Radical Polymerization

Abstract

Approximately 50% of all synthetic polymers are currently made via radical polymerization (RP) processes. The commercial success of RP can be attributed to the large range of radically polymerizable monomers, their facile copolymerization, the convenient reaction conditions employed (typically room temperature to 100 • C, ambient pressure), and very minimal requirements for purification of monomers and solvents. RP is not affected by water and protic impurities and can be carried out in bulk, solution, aqueous suspension, emulsion, dispersion, etc. The range of monomers is larger for RP than for any other chain polymerization because radicals are tolerant to many functionalities, including acidic, hydroxy, and amino groups. In conventional RP, high molecular weight (MW) polymers are formed at the early stages of the polymerization, and neither long reaction times nor high conversions are required, in sharp contrast to step-growth polymerization.However, RP has some limitations, especially in comparison with ionic processes that provide facile routes to living polymers. For a polymerization to be considered ''living,'' the contribution of chain breaking reactions such as termination and transfer should be negligible. While transfer is not a major issue in RP when the appropriate conditions are applied, termination is much less avoidable and is a major limitation of RP. In contrast to ionic reactions, in which cations or anions do not react via bimolecular termination, radicals terminate with a diffusion controlled rate. To form high MW polymers, the relative probability of such termination must be minimized. This is accomplished by running RP at very low concentrations (from ppb to ppm) of propagating radicals. The concentrations of growing species are thus much lower in RP than in carbanionic or ring-opening polymerizations. In RP, a steady concentration of radicals is established by balancing the rate of termination with that of initiation. The rate of propagation is much faster than the rate of initiation/termination (as required to keep radical concentration low). Therefore, Controlled and Living Polymerizations. Edited a Measurements in bulk at 40 • C and ambient pressure, unless otherwise stated. b k t reported at 1000 bar. c Solvent = chlorobenzene. E p and A p represent the Arrhenius activation energy for propagation and frequency factor for propagation, respectively. Typical Features of Radical Polymerization 107coefficients of termination depend on chain length and conversion (viscosity), the values in Table 3.1 are given for low conversion and DP ≈ 100. CopolymerizationFacile statistical copolymerization is one of the main advantages of RP. In contrast to ionic polymerization, the reactivities of many monomers are relatively similar, which makes them easy to copolymerize statistically. For monomers with opposite polarities, there is a tendency for alternation (r A r B < 1, where r A = k AA /k AB and r B = k BB /k BA ). Electrophilic radicals (i.e., those with -CN, -C(O)OR, or Cl groups) prefer to react ...

Synthesis and copolymerization of vinylidene fluoride (VDF) with Trifluorovinyl Monomers, 11

“…However, the selectivity of monoaddition confirms previous work when these catalysts were used at 130 °C at least 51. Even in the presence of various ligands which favor a control of radical polymerization, especially the 2,2′‐bipyridine52,53 or the 2‐pyridinecarbaldehyde n ‐pentyl imine called “Haddleton's ligand”54 (run 5), no improvement was noted.…”

Studies on controlled radical polymerisation using 5-membered cyclic PROXYL nitroxides and corresponding alkoxyamines

“…This may be indicative of transfer reactions occurring during polymerisation. [1] In Tab. 1 are shown M n and polydispersity data for each polymerisation after 24 h, without reprecipitation of the samples.…”

“…The various CRP methods all involve reversible termination of growing polymer chains by some mediator, and a fast equilibrium between active and dormant polymerising species leading to a linear growth in chain length as a function of monomer conversion. [1] Systems capable of achieving this include dithiocarbamate iniferters, [2] various metalbased atom-transfer processes (ATRP), [3][4][5][6] nitroxide stable free radicals (SFRs), [7][8][9][10][11][12] cobalt porphyrin complexes [13] and certain chain transfer agents (RAFT). [14][15][16] Arguably, ATRP using Cu(I) complexes and nitroxidemediated CRP have been the most popular and successful methods.…”

Kinetics and electron spin resonance study of the radical polymerization of n‐butyl acrylate mediated by a nitroxide precursor: C‐phenyl‐N‐tert‐butylnitrone

“…New synthetic routes to well‐defined polymers with a low polydispersity have emerged as result of recent progress in controlled radical polymerization (CRP), mainly nitroxide‐mediated polymerization (NMP), atom transfer radical polymerization, and reversible‐addition fragmentation chain transfer 1–16. NMP is one of the most extensively studied CRPs and involves the use of either an appropriate nitroxide or an alkoxyamine.…”