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.

     

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.     

Polymerization of 1,3‐Pentadiene Initiated by Aluminium Trichloride in Nonpolar Solvent: Pseudo‐Control is Explained by Continuous Grafting

Summary: The cationic polymerization of 1,3‐pentadiene initiated by AlCl₃ was studied in nonpolar solvent. It was previously shown that at room temperature the active species were long‐lived and that the number‐average molar mass of the polymer chains was increasing with the polymerization yield. In order to explain this apparent control, the macromolecules were labeled with a transfer agent, triphenylamine (NPh₃). The latter binds to active species by electrophilic aromatic substitution. The labeling of the polymer chains indicated that at 20 °C the polymer chains mainly contained one NPh₃ molecule per macromolecule while the NPh₃ content was higher for the high molar mass chains due to a “grafting from” polymer transfer mechanism. Thus, the pseudo‐control was assigned to the branching reactions. The labeling process by NPh₃ also succeeded at ?10 °C. Whereas at ?10 °C a dialkylation of NPh₃ was observed, a trialkylation at 20 °C was obtained. The analysis of the polymer microstructure at both temperatures highlighted an interaction between the active centers and NPh₃. This paper also describes a process to synthesize tri‐arm stars polymers by cationic polymerization.

RI SEC chromatograms of soluble polymers synthesized at 20 °C in the presence of NPh₃ with increasing reaction times (r = [NPh₃]/[AlCl₃] = 1); (a) t = 0.25 h, (b) t = 0.5 h, (c) t = 1 h, (d) t = 2 h, (e) t = 18 h, (f) t = 48 h; [AlCl₃] = 2.3 × 10^?2 mol · L^?1, [1,3‐pentadiene] = 1.6 mol · L^?1, pentane.     

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).     

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).     

A New Insight Into the Mechanism of the Ring‐Opening Polymerization of Trimethylene Carbonate Catalyzed by Methanesulfonic Acid

The ring‐opening polymerization (ROP) of trimethylene carbonate (TMC) initiated by a monoalcohol and catalyzed by CH₃SO₃H is investigated, in an effort to reveal extra features of the known activated monomer/active chain‐end (AM/ACE) combined mechanism. Size‐exclusion chromatography (SEC) profiles obtained with high‐molar‐mass samples show a poly(trimethylene carbonate) (PTMC) fraction generated by AM/ACE with a molar mass that is exactly twice that of the PTMC fraction coming from pure AM. Conversely, PTMC prepared with a diol is perfectly unimodal and keeps its molar mass dispersity below 1.1. This suggests that the side AM/ACE mechanism may be a bidirectional AM mechanism, and that PTMC with a narrow unimodal molar‐mass distribution can be obtained easily from a diol regardless of this side propagation.     

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.     

Propylene Polymerization with rac‐SiMe2(2‐Me‐4‐PhInd)2ZrMe2/MAO: Polymer Characterization and Kinetic Models

Poly(propylene)s were prepared with metallocene catalyst rac‐SiMe₂(2‐Me‐4‐PhInd)₂ZrMe₂/MAO (rac‐dimethylsilylbis(2‐methyl‐4‐phenylindenyl)dimethylzirconium/methylaluminoxane) in heptane solution at temperatures from 50 to 80 °C with varying concentrations of monomer, hydrogen, triisobutylaluminium (TIBA) and MAO. Polymer molar mass depended on the monomer, MAO, TIBA, and hydrogen concentrations and on polymerization temperature. The isotacticity was very high (mmmm > 95%), and only a slight decrease was detected at high temperatures. Regio selectivity was also high; the total amount of 2,1‐ and 3,1‐insertions was less than 0.4 mol‐%. Lowering the monomer concentration and raising the temperature increased the amount of 3,1 defects over the amount of 2,1 defects. End‐group analysis by ¹³C NMR spectroscopy revealed isobutyl and allyl end‐groups. Chain transfer to aluminium and β‐CH₃ elimination were concluded to be the dominating chain‐termination mechanisms. The importance of β‐CH₃ elimination increased with temperature. Hydrogen addition changed both the initiation and termination mechanisms as indicated by the presence of propyl, butyl and 2,3‐dimethylbutyl end‐groups. According to modeling studies, the molar mass follows a first‐order relationship with propylene and hydrogen concentrations, and a half‐order relationship with MAO concentration. Arrhenius‐type activation energy coefficients were 125 kJ · mol^?1 for β‐CH₃ elimination, 66 kJ · mol^?1 for chain transfer to aluminium, and 53 kJ · mol^?1 for chain transfer to hydrogen. A value of 45 kJ · mol^?1 was used for the propagation.

