Daniel Colombani

Chain-growth control in free radical polymerization

Abstract The present review relates generally to improved processes in which it is possible to control the growth steps of a radical polymerization to produce relatively monitored chain-length homopolymers and copolymers, including block, star and graft copolymers. The review, which contains more than 360 references to original works, has only to be taken as a snapshot of this rapidly developing area. Investigations summed up in this article are mainly those carried out by ‘polymer synthesis schools’ over the world over the past 20 years, with a detailed analysis of results obtained over the past three years. Indeed, works pertaining to the possibility of continuous radical polymerization to be achieved through the complexing and stabilization of free radicals, often called ‘living’ radical polymerizations, have witnessed an explosive growth in the past decade. These latter are chain-growth polymerizations, free from chain transfer, termination and other side reactions, and are promising methods for the synthesis of well-defined polymers. Despite the inherent instability of the growing macroradicals, ‘living’ radical polymerizations of vinyl monomers have recently been investigated based on the general principle of ‘radicophilic stabilization of the growing macroradical’. Three approaches have been followed in efforts to develop radical polymerization systems exhibiting living characters: • • The first uses physical methods to prevent radicals from contacting each other, thereby preventing their termination. The process is generally conducted in a heterogeneous medium. • • The second involves chain-growth control by chemical stabilization of the growing macroradical in homogeneous polymerization. • • The third approach, which involves no living process but exhibits some living characteristics, is based on the repeated reinitiation of polymer chains either by homolysis (thermo- or photolysis) of weak and/or reversible linkages that are built into the chains or by addition-fragmentation processes on macromonomers.

Chain transfer by addition-substitution-fragmentation mechanism: 3. Access to low-molecular-weight telechelic polymers using ethyl 2-[1-(1-methoxy-1-cyclohexylperoxy)-ethyl]propenoate

Abstract Low-molecular-weight di-end-functional (telechelic) telomers were prepared by radical addition-substitution-fragmentation transfer reactions on ethyl 2-[1-(1-methoxy-1-cyclohexylperoxy)ethyl]propenoate (EMCPEP), used as a new chain-transfer regulator in the free-radical polymerization of methyl methacrylate (MMA), styrene (St) and butyl acrylate (BA). The chain-transfer constant obtained in MMA polymerization at 60°C ( C tr = 0.102) was very low compared to those obtained in St and BA polymerization in the same conditions ( C tr = 1.02 and 0.88, respectively). Such a result was attributed to the allylic steric effect of both monomer and transfer agent in the addition step of the transfer reaction. EMCPEP behaves as an ideal transfer agent for St and BA at 60°C. The activation energy for the transfer reaction of EMCPEP with poly(methyl methacrylate) radicals ( Ea tr = 31.3 kK mol −1 ) was determined from transfer constants measured in MMA polymerization performed at 50, 55, 60, 70 and 80°C. The differential scanning calorimetry study of the thermal stability of peroxyketal EMCPEP gave an estimation of the rate constants and the activation energy of the thermolysis ( Ea th = 130.3 kJ mol −1 ) at various reaction temperatures.

Chain transfer by addition-fragmentation mechanism—9. access to diene-functional macromonomers using 5-(substituted)-1,3-pentadiene-type addition-fragmentation chain-transfer agents in radical polymerization

Abstract Radical polymerizations of methyl methacrylate (MMA) and styrene (St) in bulk at low conversion were carried out in the presence of pentadienic chain transfer agents (CTA), 5-bromo-1,3-pentadiene (1), 5-benzenesulphonyl-1,3-pentadiene (2) and methyl 2-bromomethyl-4-methyl-2,4-pentadienoate (3), to produce conjugated diene-end capped macromonomers by a addition-fragmentation mechanism. The chain-transfer constants (Cir) of 1, 2 and 3 for MMA polymerization were obtained from the Mayo equation, respectively. Correction to zero conversion afforded an accurate value of the chain transfer constant for 1. The chain transfer was found to be degradative. The pentadienyl group formed by fragmentation of the macroradical abduct is quantitatively introduced at the ω-end of the polymer.

Synthesis, characterization and hydrolysis of poly[styrene-co-(6-methylene-1,4-oxathiepane-7-one)] and poly[styrene-co-(6-methylene-5-methyl-1,4-oxathiepane-7-one)]

Vinylic-type polymers bearing ester groups inside the polymer backbone have been synthesized by free radical copolymerization of styrene (St) and 6-methylene-1,4-oxathiepane-7-one (MOTPO) or 6-methylene-5-methyl- 1,4-oxathiepane-7-one (MMOTPO). The addition-fragmentation ring-opening polymerization of both MOTPO and MMOTPO leads to the formation of ester linkages located inside the vinylic polymer backbone. A strong decrease of the molar mass of the copolymer has been observed when the copolymers were dissolved in a mixture of THF and water in the presence of sodium hydroxide. This decrease can be attributed to the hydrolysis of the ester linkages, as followed by size exclusion chromatography (SEC). The molar mass of the degraded polymer samples was correlated with the number of ester linkages in the backbone, showing that only a fraction of these ester groups have been hydrolyzed.

