Chain Transfer Constants Research Articles

Determination of Chain Transfer Constants of Alcohols for High‐Pressure Polyethylene Polymerization

AbstractIn low‐density polyethylene (LDPE) production, low‐molecular‐weight species are used as chain transfer agents (CTAs). They reduce the degree of polymerization, which has an impact on the polymer properties, such as processability. To specifically adjust the product properties, a detailed understanding of the transfer kinetics to CTAs is indispensable. Thus, the transfer activity of various primary, secondary, and tertiary alcohols as well as diols was investigated experimentally between 120 °C and 250 °C at 2000 bar. The results were evaluated using the Mayo and chain length distribution (CLD) method, and the latter was found to give more reliable results. It was observed that the hydroxy group systematically increases transfer activity compared to alkanes. Additionally, a linear increase of transfer activity with the number of hydrogen atoms is found.

α-Amido Trifluoromethyl Xanthates: A New Class of RAFT/MADIX Agents.

Xanthates have long been described as poor RAFT/MADIX agents for styrene polymerization. Through the determination of chain transfer constants to xanthates, this work demonstrated beneficial capto-dative substituent effects for the leaving group of a new series of α-amido trifluoromethyl xanthates, with the best effect observed with trifluoroacetyl group. The previously observed Z-group activation with a O-trifluoroethyl group compared to the O-ethyl counterpart was quantitatively established with Cex = 2.7 (3-4 fold increase) using the SEC peak resolution method. This study further confirmed the advantageous incorporation of trifluoromethyl substituents to activate xanthates in radical chain transfer processes and contributed to identify the most reactive xanthate reported to date for RAFT/MADIX polymerization of styrene.

Ethylene Polymerization over Supported Vanadium-Magnesium Catalysts with Different Vanadium Content: The Effect of Hydrogen on Molecular Weight Characteristics of the Produced Bimodal Polyethylene

Data are presented on the activity of supported vanadium-magnesium catalysts (VMCs) with different vanadium content in ethylene polymerization and the molecular weight characteristics of the produced polyethylene. The VC1 catalyst, with a very low vanadium content (0.12 wt.%), showed a sixfold higher activity per unit weight of vanadium than the VC2 catalyst with a high-vanadium content (4.0 wt.%). Additionally, the total activity of VC2 (kg PE/g cat·h) was fivefold higher when compared to VC1. The introduction of hydrogen in polymerization leads to a considerable decrease in the activity of both catalysts. The polyethylene obtained in the presence of hydrogen over both catalysts has a broad bimodal molecular weight distribution (MWD) with a distinct shoulder in the high-molecular region (Mw ≥ 106 g/mol). Decomposition of the MWD curves of bimodal polyethylene into two fractions (high- and low-molecular fractions) made it possible to determine for the first time the ratio of the reaction rate constants of chain transfer with hydrogen (KtrH) and polymer chain propagation (Kp) for two groups of the VMC active sites producing low- and high-molecular fractions of bimodal polyethylene.

Effect of solvent on the microstructure of polyvinyl alcohol

AbstractThe method for producing polyvinyl alcohol (PVA) by the solution polymerization of vinyl acetate first and then alcoholysis strategy is industrial. Where solvent plays a vital role in control over stereoregularity, molecular weight and its distribution, crystallinity and so on. In this report, when methyl acetate was used as a solvent in the polymerization of vinyl acetate instead of methanol, prepared PVA was characterized by Fourier‐transform infrared spectroscopy (FT‐IR), nuclear magnetic resonance (NMR), gel permeation chromatography (GPC), thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The results showed that PVA obtained in methyl acetate has a higher content of syndiotactic structure, crystallinity melting temperature and enhanced thermal stability compared with PVA obtained in methanol due to the formation of Π ‐ Π complex between methyl acetate and vinyl acetate and the enhanced steric hindrance effect during polymerization. Additionally, methyl acetate has a smaller chain transfer constant (1.31 × 10−4), which benefits to decrease the branching degree and molecular weight distribution index of PVA. This research provides more ideas for the choice of solvents in the industrial production of polyvinyl alcohol.

