and applied for the preparation of the isobutylene polymerization. Low and medium molecular weight polyisobutylenes containing high amount of exo‐type double bond end groups (70–80%) were successfully prepared using these manganese(II) complexes as catalysts at room temperature. The influence of monomer and catalyst concentration was intensively analyzed for achieving high monomer conversion and high exo double bond content of the products. Details on end group distribution in the products and development of the exo‐type end group content with reaction time were evaluated by ¹H NMR. The catalysts are also active for the homopolymerization of styrene and the copolymerization of isobutylene and styrene. The highly reactive polyisobutylene products obtained by these manganese complexes show features similar to products obtained by conventional cationic polymerization, but the polymerization characteristics clearly deviate. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5636–5648, 2007     

Synergistic interaction of epoxides and N‐vinylcarbazole during photoinitiated cationic polymerization

A kinetic study was conducted of the independent photoinitiated cationic polymerization of a number of epoxide monomers and mixtures of these monomers with N‐vinylcarbazole. The results show that these two different classes of monomers undergo complex synergistic interactions with one another during polymerization. It was demonstrated that N‐vinylcarbazole as well as other carbazoles are efficient photosensitizers for the photolysis of both diaryliodonium and triarylsulfonium salt photoinitiators. In the presence of large amounts of N‐vinylcarbazole, the rates of the cationic ring‐opening photopolymerization of epoxides are markedly accelerated. This effect has been ascribed to a photoinitiated free‐radical chain reaction that results in the oxidation of monomeric and polymeric N‐vinylcarbazole radicals by the onium salt photoinitiators to generate cations. These cations can initiate the ring‐opening polymerization of the epoxides, leading to the production of copolymers. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3697–3709, 2000     

Living cationic polymerization of isobutylene coinitiated by FeCl3 in the presence of isopropanol

A simple but effective FeCl₃‐based initiating system has been developed to achieve living cationic polymerization of isobutylene (IB) using di(2‐chloro‐2‐propyl) benzene (DCC) or 1‐chlorine‐2,4,4‐trimethylpentane (TMPCl) as initiators in the presence of isopropanol (iPrOH) at ?80 °C for the first time. The polymerization with near 100% of initiation efficiency proceeded rapidly and completed quantitatively within 10 min. Polyisobutylenes (PIBs) with designed number‐average molecular weights (M_n) from 3500 to 21,000 g mol^?1, narrow molecular weight distributions (MWD, M_w/M_n ≤ 1.2) and near 100% of tert‐Cl terminal groups could be obtained at appropriate concentrations of iPrOH. Livingness of cationic polymerization of IB was further confirmed by all monomer in technique and incremental monomer addition technique. The kinetic investigation on living cationic polymerization was conducted by real‐time attenuated total reflectance Fourier transform infrared spectroscopy. The apparent constant of rate for propagation (k_p^A) increased with increasing polymerization temperature and the apparent activation energy (ΔE_a) for propagation was determined to be 14.4 kJ mol^?1. Furthermore, the triblock copolymers of PS‐b‐PIB‐b‐PS with different chain length of polystyrene (PS) segments could be successfully synthesized via living cationic polymerization with DCC/FeCl₃/iPrOH initiating system by sequential monomer addition of IB and styrene at ?80 °C. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Calcification resistance of polyisobutylene and polyisobutylene‐based materials

