Demonstration of the Existence of Long‐Lived Species in the Cationic Polymerization of 1,3‐Pentadiene Initiated by Aluminium Trichloride in Non Polar Solvent

Summary: A kinetic study of the polymerization of a mixture of 1,3‐pentadiene isomers initiated by AlCl₃ was carried out in pentane at ?10 and 20 °C. A second apparent monomer order was found mainly at room temperature and was explained by a complexation of propagating active centers by the polymer, the true monomer order being first. This apparent order did not result from the presence of the two isomers in the monomer mixture. Kinetic simulations confirmed this interpretation and pointed out the fact that the interaction between the polymer and the active centers was stronger at ?10 °C, thus limiting the polymer conversion at this temperature. The novelty of these findings lies in the fact that the inactive complexed centers can be activated by reaction with the monomer, providing active species when necessary. The existence of active centers even after a long reaction time at room temperature, and the reactivation of the complexed centers, was evidenced by incremental monomer addition and by the formation of sulfonium ions after quenching the polymerization by an excess of dimethyl sulfide. The latter were characterized by ¹H NMR spectroscopy.

Complexation of propagating active centers by the polymer giving dormant species (C⁺/P), with following reactivation by a new monomer charge.     

A New Insight into the Mechanism of 1,3‐Dienes Cationic Polymerization I: Polymerization of 1,3‐Pentadiene with tBuCl/TiCl4 Initiating System: Kinetic and Mechanistic Study

The cationic polymerization of 1,3‐pentadiene using a tert‐butyl chloride (^tBuCl)/TiCl₄ initiating system in CH₂Cl₂ at different reaction conditions is reported. It is shown that the reaction rate increases with the increase of the ^tBuCl/TiCl₄ molar ratio, while the molecular weight distribution becomes narrower. Well‐defined oligo(1,3‐pentadiene)s ( ≤ 3500 g mol^?1; / ≤ 3.0) are obtained at high ^tBuCl/TiCl₄ molar ratio (340) and low temperature (–78 °C). ¹H and ¹³C NMR spectroscopy studies reveal the presence of tert‐butyl head and –CH₂–Cl end groups. The number‐average functionalities (F_ns) at the α‐ and ω‐ends are calculated to be F_n(^tBu) > 1 and F_n(Cl) < 1, respectively. The general mechanism of 1,3‐pentadiene polymerization is proposed.

     

A New Insight into the Mechanism of 1,3‐Dienes Cationic Polymerization III: Polymerization of 1,3‐Pentadiene with CF3COOD/TiCl4 Initiating System: Chain‐Ends Structure and Kinetics

A novel method for the investigation of the chain‐end structure of poly(1,3‐pentadiene)s synthesized using the CF₃COOD/TiCl₄ initiating system is developed. It is shown for the first time that the content of trans‐1,2‐structures in the first monomer unit is considerably higher than the content of trans‐1,4‐structures, whereas the content of trans‐1,4‐units is substantially higher than trans‐1,2‐units for the polymer chain as a whole. Another important observation is that chain transfer to monomer is significant even at the earlier stages of the 1,3‐pentadiene polymerization (after 1 s of reaction). The very low functionality at the ω‐end (F_n (Cl) < 0.15) confirms the intensive chain transfer to monomer. This method is also applied for the estimation of the concentration of active species and the rate constant for propagation (k _p) for the cationic polymerization of 1,3‐pentadiene using the CF₃COOD/TiCl₄ initiating system: rate constants for propagation, k _p, of 1.5 × 10³ and 3.3 × 10³ L mol^?1 min^?1 are determined for 1,3‐pentadiene polymerization at 20 and –78 °C, respectively.

     

Polymerization of 4‐Methyl‐1,3‐pentadiene with Catalysts Based on Cyclopentadienyl Titanium Chlorides: Effect of anti/syn Isomerism of the Allylic Group on the Chemoselectivity and the Role of Backbiting Coordination in 1,3‐Diene Polymerization

Summary: Homopolymerization of 4‐methyl‐1,3‐pentadiene (MP) and copolymerization of 4‐methyl‐1,3‐pentadiene with alkenes (ethylene, 1‐pentene, 4‐methyl‐1‐pentene) were performed to investigate the effect of the so‐called backbiting coordination on the chemoselectivity of 1,3‐diene polymerization. Three homogeneous catalyst systems were used: CpTiCl₃‐MAO, Cp₂TiCl₂‐MAO and Cp₂TiCl‐MAO. Backbiting coordination is possible with the first catalyst, but not with the other two. The three catalysts gave similar results, which indicates that backbiting has no effect on the polymerization chemoselectivity, contrary to what has been reported in recent literature. An interpretation is presented for the formation of 1,4 units in MP/alkene copolymers. This interpretation is based on the fact that allyl groups have predominantly a syn configuration in MP homopolymerization, whereas allyl groups of anti configuration are formed in MP/alkene copolymerization. The role of backbiting in diene polymerization is discussed.