Study on polymerization of bio-based isosorbide monomethacrylate for the formation of low-T and high-T sustainable polymers

Isosorbide monomethacrylate (IMMA) is a biobased monomer that appears as a potential substitute for current petrobased methacrylates to prepare high-performance sustainable polymers by free-radical polymerization technique. However, as the molecular weight of polymers can markedly affect their physical, mechanical and processing properties, it is crucial to understand the nature of the processes that control the growing of IMMA based polymer chains. To this end, the free radical polymerizability of IMMA has been investigated in depth under various conditions (solvent, conversion rate, [IMMA]/[AIBN] ratio). This study, based on the determination of chain transfer constants using the Mayo method, highlighted the key role of monomer and solvent transfer reactions for controlling molecular weight of PIMMA (C tr M = 5.0x10 -4 ; C tr S (DMSO) = 4.0x10 -4 ; C tr S (DMF) = 5.0x10 -4 ; C tr S (Isopropanol/dioxane (1:6)) = 1.0x10 -3 ). Knowing the parameters governing the free radical polymerization of IMMA, PIMMA with tailored molecular weight (from 2100 to 80200 g.mol −1 ) and T g have been prepared and characterized. In particular, DSC analyses of PIMMA revealed the opportunity to prepare from low- T g to high- T g materials with different potential applications.

Expanding the Scope of Polymerization‐Induced Self‐Assembly: Recent Advances and New Horizons

Over the past decade or so, polymerization-induced self-assembly (PISA) has become a versatile method for rational preparation of concentrated block copolymer nanoparticles with a diverse set of morphologies. Much of the PISA literature has focused on the preparation of well-defined linear block copolymers by using linear macromolecular chain transfer agents (macro-CTAs) with high chain transfer constants. In this review, a recent process is highlighted from an unusual angle that has expanded the scope of PISA including i) synthesis of block copolymers with nonlinear architectures (e.g., star block copolymer, branched block copolymer) by PISA, ii) in situ synthesis of blends of polymers by PISA, and iii) utilization of macro-CTAs with low chain transfer constants in PISA. By highlighting these important examples, new insights into the research of PISA and future impact these methods will have on polymer and colloid synthesis are provided.

Kinetics and Mechanism of Synthesis of Carboxyl-Containing N-Vinyl-2-Pyrrolidone Telehelics for Pharmacological Use.

It was found that sulfanylethanoic and 3-sulfanylpropanoic acids are effective regulators of molecular weight with chain transfer constants of 0.441 and 0.317, respectively, and show an unexpected acceleration effect on the radical polymerization of N-vinyl-2-pyrrolidone, initiated by 2,2’-azobisisobutyronitrile. It was determined for the first time that the thiolate anions of mercapto acids form a high-temperature redox initiating system with 2,2’-azobisisobutyronitrile during the radical polymerization of N-vinyl-2-pyrrolidone in 1,4-dioxane. Considering the peculiarities of initiation, a kinetic model of the polymerization of N-vinyl-2-pyrrolidone is proposed, and it is shown that the theoretical orders of the reaction rate, with respect to the monomer, initiator, and chain transfer agent, are 1, 0.75, 0.25, and are close to their experimentally determined values. Carboxyl-containing techelics of N-vinyl-2-pyrrolidone were synthesized so that it can slow down the release of the anticancer drug, doxorubicin, from aqueous solutions, which can find its application in the pharmacological field.

АКРИЛОВІ МОНОМЕРИ НА ОСНОВІ РОСЛИННИХ ОЛІЙ ІЗ ВИСОКИМ ВМІСТОМ ЕСТЕРІВ ОЛЕЇНОВОЇ КИСЛОТИ

New acrylic monomers were obtained via transesterification of olive, canola, and high-oleic soybean oils by N-hydroxyethylacrylamide. The kinetic features of homopolymerization of these monomers were studied and the influence of linoleic (C18: 2) and linolenic (C18: 3) acid esters on the polymerization rate and the molecular weight of homopolymers were compared. It was found that the chain transfer and propagation rate constants increase in monomer’s range: olive (CM = 0.016) <high-oleic soybean (CM = 0.018) <canola (CM = 0.025). Features of homopolymerization are associated with varying degrees of unsaturation of fatty acid fragments.

Controlled Synthesis of Polyphosphazenes with Chain-Capping Agents.

N-alkyl phosphoranimines were synthesized via the Staudinger reaction of four different alkyl azides with tris(2,2,2-trifluoroethyl) phosphite. N-adamantyl, N-benzyl, N-t-butyl, and N-trityl phosphoranimines were thoroughly characterized and evaluated as chain-capping compounds in the anionic polymerization of P-tris(2,2,2-trifluoroethoxy)-N-trimethylsilyl phosphoranimine monomer. All four compounds reacted with the active chain ends in a bulk polymerization, and the alkyl end groups were identified by 1H-NMR spectroscopy. These compounds effectively controlled the molecular weight of the resulting polyphosphazenes. The chain transfer constants for the monomer and N-benzyl phosphoranimine were determined using Mayo equation.