Calcification of implanted biomaterials is highly undesirable and limits clinical applicability. Experiments were carried out to assess the calcification resistance of polyisobutylene (PIB), PIB‐based polyurethane (PIB‐PU), PIB‐PU reinforced with (CH₃)₃N⁺CH₂CH₂CH₂NH₂ I^?‐modified montmorillonite (PIB‐PU/nc), PIB‐based polyurethane urea (PIB‐PUU), PIB‐PU containing S atoms (PIB_S‐PU), PIB_S‐PU reinforced with (CH₃)₃N⁺CH₂CH₂CH₂NH₂ I^?‐modified montmorillonite (PIB_S‐PU/nc), and poly(isobutylene‐b‐styrene‐b‐isobutylene) (SIBS), relative to that of a clinically widely implanted polydimethylsiloxane (PDMS)–based PU, Elast‐Eon (the “control”). Samples were incubated in simulated body fluid for 28 days at 37°C, and the extent of surface calcification was analyzed by scanning electron microscopy (SEM), atomic force microscopy (AFM), energy‐dispersive X‐ray spectroscopy (EDX), X‐ray photoelectron spectroscopy (XPS), and Fourier‐transform‐infrared (FT‐IR) spectroscopy. Whereas the PDMS‐based PU showed extensive calcification, PIB and PIB‐PU containing 72.5% PIB, ie, a polyurethane whose surface is covered with PIB, were free of calcification. PIB_S‐PU and PIB‐PUU, ie, polyurethanes that contain S or urea groups, respectively, were slightly calcified. The amine‐modified montmorillonite‐reinforcing agent reduced the extent of calcification. SIBS was found slightly calcified. Evidently, PIB and materials fully coated with PIB are calcification resistant.     

Construction of excellent thermal latent system for the synthesis of networked epoxide polymers by sulfonium salts

An efficient thermally latent initiation system using dual sulfonium salts, consisting S‐benzylsulfonium salt 1 bearing counter anion and S,S‐dimethylsulfonium salt 2 bearing CH₃ counter anion, has been developed for the cationic polymerization of epoxides. Compared to the conventional system using 1 as a thermally latent initiator, the newly developed system allowed significant improvement of stability of epoxy formulations during storage at ambient temperature without sacrificing their curability at elevated temperatures. Such a remarkable performance is attributable to the nucleophilic attack of CH₃ to cationic species formed unavoidably by the reaction of 1 with epoxide. Such entrapment of cationic species into the corresponding dormant led to the inhibition of undesirable chain growth of polymers during storage of epoxy formulations. In addition, the dormant can undergo dissociation at elevated temperature to give cationic species, which can readily initiate the polymerization of epoxide. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 2096–2102     

Synthesis and characterization of two novel star blocks: tCum[poly(isobutylene‐b‐norbornadiene)]3 and tCum[poly(norbornadiene‐b‐isobutylene)]3

Two structurally closely related three‐arm star blocks were synthesized and characterized: tCum(PIB‐b‐PNBD)₃ and tCum(PNBD‐b‐PIB)₃ [where tCum (tricumyl) stands for the phenyl‐1,3,5‐tris(‐2‐propyl) fragment and PIB and PNBD are polyisobutylene and polynorbornadiene, respectively]. The syntheses were accomplished in two stages: (1) the preparation of the first (or inner) block fitted with appropriate chlorine termini capable of initiating the polymerization of the second (or outer) block with TiCl₄ and (2) the mediation of the polymerization of the second block. Therefore, the synthesis of tCum(PIB‐b‐PNBD)₃ was effected with tCum(PIB‐Cl^t)₃ [where Cl^t is tert‐chlorine and number‐average molecular weight (M_n) = 102,000 g/mol] by the use of TiCl₄ and 30/70 CH₃Cl/CHCl₃ solvent mixtures at ?35 °C. PNBD homopolymer contamination formed by chain transfer was removed by selective precipitation. According to gel permeation chromatography, the M_n's of the star blocks were 107,300–109,200 g/mol. NMR spectroscopy (750 MHz) was used to determine structures and molecular weights. Differential scanning calorimetry (DSC) indicated two glass‐transition temperatures (T_g's), one each for the PIB (?65 °C) and PNBD (232 °C) phases. Thermogravimetric analysis thermograms showed 5% weight losses at 293 °C in air and at 352 °C in N₂. The synthesis of tCum(PNBD‐b‐PIB)₃ was achieved by the initiation of isobutylene polymerization with tCum(PNBD‐Cl^sec)₃ (where Cl^sec is sec‐chlorine and M_n = 2900 g/mol) by the use of TiCl₄ in CH₃Cl at ?60 °C. DSC for this star block (M_n = 14,200 g/mol) also showed two T_g's, that is, at ?67 and 228 °C for the PIB and PNBD segments, respectively. It is of interest that the Cl^sec terminus of PNBD, , readily initiated isobutylene polymerization. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 740–751, 2003