     

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.

     

Effect of Et3Al and 9,9‐Bis(methoxymethyl)fluorine on Propylene Polymerization at High Temperature with TiCl4/MgCl2 Catalysts

Summary: A study of the effect of Et₃Al and a diether, 9,9‐bis(methoxymethyl)fluorine (BMMF), on propylene polymerization at high temperature with the use of MgCl₂‐supported catalysts is reported. BMMF could be extracted from a TiCl₄/MgCl₂/BMMF catalyst by Et₃Al at 100 °C, and Et₃Al is an efficient chain‐transfer agent in the presence of BMMF at 100 °C. The results obtained from differential scanning calorimetry (DSC) and ¹³C NMR spectroscopy showed that the isotactic poly(propylene) (iPPs) produced by the catalysts containing BMMF as an internal or external donor, contained ethylene units. This phenomenon was not found in the iPP chains obtained with donor‐free TiCl₄/MgCl₂ catalyst in the absence of any external donor at 100 °C. It is suggested that the decomposition of Et₃Al did not take place in the absence of any donor and took place in the presence of BMMF at 100 °C.

¹³C NMR spectrum of an iPP sample.     

Cyclometallated Pt(II) Complexes in Visible‐Light Photoredox Catalysis: New Polymerization Initiating Systems

A household halogen lamp is used to promote the ring‐opening photopolymerization of epoxides in the presence of coumarin moiety containing Pt(II) complex/silane/iodonium salt combinations (epoxide/acrylate interpenetrated polymer networks can also be synthesized). Excellent polymerization profiles are obtained (conversions >70%) under this selected very soft irradiation. These Pt(II) complexes behave as photocatalysts in an oxidative cycle. A photoluminescent polymer can be in situ synthesized thanks to the luminescence properties of these complexes. The mechanisms are investigated by ESR, laser flash photolysis, and luminescence experiments.     

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.     

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%).

     

Low Conversion 4‐Acetoxystyrene Free‐Radical Polymerization Kinetics Determined by Pulsed‐Laser and Thermal Polymerization

Summary: The free‐radical polymerization kinetics of 4‐acetoxystyrene (4‐AcOS) is studied over a wide temperature range. Pulsed‐laser polymerization, in combination with dual detector size‐exclusion chromatography, is used to measure k_p, the propagation rate coefficient, between 20 and 110 °C. Values are roughly 50% higher than those of styrene, while the activation energy of 28.7 kJ · mol⁻¹ is lower than that of styrene by 3–4 kJ · mol⁻¹. With known k_p, conversion and molecular weight data from 4‐AcOS thermal polymerizations conducted at 100, 140, and 170 °C are used to estimate termination and thermal initiation kinetics. The behavior is similar to that previously observed for styrene, with an activation energy of 90.4 kJ · mol⁻¹ estimated for the third‐order thermal initiation mechanism.

Joint confidence (95%) ellipsoids for the frequency factor A and the activation energy E_a from non‐linear fitting of k_p data for 4‐AcOS (black) and styrene (grey).     

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.

     

A Novel Branched Polyoxymethylene Synthesized by Cationic Copolymerization of 1,3,5‐Trioxane with 3‐(Alkoxymethyl)‐3‐ethyloxetane

Novel branched polyoxymethylene copolymers are synthesized by cationic copolymerization of 1,3,5‐trioxane (TOX) with 3‐(alkoxymethyl)‐3‐ethyloxetane (ROX) using BF₃·Et₂O as an initiator. Four oxetane derivatives with different side‐chain lengths (from 1 to 6 carbons) are tested for copolymerization. The copolymer composition is controlled by the feed ratio of ROX, and influenced by the chain length of alkyl group on ROX. The incorporation ratio and side‐chain length of the ROX unit have great influence on the thermomechanical properties and crystallinity of the copolymers.