The effect of anti/syn isomerism on the chemoselectivity in the different polymerizations.     

Anionic Polymerization of 1,3‐Cyclohexadiene: Reactivity of Poly(1,3‐cyclohexadienyl)lithium

Summary: The reactivity of poly(1,3‐cyclohexadienyl)lithium (PCHDLi), as a species for the propagation of the anionic polymerization of 1,3‐cyclohexadiene (1,3‐CHD), was examined using a post‐polymerization reaction of PCHDLi and 9‐bromofluorene (9‐BFL). The degree of nucleophilicity of the PCHDLi systems was determined as PCHDLi/1,4‐diazabicyclo[2,2,2]octane (DABCO) > PCHDLi/N,N,N′,N′‐tetramethylethylenediamine (TMEDA) > PCHDLi. The nucleophilicity of PCHDLi was strengthened by the complexation of TMEDA to Li, and was saturated when the TMEDA/Li molar ratio was over 1.0. The rate of polymerization increased with increasing nucleophilicity of PCHDLi. In addition, the molar ratio of the 1,2‐addition (1,2‐CHD unit)/1,4‐addition (1,4‐CHD unit) seemed to be strongly affected by both the nucleophilicity of PCHDLi and the steric hindrance of C? Li bonds in PCHDLi.

Post‐polymerization reaction of poly(1,3‐cyclohexadienyl)lithium (PCHDLi) and 9‐bromofluorene (9‐BFL).     

Novel Methylaluminoxane‐Activated Neodymium Isopropoxide Catalysts for 1,3‐Butadiene Polymerization and 1,3‐Butadiene/Isoprene Copolymerization

The homo‐ and copolymerizations of 1,3‐butadiene and isoprene are examined by using neodymium isopropoxide [Nd(Oi‐Pr)₃] as a catalyst, in combination with a methylaluminoxane (MAO) cocatalyst. In the homopolymerization of 1,3‐butadiene, the binary Nd(Oi‐Pr)₃/MAO catalyst works quite effectively, to afford polymers with high molecular weight ( ≈ 10⁵ g mol^‐1), narrow molecular‐weight distribution (MWD) (/ = 1.4–1.6), and cis‐1,4‐rich structure (87–96%). Ternary catalysts that further contain chlorine sources enhance both catalytic activity and cis‐1,4 selectivity. In the copolymerization of 1,3‐butadiene and isoprene, the copolymers feature high , unimodal gel‐permeation chromatography (GPC) profiles, and narrow MWD. Most importantly, the ternary Nd(Oi‐Pr)₃/MAO/t‐BuCl catalyst affords a copolymer with high cis‐1,4 content in both monomer units (>95%).

     

Polymerization of 1,3‐Butadiene Initiated by Neodymium Versatate/Diisobutylaluminium Hydride/Ethylaluminium Sesquichloride: Kinetics and Conclusions About the Reaction Mechanism

The kinetics of the polymerization of 1,3‐butadiene initiated by the ternary Ziegler–Natta catalyst system comprising neodymium versatate (NdV), diisobutylaluminium hydride (DIBAH) and ethylaluminium sesquichloride (EASC) have been studied in order to quantify the impact of the catalyst components EASC and DIBAH on the polymerization rate, the control of molar masses, the molar mass distributions as well as on the microstructure of the resulting polymer (cis‐1,4, trans‐1,4 and 1,2 content). A further focus of the work was on the elucidation of the living nature of the polymerization. It has been found that the catalyst component EASC influences the reaction rate and the microstructure of the obtained polybutadiene. The main effect of the variation of DIBAH is on the molar mass, but the polymerization rate and the microstructure are also influenced. Straight lines are obtained for the dependence of the molar masses on monomer conversion revealing the living nature of the polymerization. The theoretically predicted molar masses are significantly higher than those experimentally found. This discrepancy is explained by the existence of dormant species resulting from the reversible transfer of living polymer chains from Nd onto Al. Only one third of the DIBAH which is not consumed by the processes of scavenging of impurities and activation of the Nd catalyst, is involved in this transfer reaction. This makes DIBAH an inefficient chain control agent for the experimental conditions applied (namely 60°C). A reaction scheme is put forward which accounts for the features observed.     

