Polymerization NIPAM

Dithiocarbamatc 16 has been used to prepare low dispersity PMAA ( Mw 1 Mn-1.2).52 Photopolymerization of S in the presence of dithiocarbamate 16 also displays some living characteristics (molecular weights that increase with conversion, ability to make block copolymer). However, 17 appears to behave as a conventional initiator in S polymerization.53 The difference in behavior was attributed to the relatively poor leaving group ability of the 2-carboxyprop-2-yI radical. This hypothesis is supported by MO calculations. Dithiocarbamatc 17 was used to control polymerizations of MMA,54 HEMA54 and NIPAM.5... 

The synthesis of block copolymers by macromonotner RAFT polymeriza tion has been discussed in Section 9.5.2 and examples are provide in Table 9.9. RAFT polymerization with thioearbonylthio compounds has been used to make a wide variety of block copolymers and examples arc provided below in Tabic 9.28. The process of block formation is shown in Scheme 9.59. Of considerable interest is the ability to make hydrophilic-hydrophobic block copolymers directly with monomers such as AA, DMA, NIPAM and DMAEMA. Doubly hydrophilic blocks have also been prepared.476 638 The big advantage of RAFT polymerization is its tolerance of unprotected functionality. 

RAFT polymerization has been used to prepare poly(ethylene oxide)-/ /wA-PS from commercially available hydroxy end-functional polyethylene oxide).4 5 449 Other block copolymers that have been prepared using similar strategies include poly(ethylene-co-butylene)-6/oci-poly(S-eo-MAH), jl poly(ethylene oxide)-block-poly(MMA),440 polyethylene oxide)-Moe -poly(N-vinyl formamide),651 poly(ethylene oxide)-Wot A-poly(NlPAM),651 polyfethylene ox de)-b ock-polyfl,1,2,2-tetrahydroperfluorodecyl acrylate),653 poly(lactic acid)-block-poly(MMA)440 and poly( actic acid)-6focA-poly(NIPAM),4 8-<>54... 

In a series of papers, Matsuda et al. [291-295] employed RAFT-SIP with immobilized benzyl N,N-diethyldithiocarbamate to form polymer brushes from styrene, methacrylamides, acrylamides and acrylates, NIPAM and N-vinyl-2-pyrrolidone on various surfaces. The SIP is initiated by UV irradiation of the surface-bonded dithiocarbamates. Thermoresponsive polymer brushes were prepared by the polymerization of NIPAM and investigated by XPS, wetting experiments and mainly SPM [294]. Patterned polymer brush layers were also prepared. When chloro-methyl styrene was used as a comonomer, RAFT-SIP resulted in branching. By control of the branching, spatio-resolved hyperbranching of a controllable stem/ branch design was realized (Fig. 9.32) [293, 295]. 

N-isopropylacrylamide (NIPAM) monomer (courtesy of Kohjin Ltd) can be purified by re-crystallization in a benzene/n-hexane mixture and azobi-sisobutyronitrile (AIBN) (from Aldrich, analytical grade) can also be purified by re-crystallization. In a typical free-radical polymerization, 18 g NIPAM monomer was first dissolved in 150 mL benzene with 1 mol % of AIBN added as the initiator. The solution mixture was then degassed through three cycles of freezing and thawing. Polymerization was carried out in an oil bath at 56 °C for 30 h under a positive nitrogen pressure. The solvent was removed... 

Comonomer l-vinyl-2-pyrrolidone (VP) comonomer can be purified by distillation at reduced pressure prior to use. Potassium persulfate (KPS) can be purified in a mixture of water and methanol. NIPAM-co-VP copolymers with different amounts of VP can be prepared at temperatures lower or higher than the LCST of PNIPAM by free radical polymerization in water with an initiator of KPS/N,N,N/,N/-tetramethylethylenediamine (TEMED) redox. The resultant copolymer can be harvested by precipitation, i.e., pouring the reaction mixture into an equal volume of methanol. Each resultant copolymer can be further purified by several cycles of re-dissolution in water and precipitation in methanol to ensure a complete removal of residual monomers. The final product can be dried under reduced pressure at 40 °C. 

Hydrophobically modified PNIPAM-seg-St segmented copolymers can be prepared by evenly inserting short styrene segments (stickers) into a PNIPAM chain backbone using the micellar polymerization. In this method, hydrophobic styrene (St) monomers is first solubilized inside small micelles made of surfactant, hexadecyltriethylammonium bromide (CTAB). KPS and TMED can be used to initiate the polymerization of hydrophibc NIPAM monomers dissolved in the continuous aqueous medium. When the free radical end of a growing PNIPAM chain enters a micelle, styrene monomers entrapped inside start to react to form a short hydrophobic segment (sticker). In this way, the coming-in-and-out of different micelles of each free-radical chain end can connect short styrene blocks on a PNIPAM chain. 

In a typical reaction, initial concentrations of NIPAM, styrene, CTAB, KPS, and TMEDA are 0.16 M, 5.24 mM, 17.3 mM, 0.34 mM, 0.67 mM, respectively. The styrene content (3.9 mol %) of the resultant segmented PNIPAM-seg-St copolymer can be determined by pyrolysis gas chromatography. The average degree of polymerization between two styrene segments can be over a wide range, mainly depending on the initial NIPAM/styrene ratio. The resultant copolymer can be purified and fractionated by a number of successive dissolution-and- precipitation cycles in a mixture of extremely dried... 

Thermally responsive polymers, such as poly( V-isopropyl acrylamide) (NI-PAm), have also been studied extensively for applications related to those previously discussed [112], De las Heras et al. described the synthesis and patterning of NIPAm brushes on SAMs and their subsequent performance during temperature-dependent adhesion assays of BSA and Streptococcus mutans (Fig. 7). The authors employed p.CP to pattern features of hydrophobic hexadecanethiol and backfilled the surface with an initiator-functionalized alkanethiol. Polymer brushes were grown via surface-initiated atom transfer radical polymerization (ATRP). FITC-BSA was then... 

Cationically-charged microgel particles Precipitation polymerization of NIPAM or NIPMAM with a crosslinker (MBA) and AEMH 200-1000 nm Cationic surface charge. Swelling capacity dependent on MBA amount [12]... 

Cationic PNIPAM-covered magnetic particles Hetero coagulation of iron oxide nanoparticles onto cationic particles. Encapsulation of preformed particles by polymerization of NIPAM, BAM and AEMH 300-1000 nm Superparamag-netic, bioreactive particles. Temperature, salinity and pH sensitive [10,18,19]... 

FIGURE 12.7 Monomer conversion vs. polymerization time for two initiator concentrations. (Example for poly(NIPAM) particles polymerization temperature T = 70°C, [NIPAM] = 48.51 mmoles, [MBA] = 3 mmol,... 

For example, a = 0.18 for NIPAM/MBA/KPS at 70°C (Figure 12.8), and a = 0.4 in the case of emulsion polymerization. With such a complex system, it is more appropriate to consider the relationship between the number of particles (Np) and the reactants concentration, then ... 

There arc exceptions to this general rule. For example, polymerizations of methacrylates with very bulky ester substituents (1-4) show a marked preference for isotacticity w hcrcas polymerizations of MMA show a significant preference for syndiotacticity (Table 4.1). Polymerization of the acrylamide derivative 5 which has a bulky substituent on nitrogen also provides a polymer that is highly isotactic. " AM and simple derivatives (NIPAM, DMAM) give polymers that are slightly syndiotactic (Table 4.1). Tacticity can be influenced by solvent and... 

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