Correction: When Mayo falls short (Ctr ≫ 1): the use of cumulative chain length distribution data in the determination of chain transfer constants (Ctr) for radical polymerizations

Correction for ‘When Mayo falls short (Ctr ≫ 1): the use of cumulative chain length distribution data in the determination of chain transfer constants (Ctr) for radical polymerizations’ by Matt K. Donald and Stefan A. F. Bon, Polym. Chem., 2020, 11, 4281–4289, DOI: 10.1039/D0PY00348D.

Effect of bulky 2,6-bis(spirocyclohexyl)-substituted piperidine rings in bis(hindered amino)trisulfide on thermal healability of polymethacrylate networks

Bulky 2,6-bis(spirocyclohexyl)-substituted piperidine rings in bis(hindered amino)trisulfide affords low chain-transfer constant and thermal healability at moderate temperature.

The thiol group modified multi-wall carbon nanotubes to enhance the dielectric properties of polystyrene

The researches on how to prepare the polymer composites with high dielectric constant (high-k) and low dielectric loss were important for the rapid development of electronic industry. In this paper, by using silane coupling agent, the multi-wall carbon nanotubes (MWCNTs) could graft the thiol groups with high chain transfer constant on the surface, which made the polystyrene (PS) chain coat more efficiently on the surface of MWCNTs to form the core-shell structure. 1H NMR, TGA and TEM results, showed polystyrene were grafted on the surface of MWCNTs. The grafted PS could reduce the dielectric loss of composite materials and promote the dispersion of the coating MWCNTs in the matrix. In addition, the insulating layer effectively improved the interaction between the MWCNTs and PS matrix, which strengthened the binding force between the filler and matrix, resulting in the increase of the thermal stability of the PS composites. In terms of AC conductivity, the insulating layer could effectively prevent the contact between adjacent MWCNTs from forming a complete conducting network, which inhibited the generation of leakage current. Therefore, compared with untreated multi-wall carbon nanotubes/polystyrene composites, the above composites had excellent dielectric properties with high dielectric constant (214) and low dielectric loss (2.23) at low CNTs mass fraction (4 wt%).

Preparation of bio-based styrene alternatives and their free radical polymerization

Three different bio-based styrene monomer alternatives, i.e., 4-vinylphenol derivatives, were synthesized starting from lignin building blocks. The synthetic route encompassed a green Knoevenagel condensation followed by two decarboxylation steps and acetylation. High isolated yields for the decarboxylation were obtained in a polar aprotic solvent, which prevented oligomerization. The polymerization of the three acetylated 4-vinylphenols was performed by free radical polymerization. The chain transfer constants of styrene and 4-acetoxy styrene 4c in toluene were evaluated in the Mayo procedure using the number-average molecular weight (Mn). After polymerization, the acetoxy group of the different polymers was efficiently hydrolyzed. The resulting phenol side groups of these polystyrene alternatives provide future opportunities to functionalize these bio-based polymers further.

Aluminum Alkyls as Highly Active Catalytic Chain Transfer Agents

AbstractDuring the production of free radical initiated low‐density polyethylene (LDPE), it was discovered that the addition of low levels of alkyl aluminum compounds caused the molecular weight of the LDPE to drop precipitously. Further investigation demonstrated that aluminum‐alkyl compounds are among the most effective chain transfer agents ever utilized. It was also shown that polymer chains, which transfer to Al alkyl species, contain almost exclusively vinyl terminated end groups. A catalytic chain transfer mechanism is proposed in which chain transfer occurs from a growing polymer chain to an aluminum center followed by beta hydride elimination to produce a vinyl terminated polymer chain and a new aluminum hydride bond. This new aluminum hydride bond can then undergo further chain transfer reactions. This is the first time such a catalytic chain transfer mechanism has been reported. As little as 10–20 mol ppm aluminum alkyl species decreased the degree of polymerization by a factor of 2 resulting in chain transfer constant (Cs) values as high as 1000–2000. Density functional theory (DFT) study elucidated the catalytic cycle of triethylaluminum (TEA). It is discovered that, depending on the reaction conditions, TEA can serve as a conventional as well as catalytic chain transfer agent.

The Kinetic Study of Ethylene Polymerization over Titanium–Magnesium Catalysts in the Presence of Hydrogen: The Number and Reactivity of Active Centers

Using the inhibition of polymerization by radioactive carbon monoxide 14СО, the effect of hydrogen (chain transfer agent) on the number of active centers and propagation rate constant in ethylene polymerization over modern highly active titanium–magnesium catalysts with different composition is studied. It is found that a decrease in the rate of ethylene polymerization in the presence of hydrogen is associated primarily with reduction in the number of active centers and these changes are reversible at the introduction/removal of hydrogen. The scheme of reversible temporary deactivation of active center precursors is advanced to explain the experimental results. According to the data on the effect of hydrogen concentration on the molecular weight of polyethylene and the values of propagation rate constants, the rate constant of polymer chain transfer by hydrogen is calculated for catalysts with different composition.