A New Approach toward Investigation of Structure of Terminal Units of Poly(1,3‐Pentadiene) using T2‐Filtered NMR Spectroscopy

A new method for the identification of the structure of terminal units in poly(1,3‐pentadiene) synthesized by cationic mechanism is developed. The conducting of NMR experiments with a T₂ filter allows the intensities of the spectral signals of carbon and hydrogen atoms of main chain groups of poly(1,3‐pentadiene) with a short relaxation time to be decreased and significantly increases the intensities of signals of carbon and hydrogen atoms of head and end groups, which are characterized by higher mobility. Using 1D NMR spectroscopy with a T₂ filter as well as 2D heteronuclear single‐quantum correlation and heteronuclear multiple‐bond correlation NMR spectroscopy, it is found that the position of four out of five carbon atoms of the head group fully coincides with the position of spectral signals of carbon atoms of trans‐1,2‐ and trans‐1,4‐ units of a main polymer chain. This new method allows the identification of the signals of all carbon atoms in head trans‐1,4‐ group of poly(1,3‐pentadiene).     

Fast Ring‐Opening Polymerization of 1,2‐Disubstituted Epoxides Initiated by a CoIII‐Salen Complex

Polyethers are an important class of polymers that find numerous applications. Ring‐opening polymerization of various 1,2‐disubstituted epoxides initiating a commercial, and well‐defined Co^III‐Salen complex is investigated in this paper. Remarkable reactivity (0.01% Co^III loadings, TOF up to 19200 h^?1) is discovered in this transformation. High molecular weight polymers (up to 80 kg mol^?1) are produced. In particular, polyether from ring‐opening polymerization of 1,4‐dihydronaphthalene oxide exhibits a glass transition temperatures (T_g) of up to 108 °C. By investigating the relationship between polymerization reactivity and counter ion in the Co^III complex, as well as the properties of the resultant polyethers, a cationic mechanism through an oxonium species is proposed.     

cis‐Specific Living Polymerization of 1,3‐Butadiene with CoCl2 and Methylaluminoxane

The polymerization of 1,3‐butadiene was conducted by CoCl₂ combined with methylaluminoxane (MAO) as a cocatalyst at 0 and 18°C. The uni‐modal molecular weight distribution curves of the resulting polymers shifted toward higher molecular weight regions and became narrower when increasing the polymerization time. The number‐average molecular weight increased linearly with polymerization time, while the polymer yield increased exponentially in the initial stage. As a consequence, the number of polymer chains, calculated from the polymer yield and M̄_n, increased gradually with polymerization time to reach a plateau value. These phenomena was interpreted based on a slow initiation system without any termination and chain transfer reaction. The microstructure of the polymer was determined by ¹H NMR and ¹³C NMR spectroscopy to be a cis‐1,4 structure in a 98–99% purity.     

Determination of the Crystal Structure of Isotactic cis‐1,4‐Poly(1,3‐hexadiene) by X‐Ray Diffraction and Molecular Mechanics

The crystal structure of isotactic cis‐1,4‐poly(1,3‐hexadiene) has been determined through a combination of X‐ray diffraction analysis and molecular mechanics. Two reasonable values for unit cell parameters were obtained from the fiber diffraction pattern of a well‐oriented sample. Energy maps performed on a molecular model of the polymer chain yielded an absolute minimum corresponding to a conformation having the internal parameters similar to those of an s(2/1) helical polymer chain. Packing energy minimizations performed for all the orthorhombic and monoclinic space groups having crystallographic s(2/1) chain symmetry indicate unequivocally that the best mode of packing is obtained for a crystal structure characterized by a unit cell with parameters a = 8.98 Å, b = 7.82 Å, and c = 8.10 Å in the P2₁2₁2₁ space group. The calculated powder and fiber diffraction patterns of this structural model are in very good agreement with the experimental diffraction patterns. Similarities and differences with the crystal structures of isotactic cis‐1,4‐poly(1,3‐pentadiene) and isotactic cis‐1,4‐poly(2‐methyl‐1,3‐pentadiene) are discussed.

     

Pseudo‐Living Anionic Telomerization of Buta‐1,3‐diene

Low‐molecular‐weight liquid polybutadienes (1 000–2 000 g · mol^?1) consisting of 60 mol‐% poly(buta‐1,2‐diene) repeating units were synthesized via anionic telomerization. Maintaining the initiation and reaction temperature at less than 70 °C minimized chain transfer and enabled the polymerization to occur in a living fashion, which resulted in well‐controlled molecular weights and narrow polydispersity indices. MALDI‐TOF mass spectrometry confirmed that the end groups of liquid polybutadienes synthesized via anionic telomerization contained one benzyl end and one protonated end. In comparison, the end groups of liquid polybutadienes synthesized via living anionic polymerization contained one sec‐butyl or butyl end and one protonated end.

