Intermolecular Chain Transfer Research Articles

Consideration of Macromonomer Reactions in n‐Butyl Acrylate Free Radical Polymerization

n-Butyl acrylate (BA) starved-feed solution semibatch experiments with varying final polymer content and monomer feed times were carried out at 138 °C. A full mechanistic model of the system implemented in Predici includes intermolecular chain transfer to polymer and macromonomer propagation as well as backbiting, chain scission, and midchain radical propagation and termination. The importance of macromonomer propagation under these conditions of industrial interest is illustrated by experiment and simulation, with the macromonomer reaction responsible for the significant increase in polymer weight-average molecular weight ($\overline M _{\rm w}$) with time. Rate coefficients for macromonomer propagation (k(mac) ) and β-scission (k(β) ) of k(mac) /k(p) = 0.55 and k(β) = 12 s(-1) (with k(p) the rate coefficient for BA chain-end propagation) provide a good representation of experimental $\overline M _{\rm w}$ and macromonomer end group data at 138 °C.

Effects of Reversible Addition Fragmentation Transfer (RAFT) on Branching in Vinyl Acetate Bulk Polymerization

A detailed mathematical model has been formulated for branching due to chain transfer to polymer in reversible addition fragmentation chain transfer (RAFT) polymerization. The traditionally adopted mechanism for RAFT polymerization has been modified by the inclusion of a relaxation process. It is proposed that this step occurs during the period immediately after a chain radical is released from the RAFT transfer intermediate. While undergoing relaxation, the so-called unrelaxed radicals are assumed to be capable of propagation, intermolecular chain transfer to polymer, and bimolecular termination, but not intramolecular chain transfer to polymer which only becomes possible when the radical is relaxed. The use of these assumptions indicates that RAFT can reduce the overall rate of branching without significantly affecting other key measurements such as conversion and average molecular weights. If relaxation is neglected, RAFT would appear not to affect the rate of branching. However, by reducing the averag...

Z-RAFT Star Polymerizations of Acrylates: Star Coupling via Intermolecular Chain Transfer to Polymer

Six-arm star polymers of methyl acrylate (MA), butyl acrylate (BA), and dodecyl acrylate (DA) were generated in bulk at 60 °C via reversible addition−fragmentation chain transfer (RAFT) polymerization. A hexafunctional trithiocarbonate was employed as mediating compound, in which the active RAFT-agent moieties are interlinked to the central core molecule via the stabilizing Z-group. Well-defined star polymers with predictable number-average molecular weights of more than 1 × 106 g·mol-1 were obtained by this Z-RAFT star polymerization of acrylates. Narrow and monomodal molecular weight distributions were found up to intermediate monomer conversions. At higher monomer conversions, an unexpected high molecular weight component occurred with increasing extent when going from MA over BA to DA polymerization. This high molecular weight material was assigned to a star−star couple containing two living cores, which formation is not in accordance to the basic Z-RAFT star polymerization mechanism, but most likely arises from an intermolecular chain transfer to polymer reaction. The amount of star−star couples, which is an indication for the extent of long-chain branching, was quantified as a function of the monomer conversion and subsequently modeled via kinetic simulations. By this approach, the rate coefficient of intermolecular transfer to polymer at 60 °C was estimated to be ktrP,inter = 0.33 L·mol-1·s-1 in BA polymerization and ktrP,inter = 7.1 L·mol-1·s-1 in DA polymerization. The living process was found to be very effective up to high monomer conversions, indicating that steric congestion next to the star polymer core, where the actual RAFT process takes place, is not significantly hampering the Z-RAFT star polymerization of acrylates.

Midchain Transfer to Polymer in Styrene−Poly(butyl acrylate) Systems: Direct Evidence of Retardative Effects

Branching in poly(butyl acrylate) (polyBA) systems is a well-accepted process that occurs via two reactions: a bimolecular reaction, intermolecular chain transfer by hydrogen atom abstraction (long chain branching, LCB), and a unimolecular reaction, intramolecular backbiting (short chain branching, SCB). The relative amount of each type of branching depends on the monomer-polymer ratio.

