Statistical and Gradient Copolymers

Spontaneous gradirait formation using the simultaneous reaction of monomer pairs with sufficiently differing reactivity ratios 

Forced gradient copolymerization using continuous feed or semibatch techniques 

Statistical and gradient copolymCTs have bear prepared successfully using NMP and ATRP in bulk, solution, and mini-emulsion, particularly when styrene is used as a monomer with acrylates, methacrylates, acrylonitrile, and substituted styrenes as comonomers. A number of BPO/TEMPO-mediated statistical polymerizations have been reported, but in some cases, control can only be effected if the initial comaiQmer feed is rich in styrene. 

---

A number of different types of copolymers are possible with ATRP—statistical (random), gradient, block, and graft copolymers [Matyjaszewski, 2001]. Other polymer architectures are also possible—hyperbranched, star, and brush polymers, and functionalized polymers. Statistical and gradient copolymers are discussed in Chap. 6. Functionalized polymers are discussed in Sec. 3-16b. 

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. 

Gradient copolymers, also known as tapered copolymers, describe a chain topology that is generally unique to living polymerizations [6,8]. They are intermediates between statistical and block copolymers, as shown in Fig.4. 

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). 

Contrary to statistical and block copolymers, gradient copolymers exhibit a composition gradient along their chain. The properties of gradient copolymers are expected to be intermediate between those of block and statistical copolymers. In particular, their kinetics of aggregation and exchange can be faster than in the case of block... 

Fig. 10 Illustrations of the microchannel confined surface-initiated polymerization (p-SIP) route for producing gradient polymer brush libraries a route for making polymer molecular weight and block copolymer libraries b route for making statistical copolymer libraries. Red arrows show the flow of monomer solution from a syringe pump used to gradually fill the microchannel. See text for details...

Statistical, gradient, and block copolymers as well as other polymer architectures (graft, star, comb, hyperbranched) can be synthesized by NMP following the approaches described for ATRP (Secs. 3-15b-4, 3-15b-5) [Hawker et al., 2001]. Block copolymers can be synthesized via NMP using the one-pot sequential or isolated macromonomer methods. The order of addition of monomer is often important, such as styrene first for styrene-isoprene, acrylate first for acrylate-styrene and acrylate-isoprene [Benoit et al., 2000a,b Tang et al., 2003]. Different methods are available to produce block copolymers in which the two blocks are formed by different polymerization mechanisms ... 

Davis KA, Matyjaszewski K (2002) Statistical, gradient, block and graft copolymers by controlled/living radical polymerizations. Springer, Berlin Heidelberg New York... 

Statistical, Gradient, Block and Graft Copolymers by Controlled/ Living Radical Polymerizations Authors Davis, K. A., Matyjaszewski, K. Vol. 159,2002... 

Statistical, Gradient, Block and Graft Copolymers by Controlled/Living Radical Polymerizations... 

Scheme 2 Schematic representation of prepared copolymer samples of P(IBA-co-nBA) with different molecular weights, compositions and architectures statistical (open squares), block (rhombi) and gradient (circles).

Copolymers composed of two monomers can be classified according to the relative arrangement of the two types of monomer units along the chain or, in other words, according to the monomer distribution. For linear chains, one can have four types of copolymers statistical/random, alternating, block, and gradient. 

The large variation in polymerization rates among the various 2-oxazoline monomers has been exploited for the preparation of quasi-diblock copolymers, namely, gradient copolymers with a narrow and steep monomer gradient, by statistical copolymerization of selected monomer combinations exhibiting large differences in reactivity. Such a one-pot quasi-diblock copolymer synthesis was first demonstrated for the statistical copolymerization of 2-phenyl-2-oxazoline and a much less reactive 2-perfluoroalkyl-2-oxazoline [174]. This living CROP (in nitromethane at 120 °C initiated by methyl p-nitrobenzenesulfonate) revealed complete consumption of the 2-phenyl-2-oxazoline after 2 min with only minor incorporation of the fluorinated monomer. Continuation of the polymerization for another 40 h led to full conversion of the fluorinated monomer. The... 

---