     

Synthesis of AB2 3‐ and AB4 5‐Miktoarm Star Copolymers Initiated from Dendritic Tri‐ and Penta‐Functional Initiators by Combination of Ring‐Opening Polymerization of ε‐Caprolactone and Nitroxide‐Mediated Radical Polymerization of Styrene

Summary: Well‐defined AB₂ 3‐ and AB₄ 5‐miktoarm star copolymers were prepared by combination of ring‐opening polymerization (ROP) and nitroxide‐mediated radical polymerization (NMRP) using dendritic tri‐ and penta‐functional initiators. Initially, two kinds of dendritic initiators having one benzylic OH and two or four TEMPO‐based alkoxyamine moieties were prepared. Using them, ROP of ε‐caprolactone was carried out at room temperature to give poly(ε‐caprolactone)s carrying two or four alkoxyamine moieties. NMRP of styrene from the poly(ε‐caprolactone)s was carried out at 120 °C to give AB₂ 3‐ and AB₄ 5‐miktoarm star copolymers, which were analyzed by ¹H NMR and SEC. The increased linearly with conversion and the were in the range 1.10–1.37, showing that well‐defined AB₂ 3‐ and AB₄ 5‐miktoarm star copolymers were formed.

Well‐defined AB₂ 3‐ and AB₄ 5‐miktoarm star copolymers were prepared by combination of ring‐opening polymerization (ROP) and nitroxide‐mediated radical polymerization (NMRP) using dendritic tri‐ and penta‐functional initiators.     

Efficient Grafting of Hyperbranched Polyglycerol from Hydroxyl‐Functionalized Multiwalled Carbon Nanotubes by Surface‐Initiated Anionic Ring‐Opening Polymerization

Biocompatible hyperbranched polyglycerol (HPG) has been covalently grafted from the surfaces of multiwalled carbon nanotubes (MWNTs) by the “grafting from” method based on in situ anionic ring‐opening polymerization. The macroinitiator of hydroxyl‐functionalized MWNTs (MWNT‐OH) with hydroxyl density of 1.39 mmol per gram of nanotubes was prepared by one‐pot nitrene chemistry in N‐methyl‐2‐pyrrolidone (NMP) at 160 °C for 12 h. The amount of HPG grafted from MWNTs can be readily adjusted in a wide range up to 90.8wt.‐% by tuning the feed ratio of glycidol to MWNT‐OH. The resulting HPG‐grafted MWNTs (MWNT‐g‐HPG) nanohybrids were characterized by TGA, FT‐IR, and NMR spectroscopy, HRTEM, and SEM. The as‐prepared nanohybrids show good dispersibility in polar solvents such as water, DMF, DMSO, and methanol. On the basis of numerous functional hydroxyl groups of the HPG grown on MWNTs, fluorescent molecules of rhodamine 6B were conjugated to the surface of MWNT‐g‐HPG by N, N′‐dicyclohexylcarbodiimide (DCC) coupling, affording fluorescent MWNTs. The multifunctional MWNT‐g‐HPG nanohybrids promise potential applications in drug delivery, cell imaging and bioprobing.

     

Kinetic and Molecular Weight Modeling of Miniemulsion Polymerization Initiated by Oil‐Soluble Initiators

A mathematical model for the miniemulsion polymerization initiated by oil‐soluble initiators is described in detail with comparisons to experiment results. The desorption of primary radicals and particle population balance are introduced to illustrate the distribution of radical per particles. The effects of different variable factors including the particle size, surfactant layer, initiator, and aqueous phase inhibitor are considered, and the model predictions coincide well with experiment results under different above‐mentioned conditions. The model is found to be useful in explaining the ongoing debate about the origin of effective radical in miniemulsion polymerization initiated by oil‐soluble initiators. It is found that the effective radicals are originated from the primary radicals desorbed from the particles, and the contribution of the small amount of initiator dissolved in the aqueous phase can be ignored.