Matrix-assisted laser desorption ionization time-of-flight mass spectroscopy of polydimethylsiloxanes prepared via anionic ring-opening polymerization

Full Paper: Poly(dimethylsiloxane) prepared by theanionic ring-opening polymerization initiated by butyl-lithium in tetrahydrofuran has been examined by MALDI TOF mas spectrometry. The molecular weight distributions hav been found to be different from those obtainedby SEC, with the MALDI spectra underestimating the contribution of the higher mass macromolecules leading to an underestimation of the average molecular weigths. Three ditributions are observed in the mass spectra that correspon to the initiation and interchain-exchange mechanisms producing three different macromolecular species. The pre-dominant species is an integral sum of the repeat unit indicating that anionic ring-opening propagation is fast when compared to intermolecular chaintransfer. This is in conflict with earlier studies and indicates that the relative rates of transfer by diffferent processes are highly dependent upon reaction conditions.

Intra‐ and intermolecular chain transfer to macromolecules with chain scission. The case of cyclic esters

AbstractChain transfer to macromolecules with chain scission is the most often observed “side” reaction in the polymerization of heterocyclics. In our previous works we analysed quantitatively the intramolecular chain transfer to the own macromolecule (back‐biting). This paper gives a general treatment of the kinetics of polymerization with propagation and intermolecular chain transfer to macromolecules, accompanied with chain scission. The numerical solution developed allows determining the kp/ktr ratio from the dependence of m̄w/m̄n on monomer conversion. This treatment was applied to the polymerization of L,L‐lactide and kp/ktr ratios were measured for covalent alcoholate active species bearing Al, Fe, Ti, Sm, and La. In this way selectivities of active species (expressed with kp/ktr) were for the first time measured and finally correlated with the atomic number of the corresponding metal atoms, related to the strength of the bond involved in the monomer addition.

Study on the cationic polymerization mechanism of 1,3‐dioxepane in the presence of ethylene glycol

AbstractThe cationic polymerization of 1,3‐dioxepane (DOP) initiated with trifluoromethanesulfonic acid (I) in the presence of ethylene glycol (EG) was investigated. At sufficiently low concentration of the initiator ([I] > 0.01 mol/L vs. [EG] < 0.20 mol/L), the molecular weights of the obtained polyacetal oligodiols are controlled by the mole ratio of consumed DOP to initial EG. Gel‐permeation chromatography studies revealed that the concentration of cyclic oligomers in the products are negligible. The mechanism of the polymerization was investigated by means of kinetic studies. The results showed that the polymerization proceeds according to the active chain end mechanism (ACF) in combination with the activated monomer mechanism (AM); thus the cyclic oligomer in the obtained polymer is reduced, and intermolecular chain transfer to EG in ACE is dominant. It was also demonstrated that as [DOP]2[I]/[EG] decreases the contribution of ACE to the polymerization decreases and that of AM increases. In addition, 1H and 13C NMR data illustrated that each macromolecule of polyDOP oligodiols contained one EG unit on average and that no EG end groups exist.

Intermolecular chain transfer to polymer with chain scission: general treatment and determination of kp/ktr in L,L‐lactide polymerization

AbstractA general kinetic treatment of the system with intermolecular chain transfer followed by fast reinitiation is given. It leads to the broadening of the molecular weight distribution (MWD), the number of growing chains being invariable. Thus, this system can be considered as a special case of living polymerization. A general method has been elaborated allowing the determination of the ratio of the rate constant of propagation (kp) to the rate constant of the bimolecular transfer (k(2)tr) from the dependence of the MWD on monomer conversion. Numerical values of kp/k(2)tr equal to ≈ 102 and 25 were thus determined for the polymerization of L, L‐lactide (L, L‐dilactide) initiated with aluminium tris(isopropoxide) trimer ({Al(OiPr)3}3) and tributyltin ethoxide (nBu3SnOEt), respectively.

Kinetic anomalies in the radical polymerization of 2-fluoro-2,2-dinitroethyl methacrylate

The kinetics of radical polymerization of the self-inhibiting monomer 2-fluoro-2,2-dinitroethyl methacrylate in bulk and in dichloroethane solution are studied. The heat of polymerization is 52·34 kJ/mole, and the total heat of chain initiation and termination is 325 kJ/mole. Both these parameters are changed by dilution. The polymerization kinetics are described by a scheme which assumes intra-molecular chain transfer together with intermolecular chain transfer; the ratios of the chain transfer constants to the chain propagation constant are determined.