When Mayo falls short (Ctr ≫ 1): the use of cumulative chain length distribution data in the determination of chain transfer constants (Ctr) for radical polymerizations

A method to determine chain transfer constants in free radical polymerizations that are >1 using molecular weight distribution data.

Significantly Suppressed Chain Transfer to Monomer Reactions in RAFT Emulsion Polymerization of Styrene

The highest accessible molecular weight of the living product in controlled/living radical polymerizations (CLRPs) is determined by the constant of chain transfer to monomer reaction. Thus, the highest molecular weight of living polystyrene had been believed around 100 kg/mol in CLRPs. In our previous report (Yan, K.; Gao, X.; Luo, Y. Well-Defined High Molecular Weight Polystyrene with High Rates and High Livingness Synthesized via Two-Stage RAFT Emulsion Polymerization. Macromol. Rapid Commun. 2015, 36 (13), 1277–1282), we surprisingly found that the livingness of polystyrene of 550 kg/mol could be up to 90% in RAFT emulsion polymerization. Herein, we experimentally measure the constant of chain transfer to monomer reaction in RAFT emulsion polymerization of styrene at various temperatures from 60–80 °C. Very interestingly, it is found that the constant of chain transfer to monomer reaction should be only about one-third of that of the corresponding conventional radical polymerization. From the derived A...

Experimental and Theoretical Comparison of Addition–Fragmentation Pathways of Diseleno- and Dithiocarbamate RAFT Agents

Theoretical studies suggest that identically substituted dithio- and diselenocarbamate reversible addition–fragmentation chain transfer (RAFT) agents will have similar reactivity but that diselenocarbamate RAFT agents are more likely to undergo intermediate radical termination. These results are supported by an experimental study of polymerizations of vinyl acetate mediated by Se-cyanomethyl N,N-dimethyldiselenocarbamate (1Se) and its sulfur analog, S-cyanomethyl N,N-dimethyldithiocarbamate (1S). While the two polymerizations had similar interchain transfer constants (3.2 for 1S and 3.9 for 1Se), significant retardation was observed for 1Se-mediated polymerizations. External chain transfer constants were also measured for 1S (5.4) and 1Se (45). The low apparent value of the chain transfer constant of 1S is attributed to the selective formation of a single monomer adduct in the early stages of the reaction.

Theoretical investigation of thiol-ene click reactions: A DFT perspective

In this study, a detailed quantum chemical investigation of the contribution of phenyl thiol derivatives in thiol-ene reaction mechanism has been carried out for the first time. DFT calculations have been used to investigate the role of substitution in thiol-ene reactions. It is well known that the reaction mechanism is strongly controlled by the kP/kCT ratio, where kP is the propagation rate constant of the thiyl radical’s addition to the alkene and kCT is the rate constant of chain transfer to a thiol. The electrophilic nature of the phenylthio radicals and the singlet-triplet (S-T) gap of alkenes are mainly responsible for the variation of the activation barriers for the propagation reaction, this demonstrates the importance of the ene functionality on the propagation reaction. A correlation between the radical stabilization energy of the carbon centered radical intermediate and the chain transfer activation energy could not be established. The transition structures of the chain transfer reactions were shown to be stabilized by intramolecular interactions, which have lowered the activation barriers. In this study, we underlie the kP/kCT ratio which is highly dependent not only on the alkene functionality, but also on the thiol functionality. Tailor-made polymers can be obtained by altering the substituents or their positions, and the computational procedure described herein is expected to guide the synthesis.

Defined High Molar Mass Poly(2-Oxazoline)s.

Poly(2-alkyl-2-oxazoline)s (PAOx) are regaining interest for biomedical applications. However, their full potential is hampered by the inability to synthesise uniform high-molar mass PAOx. In this work, we proposed alternative intrinsic chain transfer mechanisms based on 2-oxazoline and oxazolinium chain-end tautomerisation and derived improved polymerization conditions to suppress chain transfer, allowing the synthesis of highly defined poly(2-ethyl-2-oxazoline)s up to ca. 50 kDa (dispersity (Ð) <1.05) and defined polymers up to at least 300 kDa (Ð<1.2). The determination of the chain transfer constants for the polymerisations hinted towards the tautomerisation of the oxazolinium chain end as most plausible cause for chain transfer. Finally, the method was applied for the preparation of up to 60 kDa molar mass copolymers of 2-ethyl-2-oxazoline and 2-methoxycarbonylethyl-2-oxazoline.