     

Nickel‐Mediated Surface Grafting From Polymerization of α‐Amino Acid‐N‐Carboxyanhydrides

Summary: The feasibility of a living grafting from polymerization of α‐amino acid‐N‐carboxyanhydrides (NCA) from a surface using nickel initiators was shown. The polymerization has been carried out on commercially available polystyrene resins as spherical substrates in two different ways. Firstly L ‐glutamic acid was bound to the surface as γ‐ester via a UV‐labile linker and transferred into the NCA by treatment with triphosgene. The grafting from polymerization was then carried out as a “block copolymerization” by reaction of the surface bound NCA with an excess of the Ni amido‐amidate complex initiator and subsequent addition of free NCA to grow the polymer chain. By this procedure polymer was formed at the surface and can be isolated after photolysis of the linker. The characterization of the polymer by size exclusion chromatography indicates a living polymerization at the surface. The second approach employs N‐alloc‐amides at the surface to prepare an initiating Ni amido‐amidate complex directly at the surface. It can be shown that the latter approach is much more straightforward and gives smaller quantities of non‐tethered polypeptide.

Surface bound polypeptides were obtained by ring opening polymerization of α‐amino acid‐N‐carboxyanhydrides initiated by nickel amido‐amidate complexes installed at surfaces of commercially available polystyrene resins.     

Acetyl Halide Initiator Screening for the Cationic Ring‐Opening Polymerization of 2‐Ethyl‐2‐Oxazoline

Kinetic investigations on the cationic ring‐opening polymerization of 2‐ethyl‐2‐oxazoline were conducted using acetyl chloride, acetyl bromide, and acetyl iodide as initiators. Various polymerization temperatures ranging from 80 to 220 °C were applied under microwave irradiation. The resulting polymerization mixtures were characterized with GC and GPC for the determination of monomer conversion and molecular weight distribution, respectively. Well defined polymers with narrow molecular weight distributions ( = 6 000 Dalton, PDI ≈ 1.10) were obtained with all three initiators.

     

New Thermosets Obtained by Thermal and UV‐Induced Cationic Copolymerization of DGEBA with 4‐Phenyl‐γ‐butyrolactone

Mixtures of DGEBA with 4‐phenyl‐γ‐butyrolactone (PhBL) were cationically copolymerized in the presence of ytterbium triflate or triarylsulfonium hexafluoroantimoniate as thermal or photo initiator respectively. Changes during curing and final properties of the cured materials were studied by means of DSC, FT‐IR/ATR, TMA, DMTA, TGA and densitometric measurements. The formation of a network containing polyether and poly(ether‐ester) moieties was demonstrated to take place through the ring‐opening polymerization of a spiroorthoester intermediate (SOE) formed during the copolymerization. An increase in the proportion of lactone resulted in an increase in the curing rate, a decrease in the shrinkage after gelation and in the thermal stability and glass transition temperature (T_g). A strong influence of the initiator on the curing mechanism was observed. As a consequence, the photocured materials exhibited superior thermal stability and T_g than those obtained thermally.

     

Thermoresponsive Poly(2‐Oxazoline) Molecular Brushes by Living Ionic Polymerization: Modulation of the Cloud Point by Random and Block Copolymer Pendant Chains

Molecular brushes (MBs) of poly(2‐oxazoline)s were prepared by living anionic polymerization of 2‐isopropenyl‐2‐oxazoline to form the backbone and living cationic ring‐opening polymerization of 2‐n‐propyl‐2‐oxazoline and 2‐methyl‐2‐oxazoline to form random and block copolymers. Their aqueous solutions displayed a distinct thermoresponsive behavior as a function of the side‐chain composition and sequence. The cloud point (CP) of MBs with random copolymer side chains is a linear function of the hydrophilic monomer content and can be modulated in a wide range. For MBs with block copolymer side chains, it was found that the block sequence had a strong and surprising effect on the CP. While MBs with a distal hydrophobic block had a CP at 70 °C, MBs with hydrophilic outer blocks already precipitated at 32 °C.     

Click‐Chemistry: An Alternative Way to Functionalize Poly(2‐methyl‐2‐oxazoline)

CROP has been used to synthesize well‐defined POXZ with a monofunctional (iodomethane) or a bifunctional (1,3‐diiodopropane) initiator. POXZ has been functionalized with an azido group at one (α‐azido‐POXZ, = 3.58 × 10³ g · mol^?1) or both ends (α,ω‐azido‐POXZ, = 6.21 × 10³ g · mol^?1) of the macromolecular chain. The Huisgen 1,3‐dipolar cycloaddition has been investigated between azido‐POXZ and a terminal alkyne on a small or larger molecule (PEG). In each case, the click reaction has been successful and quantitative. In this way, different telechelic polymers (polymers bearing different functions such as acrylate, epoxide, or carboxylic acid) and block copolymers of POXZ and PEG have been prepared. The polymers have been characterized by means of FTIR, ¹H NMR, and SEC.