Multifunctional polymerization initiators

Multifunctional polymeric initiators are also formed by the reaction of ozone with polyalkenes [109, 110]... 

Matyjaszewski, K., Ga5mor, S. G., and Coca, S. (1998). Controlled atom or group-transfer radical pol5mierization, coupling of molecules, multifunctional polymerization initiators, and formation of telechelic functional material. In PCT Int. Appl. WO 9840415, Carnegie Mellon University, USA, 230 pp. 

Many types of peroxides (R-O-O-R) are known. Those in common use as initiators include diacyl peroxides (36), pcroxydicarbonatcs (37), peroxyesters (38), dialkyl peroxides (39), hydroperoxides (40), and inorganic peroxides [e.g. persulfate (41)1, Multifunctional and polymeric initiators with peroxide linkages are discussed in Sections 3.3.3 and 6.3.2.1. 

The multifunctional initiators may be di- and tri-, azo- or peroxy-compounds of defined structure (c.g. 20256) or they may be polymeric azo- or peroxy-compounds where the radical generating functions may be present as side chains 57 or as part of the polymer backbone."58"261 Thus, amphiphilic block copolymers were synthesized using the polymeric initiator 21 formed from the reaction between an a,to-diol and AIBN (Scheme 7.22).26 Some further examples of multifunctional initiators were mentioned in Section 3.3.3.2. It is also possible to produce less well-defined multifunctional initiators containing peroxide functionality from a polymer substrate by autoxidalion or by ozonolysis.-0... 

The success of the multifunctional initiators in the preparation of block and graft copolymers depends critically on the kinetics and mechanism of radical production. In particular, the initiator efficiency, the susceptibility to and mechanism of transfer to initiator, and the relative stability of the various radical generating functions. Each of these factors has a substantial influence on the nature and homogeneity of the polymer formed. Features of the kinetics of polymerizations initiated by multifunctional initiators have been modeled by O Driscoll and Bevington 64 and Choi and Lei.265... 

He also prepared a poly(styrene-g-styrene) polymer by this technique [114], The lack of crosslinking in these systems is indeed proof of the control achieved with this technique. An eight-arm star polystyrene has also been prepared starting from a calixarene derivative under ATRP conditions [115]. On the other hand, Sawamoto and his coworkers used multifunctional chloroacetate initiator sites and mediation with Ru2+ complexes for the living free-radical polymerization of star poly(methylmethacrylate) [116,117]. More recent work by Hedrick et al. [84] has demonstrated major progress in the use of dendritic initiators [98] in combination with ATRP and other methodologies to produce a variety of structure controlled, starlike poly(methylmethacrylate). 

In a different approach, Hedrick et al. have studied multifunctional dendritic initiators for the synthesis of multiarm star-shaped copolymers [102]. Several dendritic initiators with hydroxyl functionality ranging from 2 to 48 have been prepared according to the method developed by Hult et al. [120]. The bulk polymerization of sCL initiated by these multifunctional macroinitiators and acti-... 

This technique is based on the use of a linear polymer with pendant functional groups that can be activated to initiate the polymerization of a second monomer. Based on this definition, the linear precursor polymer can be considered as a multifunctional macromolecular initiator. The importance of the grafting from technique by cationic polymerization of the second monomer increased considerably with the advent of living cationic polymerization. The advantage is the virtual absence of homopolymer formation via chain transfer to monomer. 

Grafting from has not been a successful method in anionic polymerization because the required low molecular weight multifunctional organometallic initiators are almost always insoluble and this is also expected to be the case when dendrimers are modified. However, in cationic polymerizations the dormant species is less polar and more soluble. For example, the hexabenzyl bromide... 

From the preceding lines it is evident that multifunctional and polymeric initiators have been the object only of laboratory work so far they have not yet progressed to larger scale production. Nevertheless, requirements of specialized polymers may dictate their wider application in the future. 

The radical mechanism generates block copolymers mainly by means of multifunctional or polymeric initiators [270] or by the combination of radical chain ends produced by the separation of two propagating monomers into an aqueous and a micellar phase (in emulsion) [271],... 

Block copolymers between alkyl or related methacrylates (B-1,132 198,357 B-2,198 and B-3115,146,148) were prepared via the ruthenium-, copper-, and nickel-catalyzed living radical polymerizations. These block copolymers can be synthesized both via sequential living radical polymerizations and via the living radical polymerization initiated from isolated polymers. For example, the ruthenium-catalyzed sequential living radical polymerization of MMA followed by nBMA affords AB block copolymers B-1 with narrow MWDs (Mw/Mn = 1.2), which can be extended further into ABA block copolymers B-2 with similarly narrow MWDs (Mw/Mn = 1.2).198 Star block copolymers with B-1 as arm chains were similarly synthesized but with multifunctional initiators.357... 