The role of the termination by the polymer chain in the cationic polymerization of oxetane and 3,3‐dimethyloxetane

AbstractThe ring opening polymerization of oxetane and 3,3‐dimethyloxetane was carried out in dichloromethane at 25°C with triethyloxonium hexafluoroantimonate as catalyst. After polymerization, the flow times were measured for active and terminated solutions in high vacuum. In the case of oxetane, the data support the presence of branched structures in the active solutions formed by an intermolecular chain transfer reaction to the polymer chains, to give non‐strained oxonium‐ions. This intermolecular side reaction was not detected in the case of 3,3‐dimethyloxetane, if the presence of intramolecular chain transfer to a small extent is not excluded.

Thermal degradation of polystyrene

Molecular weight change studies have shown that the thermal degradation of random copolymers of styrene — namely HIPS, SAN, and ABS-at low temperatures and in air involves random chain scission. The dominant process in the degradation of HIPS is random chain scission due to weak links, whereas in SAN it is intermolecular chain transfer. In ABS, the degradation is initially random scission due to weak links and then mainly intermolecular chain transfer. The infrared spectra show that during degradation the labile weak links are attacked by oxygen and peroxidic free radicals are produced. Via hydrogen abstraction or autoxidation of olefinic links, these free radicals are responsible for the formation of aliphatic ketonic or peroxyester structures, and for isomerization and cyclization. The activation energies of overall degradation of HIPS, SAN, and ABS are 134, 142, and 92 kJ.mol−1 respectively.

Calculation and analysis of the molecular weight distribution (MWD) allowing for chain transfer to polymer in homogeneous radical polymerization processes

A method is suggested for derivation of the statistical molecular weight distribution function of macromolecules, allowing for intermolecular chain transfer to polymer in radical polymerization processes. In the derivation the polymer macromolecule is represented as a “tree”. Expressions have been derived for calculation of MWDs of branched polymers. In the case of termination solely due to disproportionation the distribution function has been obtained in an explicit form in terms of standard functions. MWDs of linear and branched polymers have been found to be interrelated.

Long chain branching in poly(vinyl chloride)

AbstractThe intrinsic viscosity [η]‐weight average molecular weight Mw, relation has been studied for fractions of a range of vinyl chloride homopolymers prepared at 40–80°C in aqueous suspension. In addition to curvature of the log [η]‐log Mw plots for Mw > 2 × 105, at constant Mw a slight but definite trend to lower [η] as polymerization temperature is increased was also observed. Both these effects are ascribed to long chain branching due to an increasing incidence of intermolecular chain transfer as polymerization temperature is increased. Limited examination of poly(vinyl chloride) prepared by emulsion polymerization indicates, by the same criterion, a greater incidence of branching than found in poly(vinyl chloride) prepared at the same temperature by the aqueous suspension process. It is concluded that no single correlation of the Mark‐Houwink type can be expected to apply to vinyl chloride homopolymers prepared over a range of conditions.

Thermal degradation of vinyl polymers III–A radiochemical study of intermolecular chain transfer in the thermal degradation of polystyrene

The importance of an intermolecular chain transfer reaction in the thermal degradation of polystyrene has been established by a radiochemical method. A polystyrene containing a specific number of thermally weak links at known points along the polymer chain was prepared by an adaptation of the Szwarc ‘living’ polymer technique. It was thermally degraded at moderate temperatures (280° to 285°C) in the presence of carbon-14 labelled ‘normal’ polystyrene as diluent. Measurement of the activity of the monomer in the distillate showed that, with a kinetic chain length of about three monomer units, 1·2 chain transfers occurred per radical. Some evidence is also given to indicate that weak link scission plays a significant role in the early stages of polystyrene degradation.

Theoretical Depolymerization Kinetics in Polymers Having an Initial ``Most Probable'' Molecular Weight Distribution

The kinetic equations describing a general chain mechanism of depolymerization are solved at steady state for a polymer having an initial ``most probable'' molecular weight distribution. Chain scission initiation, end group initiation, depropagation, intermolecular chain transfer, and several types of termination are considered. In the past, solutions have been limited to special cases and approximations. Now general expressions for molecular weight change and rate of weight loss are available for polymers with an initial molecular weight distribution of wide interest. The relationships to the previously considered special cases are discussed.