Free radical polymerization poly modifiers

Emulsion Adhesives. The most widely used emulsion-based adhesive is that based upon poly(vinyl acetate)—poly(vinyl alcohol) copolymers formed by free-radical polymerization in an emulsion system. Poly(vinyl alcohol) is typically formed by hydrolysis of the poly(vinyl acetate). The properties of the emulsion are derived from the polymer employed in the polymerization as weU as from the system used to emulsify the polymer in water. The emulsion is stabilized by a combination of a surfactant plus a coUoid protection system. The protective coUoids are similar to those used paint (qv) to stabilize latex. For poly(vinyl acetate), the protective coUoids are isolated from natural gums and ceUulosic resins (carboxymethylceUulose or hydroxyethjdceUulose). The hydroHzed polymer may also be used. The physical properties of the poly(vinyl acetate) polymer can be modified by changing the co-monomer used in the polymerization. Any material which is free-radically active and participates in an emulsion polymerization can be employed. Plasticizers (qv), tackifiers, viscosity modifiers, solvents (added to coalesce the emulsion particles), fillers, humectants, and other materials are often added to the adhesive to meet specifications for the intended appHcation. Because the presence of foam in the bond line could decrease performance of the adhesion joint, agents that control the amount of air entrapped in an adhesive bond must be added. Biocides are also necessary many of the materials that are used to stabilize poly(vinyl acetate) emulsions are natural products. Poly(vinyl acetate) adhesives known as "white glue" or "carpenter s glue" are available under a number of different trade names. AppHcations are found mosdy in the area of adhesion to paper and wood (see Vinyl polymers). 

Summary Graft copolymers with poly(organosiloxane) backbone and thermoplastic side chains have been synthesized via the "grafting fi om" method based on azo- and triazene modified poly(organosiloxane)s. Initiation of free radical polymerization is possible from the polymeric azo and triazene initiators after thermal decomposition of the labile frmctions in solution. The graft products have been characterized by NMR, GPC, and DSC. Stable, free standing films can be cast from the graft copolymers. 

In a quite different but very important industrial area, free-radical polymerizations have made great inroads In the optimization of the desired commercial properties of impact-modified poly(vinyl chloride) (PVC). In a most sophisticated variation, grafted impact modifiers based on the quaterpolymerization of acrylic esters, butadiene, styrene, and acrylonitrile have been produced and almost precisely match the refractive index of PVC. The blending of the Impact modifier with PVC yields a completely clear polymer suitable for shampoo bottles and food containers. In addition to excellent clarity these polymers have extremely good impact strength combined with improved fabricability by flow molding equipment. 

Fluorescence microscopy was used, for instance, to gain spatially resolved information on the modification of ETFE with a sulfonated brush. The brush was prepared by free radical polymerization following the irradiation of the substrate with EUV radiation in an interference setup. Fluorescently labeled poly(allylamine) was deposited on the brush-modified surface and specifically adsorbed on sulfonated regions by electrostatic interactions between sulfonate and amino groups [6]. Observation of the surface clearly showed increased fluorescence intensity in brush-modified regions (Figure 5.5). 

In the free radical polymerization of vinyl acetate, CH2=CH(OOCCH3), on the other hand, there are weak dipole-dipole interactions between the ester groups in the transition state, which facilitates an occasional attack on the a-carbon atom despite steric hindrance by these groups. Poly(vinyl acetate) therefore contains l%-2% head-to-head structures, that is, the ajP orientation ratio is 0.01-0.02. The orientation ratio depends on the attacking species, as well as on the nature of the attacked monomer (Table 15-3). The attack is even almost exclusively at the a position with certain initiators, as, for example, in the copolymerization of butadiene and propylene with certain modified Ziegler catalysts. 

Although the free-radical polymerization of MMA typically exhibits a syndiotactic bias (rr triad content = 60%-70%), it has long been known that the stereochemical interactions between the chain-end radical and vinyl monomers in free-radical polymerization can be modified by using chiral protecting groups. For example, the 80 °C AIBN-initiated polymerization (AIBN = 2,2 -azobisisobutyronitrile) of oxazolidine acrylamides based on valine and t rt-leucine ultimately yields highly isotactic (92% m dyad content) poly(acrylic acid) and PMA after chemical modification (Scheme 23.23). " ... 

The polymers produced through emulsion free radical polymerization are synthesized from modified alkenes (eg, styrene, methyl methacrylate, methyl acrylate, butyl acrylate, vinyl acetate) and are characterized by the presence of C—C bonds that bind the monomers used. As a result, the obtained materials can be biocompatible [as for poly(methyl methacrylate)], but they are not biodegradable [9]. 

Sen et al. [75] prepared PS nanocomposites by free radical polymerization in the presence of organically modified MMT with low-molecular weight quartemized poly(styrene-b-4-vinylpyridine) (SVP) (Table 3.6). Clay modification was carried out in different compositions of THF and water. Copolymer intercalation and thermal stability of the resulting organoclays depended on the THF/water proportion. Greater distances were obtained... 

A recent report describes the in situ polymerization of MMA on modified cadmium sulfide (CdS) nanocrystals (NCs) along with copolymers using poly(methacryloxypropyltrimethoxysi-lane) (PMPS). The terminal functional groups on the surface were then cross-linked by free radical polymerization to form CdS NC-polymer networks. This work has been continued with incorporation of poly(styrene)-co-poly(methactylic acid) microspheres by surfactant-free emulsion polymerization. 

Very recently, Yu et al reported the synthesis of a series of PVA/MMT nanocomposites via in-situ intercalative polymerization with AIBN as initiator. Organic vinyl acetate monomers were first intercalated into the organically modified MMT galleries and followed by a one-step free radical polymerization. The prepared poly (vinyl acetate)/OMLS solution were then saponified via direct hydrolysis with NaOH solution to form PVA/MMT nanocomposites. The synthesized nanocomposites were characterized by FTIR, XRD, SEM, OPM, and TEM. XRD patterns and TEM images estabhshed the formation of mixed intercalated/exfoliated structure of the PVA/MMT nanocomposites. 

Resin-modified glass—ionomer lining and restorative materials add a multifunctional acidic monomer to the poly(acryhc acid) [9003-01 Hquid component of the system. Once the glass powder and Hquid are mixed, setting can proceed by the acid—glass—ionomer reaction or the added monomer can be polymerized by a free-radical mechanism to rapidly fix the material in place (74,75). The cured material stiH retains the fluoride releasing capabiHties of a glass—ionomer. 

Recently, biodiesel has been used as a solvent in free radical-initiated polymerization reactions (Figure 5.8). It should be noted that in contrast to polymerization reactions in some other green solvents, including SCCO2, there is no need to modify the initiator for reactions in biodiesel. All the resulting polymers except poly(methyl methacrylate) were soluble in the biodiesel. Lower molecular weights were obtained compared with conventional polymerization... 

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