Chain transfer by addition-fragmentation mechanism, 8 Study of transfer agents designed to allow 1,5-intramolecular homolytic substitutions

Some substituted olefins and dienes bearing weak bonds located in appropriate locations were synthesized and added to vinylic monomer polymerization media, i.e., cumyl 4,6-heptadienyl peroxide (CHP), ethyl 5-cumylperoxy-5-methoxy-2-methylenehexanoate (ECMMH), 6-cumylperoxy-6-methoxy-3-methylene-2-oxoheptane (CMMOH), N-t-butyl-N-(2,2-diethoxyethyl)acrylamide (tBEEA), N-t-butyl-N-(2,2-diethoxyethyl)methacrylamide (tBEEMA). Chemistry aspects of synthesis and stability of the compounds are discussed. The thermolysis activation energies of the peroxidic compounds were estimated from DSC measurements to adapt the reaction conditions to the stability of these compounds. These compounds were tested as potential new chain transfer agents, involving a radical addition on activated unsaturation and a subsequent substitution on O-O or H-C bonds. In the first case, an oxyl radical was expelled and, in the second one, the generated carbon-centered radical was expected to evolve by a fast β-scission of the adjacent C-O bond to yield an alkyl radical. In both cases, these radicals would re-initiate efficiently the polymerization cycle. It was found that, in contrast to previously studied compounds allowing efficient 1,3-intramolecular homolytic substitutions (1,3-S H i), the transfer properties of these 1,5-substituted compounds in the free radical polymerization of methyl methacrylate, styrene or butyl acrylate are poor in most cases. This behavior is discussed in terms of competition between intermolecular cross-addition reaction (copolymerization) and 1,5-intramolecular homolytic substitution (1,5-S H i).

Chain transfer by addition‐fragmentation mechanism. VII. Radical polymerization of vinyl monomers in the presence of ethyl 2‐[1‐(trimethylsilylperoxy)ethyl]propenoate and related compounds

Ethyl 2-[1-(trimethylsilylperoxy)ethyl]propenoate 1, ethyl 2-[1-(dimethylvinylsilylperoxy)-ethyl]propenoate 2, ethyl 2-[1-(1-(2-ethoxycarbonyl-1-methyl-2-propenylperoxysilyl)-1-methylethylperoxy)ethyl]propenoate 3, and 2-phenyl-2-trimethylsilylperoxypropane 4 were synthesized and added to the free radical polymerization of vinylic monomers. 1 and 2 were found to show no homopolymerizability but act as effective chain transfer reagents in radical polymerizations of methyl methacrylate (MMA), styrene (St), and n-butyl acrylate (BA). The estimated chain transfer constants (Ctr) are as follows: Ctr (1) = 0.15 for MMA, 0.90 for St, and 2.03 for BA at 60°C; Ctr (2) = 0.12 for MMA, 1.16 for St, and 1.9 for BA at 60°C. 1H–NMR spectra of poly(St) formed in the presence of 1 is consistent with the view that the polymers bear an oxirane at one terminal and an trimethylsilyloxy fragment at the other end. Moreover, peroxysilane 4 showed very low transfer properties by direct homolytic substitution (SH2). These findings indicate that the ethyl 2-[1-(substituted dimethylsilylperoxy)ethyl]-propenoates 1–3 undergo chain transfer reaction via a intramolecular homolytic substitution (SHi) following an addition process. Preparation of poly(styrene) up to high conversion in the presence of 3 yielded to the formation of the corresponding polymeric structures bearing hydrolysable C(SINGLE BOND)O(SINGLE BOND)Si(SINGLE BOND)O(SINGLE BOND)C bonds.

Synthesis of methacrylic-type peroxidic compounds and study of their homolytic induced decomposition in solution

Abstract Various unsaturated peroxidic compounds have been prepared, characterized and inductively decomposed in various solvents to afford the corresponding oxiranes. The reaction proceeded by a radical chain mechanism and was initiated either by thermolysis of added t -butyl peracetate at 110°C or AIBN at 80°C, or by autoxidation of BEt 3 at 20°C. The studied peroxyderivatives were designed to generate oxyl radicals reacting either by isomerization ( e.g. intramolecular 1,5-hydrogen atom transfer, cyclization or β-scission of a cyclic structure), fragmentation or hydrogen atom transfer to solvents to yield functional alkyl radicals.

Chain transfer by addition-fragmentation mechanism 10. Comparison of dienic peroxides designed to allow transfer reactions

A new substituted dienic peroxide, 5-cumylperoxy-1,3-pentadiene (1), bearing a weak bond in a specific location inside the molecule (i.e. in the allylic position) has been synthesized and added to vinylic monomer polymerizations. Peroxide 1 has been tested as a new potential chain transfer agent and has been compared with the reactivity of another previously reported compound, 7-cumylperoxy-1,3-heptadiene (2). These two peroxides may involve a radical addition onto the conjugated double bonds and a subsequent intramolecular homolytic substitution (SHi) on the O-O bonds, followed by the re-initiation of the polymerization cycle through the expelled oxyl radicals. In contrast to 2, which allows a rather poor 1,5-SHi, the transfer properties of 1 in the free radical polymerization of methyl methacrylate, styrene or butyl acrylate through a fast intramolecular 1,3-SHi process are particularly efficient. This behaviour is discussed in terms of the competition between the 1,5-intramolecular homolytic substituti...