ABSTRACT:The controlled radical polymerization of a variety of acrylate monomers is reported using an Ir-catalyzed visible light mediated process leading to well-defined homo-, random, and block copolymers. The polymerizations could be efficiently activated and deactivated using light while maintaining a linear increase in molecular weight with conversion and first order kinetics. The robust nature of the fac-[Ir(ppy) 3 ] catalyst allows carboxylic acids to be directly introduced at the chain ends through functional initiators or along the backbone of random copolymers (controlled process up to 50 mol % acrylic acid incorporation). In contrast to traditional ATRP procedures, low polydispersity block copolymers, poly(acrylate)-b-(acrylate), poly(methacrylate)-b-(acrylate), and poly(acrylate)-b-(methacrylate), could be prepared with no monomer sequence requirements. These results illustrate the increasing generality and utility of light mediated Ircatalyzed polymerization as a platform for polymer synthesis. have revolutionized the field of polymer chemistry, allowing for the synthesis of well-defined macromolecular structures with excellent functional group tolerance. Perhaps of greater importance is the facile reaction conditions that allow nonexperts access to these materials, enabling significant advances across a number of fields. More recently, additional control over living radical polymerizations has been achieved through regulation of the chain growth process by an external stimulus. 5 For example, electrochemical ATRP has been used to pattern polymer brushes on surfaces, 6− 8 as well as gain control over aqueous polymerizations.9 While the employment of externally regulated polymerizations is in its infancy, the potential for further innovation is significant.In considering the wide range of possible external stimuli, light offers many attractive features such as readily available light sources, tunability, and both spatial and temporal control. On this basis, significant work has been dedicated to the development of photoinitiated 10− 17 and photoregulated radical polymerizations (i.e., photocontrolled RAFT, 29 This approach uses a simple reaction setup with only ppm levels of Ir(ppy) 3 and enables efficient activation and deactivation of polymerization leading to control over molecular weight and molecular weight distributions. A fundamental element of this process is that in the absence of irradiation, the chain end rests as the dormant alkyl bromide, protected from deleterious radical reactions but available for reactivation upon re-exposure to light. Moreover, the spatial and temporal control of Ir-catalyzed photomediated processes has been exploited for patterning polymer brushes on surfaces to give novel, 3-D nanostructures. 30Our previous reports on photomediated radical polymerizations focused exclusively on methacrylates. In order to increase the scope and applicability of this strategy, extension to other monomer families is required. Our attention was therefore drawn to acrylate-based polymer...

We report herein the modular synthesis and nanolithographic potential of poly­(dimethylsiloxane-block-methyl methacrylate) (PDMS-b-PMMA) with self-assembled domains approaching sub-10 nm periods. A straightforward and modular coupling strategy, optimized for low molecular weight diblocks and using copper-catalyzed azide–alkyne “click” cycloaddition, was employed to obtain a library of PDMS-b-PMMA and poly­(dimethylsiloxane-block-styrene) (PDMS-b-PS) diblock copolymers. Flory–Huggins interaction parameters, determined from small-angle X-ray scattering experiments, were high for PDMS-b-PMMA (χ ∼ 0.2 at 150 °C), suggesting this diblock copolymer system has promise for sub-10 nm lithographic applications when compared to the corresponding PDMS-b-PS diblock copolymers (χ ∼ 0.1 at 150 °C). Performance evaluation in thin film self-assembly experiments allowed domain periods as small as 12.1 nm to be obtained, which is among the smallest highly ordered nanoscale patterns reported hitherto for thermally annealed materials.

Stopped-flow NMR measurements suitable for determination of reaction kinetics on time scales of 100 ms or longer have been achieved by adaptation of a commercial NMR flow probe with a high-efficiency mixer and drive system. Studies of metallocene-catalyzed alkene polymerization at room temperature have been complicated by high rates, imprecise knowledge of the distribution of different catalyst species with time, and the high sensitivity of the catalysts to low concentrations of impurities. Application of the stopped-flow NMR method to the study of the kinetics of 1-hexene polymerization in the presence of (EBI)ZrMe[MeB(C(6)F(5))(3)] demonstrates that NMR spectroscopy provides an efficient method for direct and simultaneous measurement of substrate consumption and catalyst speciation as a function of time. Kinetic modeling of the catalyst and substrate concentration time courses reveal efficient determination of initiation, propagation, and termination rate constants. As first suggested by Collins and co-workers (Polyhedron 2005, 24, 1234-1249), a kinetic model in which Zr-HB(C(6)F(5))(3) forms rapidly upon beta-hydride elimination but reacts relatively slowly with alkene to reinitiate chain growth is supported by these data.

In addition to the traditional parameters of chi (χ) and degree of polymerization (N), we demonstrate that the segregation strength of a diblock copolymer can be increased by introduction of an ionic unit at the junction of the two blocks. Compared to neutral linking groups, the electrostatic interactions between counterions of adjacent domain junctions leads to increased enthalpy, segregation strength, and phase separation. As a result, the order disorder transition temperatures of block copolymers with a 1,2,3triazolium ionic junction were observed to be significantly higher than the corresponding neutral block copolymers. To demonstrate the potential of block copolymers with ionic junctions for nanopatterning, block copolymers were prepared by click coupling of homopolymers and then used to fabricate well-defined sub-10 nm line features. We believe that the concept of improved thin-film assembly through the introduction of ionic junctions is a powerful tool for block copolymer lithography and complements chi (χ) and degree of polymerization (N) in the design of macromolecular systems with enhanced phase separation.

Chromophore-containing quench agents 2 and 3 enable quantitative active site counting and determination of the mass distribution of active catalyst polymeryls by refractive index (RI) and UV detected gel permeation chromatography (GPC) for the polymerization of 1-hexene catalyzed by (EBI)ZrMe 2 /B(C 6 F 5 ) 3 . Time evolution of catalyst speciation data and the time profiles of monomer consumption, end-group generation, and bulk molecular weight distribution data have been analyzed by kinetic modeling to determine rate constants for initiation by insertion of hexene into a Zr−Me bond (k i ), propagation (k p ), chain transfer to form vinylidene (k 1,2 ) and vinylene (k 2,1 ) end groups, and reinitiation from a Zr−H bond (k r ). Unlike previous models that assumed fast catalyst reinitiation, this analysis reveals that k r is considerably slower than k p ; catalyst speciation data are critical to making this distinction. This study demonstrates that chromophore quench-labeling with 2 and 3 enables rapid, quantitative analysis of detailed kinetic models for catalytic olefin polymerization reactions using GPC with UV and RI detectors.