     

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.

     

Role of tert‐Butyl Methyl Ether (TBME) as an External Donor in Propene Polymerization with Dibutyl Phthalate (DBP)‐Containing MgCl2‐Supported Ti Catalysts Activated with Al(i‐C4H9)3

Summary: Stopped‐flow polymerization of propene was first conducted at 40 °C using the TiCl₄/DBP/MgCl₂ catalyst (DBP = dibutyl phthalate) combined with Al(C₂H₅)₃. An induction period was observed at the beginning of polymerization, and the resulted polymer contained a considerable amount of the isolated ethylene unit. The formation of such unusual structures was concluded to be the result of copolymerization with ethylene originating from the ethylated Ti⁴⁺ species. Catalyst washing with toluene and stopped‐flow polymerization at 70 °C brought about a drastic decrease in the contents of both Ti and DBP as well as a disappearance of both the induction period and isolated ethylene unit. The microstructures of polymers revealed that the highly stereoregular polymers are produced at the initial stage of polymerization. Changes in yield and molecular weight of polymers with polymerization time showed that the addition of tert‐butyl methyl ether (TBME) brought about an increase in the concentration of active sites, but did not affect the propagation rate of propene polymerization. Such an increase in the active site concentration caused by an external donor has hardly been reported so far in kinetic studies using the stopped‐flow method; thus, the present result is believed to be a unique example.

Kinetic curves of propene polymerization with the Cat. IV‐Al(i‐C₄H₉)₃ catalyst. (?): with TBME/Al = 0.1 and hydrogen, (?): with TBME/Al = 0.1 and without hydrogen, (○): without TBME and with hydrogen, (?): without TBME and hydrogen.     

Synthesis of Elastomeric Poly(propylene) (ELPP) Using the Highly Active TiCl4/Dibutyl phthalate (DBP)/MgCl2‐Al(i‐C4H9)3/1‐Allyl‐3,4‐dimethoxybenzene (ADMB) Catalyst

Propene polymerization was first carried out at 70°C using the TiCl₄/DBP/MgCl₂ catalyst in combination with a mixture of ADMB and various alkylaluminium compounds. Both the activity and the molecular weight of the polymer were strongly dependent on the kinds of alkylaluminiums. However, the isotactic index (I.I.) of the polymer was hardly affected by them. In order to control the I.I. value, the propene polymerization was then conducted with the TiCl₄/DBP/MgCl₂‐Al(i‐C₄H₉)₃/ADMB catalyst by changing the polymerization temperature. A suitable selection of polymerization temperatures gave a higher molecular weight PP containing various amounts of APP, i. e., I.I. 4–59%. Mechanical testing indicated that the obtained polymers exhibited a wide range of physical properties from a low modulus thermoplastic elastomer to flexible plastics depending on their stereoregularities. The improved elastic recovery of the obtained PP in the present study could be simply interpreted by the decrease in the [mmmm] or crystallinity of the polymer. Thus, the polymer having the lowest such values was found to have an excellent elastic recovery, which was comparable to that of ELPP as previously reported. The present result is the first example of the synthesis of an elastomeric poly(propylene) using a highly active MgCl₂‐supported Ti catalyst.     

Ni‐Catalyzed Polymerization of Poly(3‐alkoxythiophene)s

The Ni‐catalyzed polymerization of P3AOTs was studied and compared with the controlled chain‐growth polymerization of P3ATs. By varying the ratio of the initial monomer concentration to the initiator concentration, no linear dependence of the molar mass was observed, revealing that the polymerization does not proceed via a controlled mechanism. This was also confirmed by analyzing the end‐groups of the polymer with MALDI‐TOF mass spectrometry. To acquire more information on the polymerization mechanism, the formation of the actual monomer and the polymerization reaction were studied into more detail. These experiments proved that the polymerization proceeds via a chain‐growth mechanism, although not in a controlled way.