Gradient copolymer, defined

Polymers prepared via CRP show promise for applications like photoresists [112], liquid-crystalline displays [147-149, 154], and photo catalysts [151]. Incorporating blocks prepared using CRP techniques into copolymers with conductive or luminescent blocks [240,241,243,251] may impart better processability and make them useful for a broader range of applications. Block or gradient copolymers with highly controlled compositions may also be industrially useful as blend compatibilizers or as surfactants [194],perhaps improving upon already existing materials. Well-defined or functional compatibilizers and stabilizers could potentially result in lower production costs if less material is needed to impart the desired properties. 

ATRP was recently reported as a new and powerful route to the synthesis of well-defined (co)polymers of such monomers as styrene, acrylates, methyl methacrylate, acrylonitrile, and isobutene. ATRP is a versatile tool for preparation of random, block, alternating, and gradient copolymers with controlled molecular weight, narrow polydispersities, and desired architectures. 

Examples of well-defined monomer sequences (Matyaszewski et al., 2012) (A) block copolymer chain, (B) linear gradient copolymer chain, (C) graft copolymer chain with all arms having the same chain length, different shades of gray (light or dark) correspond to different comonomer types. 

Living radical polymerization (atom transfer radical pol5mierization) has been developed which allows for the controlled polymerization of acrylonitrile and comonomers to produce well defined linear homopolymer, statistical copolymers, block copolymers, and gradient copolymers (214-217). Well-defined diblock copolymers with a polystyrene and an acrylonitrile-styrene (or isoprene) copolymer sequence have been prepared (218,219). The stereospecific acrylonitrile polymers are made by solid-state urea clathrate polymerization (220) and organometallic compounds of alkali and alkaline-earth metals initiated polymerization (221). 

Due to the high level of end-group functionality and the lifetime of the polymer chains, NMP is an attractive methodology for synthesizing not only homopolymers but also block copolymers and well-defined gradient copolymers. 

Gradient copolymers are defined as copolymers of two or more monomers, whose composition profile varies along the chain, reflecting variation in monomer concentrations as conversion proceeds. As a conseqnence, gradient copolymers combine the properties of the homopolymers in a way that depends on the nature of the composition profile. 

All above homopolymers are used also for the identification of suitable conditions for the coupled polymer HPLC techniques. Typical examples are liquid chromatography under critical (LC CC) and limiting (LC LC) conditions, and eluent gradient liquid chromatography (EG LC). For the development of latter methods, several defined statistical and block copolymers are available. 

RAFT has also been used to prepare copolymers. The copolymerization of MMA with nBA in the presence of cumyl dithiobenzoate as the transfer agent resulted in a polymer with a gradient of composition along the backbone, well-defined molecular weights, and low polydispersities [53]. Several copolymers were made by degenerative transfer with alkyl iodides [133]. 

The early patents also disclose and protect a nnmber of novel materials. Researchers at CMU were the first to prepare many gradient (52) and segmented copolymers by any CRP process, thereby exemplifying well defined copolymers with controllable monomer distribntions nnatlainable by prior art procedures, and were the first to nse a CRP initiator tethered to a sohd. 

In the area of novel materials CMU protected (co)polymers prepared by ATRP except with CCI4 initiator and telechelic polymers prepared by CRP with MW > 20,000 (49) copolymers with a tme gradient segment (30) polar ABA block copolymers, (30) and well defined graft copolymers and segmented copolymers with one or more CRP blocks where the macroinitiator had been prepared by another polymerization process. (36) In addition, the use of tethered initiators allowed synthesis of hybrid core/shell copolymers. Pending applications disclose other novel polymeric materials. 

Because there are well-defined equations that relate retention to the underlying transport coefficients, ThFFF retention can be used to measure those transport coefficients. In fact, the measurement of thermal diffusion coefficients by ThFFF can be used to obtain compositional information on polymer blends and copolymers (see Polymers and Particles ThFFF, p. 1869). ThFFF is also used in fundamental studies of thermal diffusion because it is a relatively fast and accurate method for obtaining the transport coefficient that quantifies the concentration of material in a temperature gradient, namely the Soret coefficient. However, the accuracy of Soret coefficients obtained from ThFFF experiments depends on appropriate accounting for several factors that involve temperature. To understand the effect of temperature on transport coefficients, as well as the effect on ThFFF calibration equations, a brief outline of retention theory is provided in the following section. 

In this section, some peculiarities of copolymerization conducted in CRP systems will initially be discussed and then examples of various types of copolymers will be provided. They will include statistical, gradient, and alternating copolymers, as well as block and graft copolymers. Some special systems will be also presented and they will include preparation of stereoblock copolymers, and various hybrid materials formed by either mechanistic transformation or by growing well-defined polymers from flat, colloidal, or irregular surfaces. 

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