Reversible addition fragmentation chain transfer polymerization of sterically hindered monomers: Toward well-defined rod/coil architectures

The sterically hindered monomers dibutyl itaconate (DBI) and dicyclohexyl itaconate (DCHI) were polymerized via reversible addition fragmentation chain transfer (RAFT) free-radical polymerization. S,S′-Bis(α, α′-dimethyl-α″-acetic acid) trithiocarbonate, cumyl dithiobenzoate, and cumyl phenyldithioacetate have been used as RAFT agents to mediate a series of polymerizations at 65°C yielding rod polymers ranging in number average molecular weight from 9000 to 92,000 g mol-1. The progress of the polymerization was followed via online Fourier transform-near infrared spectroscopy. The polydispersity indices of the obtained rod polymers were relatively high at 1.4-1.7. The RAFT polymerizations of the hindered monomers used in the present study displayed both ideal living and hybrid behavior between conventional and living polymerization, depending on the RAFT agent used. DCHI rod polymers generated via the RAFT process were subsequently reinitiated in the presence of styrene to produce a range of BAAB and A-B rod-coil block copolymers of molecular weights up to 164,000 g mol-1. The chain extension yields molecular weight distributions that progressively shift to higher molecular weights and are unimodal. Thermogravimetric analysis of the pDCHI-block-pStyrene copolymers indicates thermal degradation in two separate steps for the pDCHI and pStyrene blocks. © 2004 Wiley Periodicals, Inc

Mapping photolysis product radical reactivities via soft ionization mass spectrometry in acrylate, methacrylate, and itaconate systems

Electrospray ionization-quadrupole ion trap mass spectrometry (ESI-MS) was utilized to access the polymeric product spectrum generated by the pulsed laser polymerization (PLP) of methyl acrylate (MA) at -35 °C in the presence of the photoinitiators 2,2-dimethoxy-2-phenylacetophenone (DMPA), benzoin, benzoin ethyl ether (BEE), and bis(2,4,6-trimethylbenzoyl)phenylphosphinoxide (Irgacure 819) to study the reactivity of primary and potential secondary derived radical fragments from photolytically induced fragmentation. Similarly, the polymeric products generated from the PLP of dimethyl itaconate (DMI) at 0 °C using the aforementioned photoinitiators as well as benzil and 2,2′- azobis(isobutyronitrile) (AIBN) were studied using ESL-MS. The PLP products of methyl methacrylate (MMA) initiated with Irgacure 819 at -25 °C were also examined: Polymerization systems utilizing Irgacure 819 give complex product spectra due to the formation of second generation radical species resulting in several initiator fragments incorporated into a single polymer chain. Termination products, both combination and disproportionation, were identified with high accuracy. The reactivity of the various derived radical fragments toward the monomers employed is summarized for the current and a previous study in tabular form. Energy deposition into the MA/photoinitiator systems is found to have no influence on the product distributions of the MA polymers produced via photoinitiation under the conditions examined. For various photoinitiators employed, products congruent to that of chain transfer to monomer species in the DMI photopolymerizations are observed, conclusively illustrating that chain transfer to monomer is a significant reaction pathway in itaconate free radical polymerizations. © 2007 American Chemical Society

Living free-radical polymerization of sterically hindered monomers: Improving the understanding of 1,1-disubstituted monomer systems

The sterically hindered, 1,1-disubstituted monomers di-n-butyl itaconate (DBI), dicyclohexyl itaconate (DCHI), and dimethyl itaconate (DMI) were polymerized with reversible addition-fragmentation chain transfer (RAFT) free-radical polymerization and atom transfer radical polymerization (ATRP). Cumyl dithiobenzoate, cumyl phenyl dithioacetate, 2-cyanoprop-2-yl dithiobenzoate, 4-cyanopentanoic acid dithiobenzoate, and S- methoxycarbonylphenylmethyl dithiobenzoate were employed as RAFT agents to mediate a series of polymerizations at 60°C yielding polymers ranging in their number-average molecular weight from 4500 to 60,000 g mol-1. The RAFT polymerizations of these hindered monomers displayed hybrid living behavior (between conventional and living free-radical polymerization) of various degrees depending on the molecular structure of the initial RAFT agent. In addition, DCHI was polymerized via ATRP with a CuCl/methyl benzoate/N,N,N′N″N‴-pentamethyldiethylenetriamine/ cyclohexanone system at 60°C. Both the ATRP and RAFT polymerization of the hindered monomers displayed living characteristics; however, broader than expected molecular weight distributions were observed for the RAFT systems (polydispersity index = 1.15-3.35). To assess the cause of this broadness, chain-transfer-to-monomer constants for DMI, DBI, and DCHI were determined (1.4 × 10-3, 1.3 × 10-3, and 1.0 × 10 -3, respectively) at 60°C. Simulations carried out with the PREDICI program package suggested that chain transfer to monomer contributed to the broadening process, In addition, the experimental results indicated that viscosity had a pronounced effect on the broadness of the molecular weight distributions. © 2006 Wiley Periodicals, Inc