Multifunctional DVB initiators were used to polymerize styrene, and the living arms were subsequently reacted with ethylene oxide to produce PS chains with terminal hydroxyl groups.57... 

Based on the findings of the arm-first strategy, it is possible to obtain stars with well-defined arm length. The main problem of this strategy is the arm number distribution. Moreover, purification may cause many difficulties in the synthesis. In contrast, the core-first strategy requires multifunctional initiators and further polymerization initiated from the core. This is shown in Scheme 3. The maximum arm numbers of the stars are determined by the number of functionalities in the core. In the ideal case, the initiating efficiency of the core is close to unity, which will produce well-defined stars with precise numbers of arms. However, due to the steric hindrance and the limit of the polymerization techniques, it can be difficult to obtain full initiating efficiency. 

In another study, reducible comb-shaped polymers were prepared by using a multifunctional dextran initiator.Dextran modified with an ATRP initiator and disulfide bonds was synthesized by the activation of hydroxyl groups of dextran using GDI followed by its reaction with cystamine (Scheme 3.10). S-S-Dextran was reacted with a-bromoisobutyric acid (BIBA) by EDC/NHS chemistry, and S-S-dextran-Br was then used to polymerize PDMAEMA by ATRP, followed by chain extension with OEGMA, producing a range of copolymers of 10-100 kDa. The comb-shaped copolymers of 86 kDa... 

Another biocompatible and biodegradable polymer, chitosan, was converted into a multifunctional macroinitiator for the ZLL-NCA polymerization. Initially, the amine groups of chitosan were protected with phthalic anhydride, followed by triphenylmethyl protection of the 6-CH2OH groups. The amine groups were deprotected by hydrazine and used to... 

More recently, multifunctional ATRP initiators with 4, 6, and 11 2-bromoisobutyrate groups were prepared for the synthesis of star polymers of styrene, methyl and n-butyl and w-hexyl acrylates. The polymerization of styrene was performed at 110 °C, using the catalytic system CuBr/bipytidine. The molecular weights of the resulting stars were up to 51 X 10 , the PDIs were lower than 1.1, and the conversions... 

Some of the earliest work on the use of imidazolium-derived NHCs as catalysts focused on their ability to serve as initiators for polymerization reaction. Pioneers of this area, Waymouth and Hedrick extensively investigated the synthesis of polyesters under the influence of an NHC catalyst. Their results have been summarized in a number of excellent accounts. In 2002, Hedrick first investigated catalytic living polymerization using NHCs for cyclic esters (Scheme 14.27). Then, Waymouth and Hedrick studied polymerization of lactides and a-caprolactones and explored the chemical and mechanical properties of the resulting polymers. Also, the use of multifunctional alcohol initiators, such as ethylene glycol or pentaerythritol, produced star polymers with good conversion and low PDI (polydispersity index). ... 

Multifunctional initiators based on, for example, cyclotriphosphazine [106], silesquioxane [107], porphyrin [108] and bipyridine metal complex [109, 110] cores were also successfully used for the living cationic ring-opening (co)polymerization of 2-oxazolines, resulting in star-shaped (co)polymers. The use of polymeric initiators also allowed the construction of well-defined complex macro molecular architectures, such as triblock copolymers with a non-poly(2-oxazo-line) middle block that is used to initiate the 2-oxazoHne polymerization after functionalization with tosylate end-groups [111-113]. In addition, poly(2-oxazoline) graft copolymers can be prepared by the inihation of the CROP from, for example, poly(chloromethylstyrene) [114, 115] or tosylated cellulose [116]. 

In resists of this class, the imaging layer contains a multifunctional monomer that can form an intercormected network upon polymerization, and a photosensitizer to generate a flux of initiating free radicals. Although not stricdy required for imaging, the composition usually includes a polymeric binder (typically an acryhc copolymer) to modify the layer s physical properties. Figure 7b shows the chemical stmctures of typical components. 

Polymeric azo-compounds and multifunctional initiators with azo-linkages are discussed elsewhere (see 3.3.3 and 7.6.1) as are azo compounds, which find use as iniferters (see 9.3.4). 

The use of initiators such as 68 has been promoted for achieving higher molecular weights or higher conversions in conventional polymerization and for the production of block and graft copolymers. The use and applications of multifunctional initiators in the synthesis of block and graft copolymers is briefly described in Section 7.6.1. 

Knowledge of kui/kii is also important in designing polymer syntheses. For example, in the preparation of block copolymers using polymeric or multifunctional initiators (Section 7.6.1), ABA or AB blocks may be formed depending on whether termination involves combination or disproportionation respectively. The relative importance of combination and disproportionation is also important in the analysts of polymerization kinetics and, in particular, in the derivation of rate parameters. 

---