CHEM514 COPOLYMERS AND POLYMER BLENDS Radical Copolymerization

Effect of Reaction Conditions Generally, monomer reactivity rations are independent of reaction conditions. However, in some cases effect of reaction medium was observed. What are these?

Inhomogeneity of the reaction medium – Emulsion or suspension polymerization Reason: different solubility of the monomers in the micelles or in the dispersing medium – Bulk or solution polymerization Reason: reduced mobility of the monomers due to increased viscosity i.e. gel effect The bulkier monomer is affected more S-MMA polymerization r S (in bulk)< r S (in benzene)

Copolymer formed poorly soluble in the reaction medium – MMA (M1)-NVC (M2) copolymerization r 1 = 1.80r 2 = 0.06in benzene r 1 = 0.57r 2 = 0.75in methanol Copolymer poorly soluble in methanol. Propagating radicals trapped in the precipitating polymer NVC monomer adsorbs on the copolymer formed in methanol Higher fraction of NVC in the copolymer; probability of reaction increased.

pH of the medium Affects ionizable monomers The monomer may exist as a mixture of ionized and unionized forms at given pH values e.g. acrylic acid Reactivity of ionized and unionized forms are different Solvent polarity Concentration Also may affect this type of monomers

Temperature Small effect on r values Reason: Activation energies of radical polymerizations relatively small r 1 =k 11 /k 22 = A 11 /A 22 exp E 12 -E 11 /RT E 12 -E 11 <10 kJ/mol At higher temperatures selectivity decreases; r values tend to unity

Reactivity of a monomer toward a radical depends upon the reactivities of both the monomer and the radical 1/r = reactivity of a monomer k 12 / k11 = rate of reaction of a radical with another monomer rate of reaction of a radical with its own monomer Chemical equation:

1/r values for Various Monomers with MMA radical Butadiene4 Styrene2.2 Acrylonitrile0.82 (0.77) Methylacrylate0.52 Vinylidene chloride 0.39 (0.46) Vinylchloride0.10 Vinylacetate0.050 r values of MMA radical for Various Monomers Butadiene0.25 Styrene0.46 Acrylonitrile1.3 Methylacrylate2.2 Vinylidene chloride 2.4 Vinylchloride9.0 Vinylacetate- BD monomer most reactive; VA monomer least reactive towards MMA radical

Resonance Effects Substituents increase the reactivity of a monomer toward radical attack in the following general order: -Ph, -CH=CH 2 > -CN, -C(O)-R > -C(O)-OH, -C(O)-OR > -Cl > -O-C(O)-R, -R > OR, H S, BD> AN> MA> VC> VA The more resonance stabilized the radical formed from a monomer, the more reactive is the monomer

k 12 Values for Various Monomers towards MMA radical BD S MMA4 180 AN2 510 MA2 090 VC 520 VA230 k 12 Values for Various Radicals towards MMA monomer BDSMMAANMAVCVA The most reactive monomer forms the least reactive radical and vice versa

The interaction of radical reactivity and monomer reactivity in determining the rate of a radical-monomer reaction can be analyzed as follows: The 4 possible reactions between resonance stabilized and non stabilized monomers and radicals: R + MR I R + M s R s II R s + M s R s III R s + MRIV

R s +M > R s +M s > R+M > R+M s Decreasing order of activation energy The energy difference between R s and R is much bigger than the energy difference between M s and M The reverse order is true for the rate constants R s +M < R s +M s < R+M < R+M s Higher activation energy = slower reaction

Copolymerization will occur primarily between Two monomers without stabilizing substituents or Two monomers with stabilizing substituents R+M R R s +M s R s

The combination of a monomer with a stabilizng substituent and a monomer without stabilizng substituent: A combination of reactions II and IV R+M s R s fast R s +MRvery slow e.g. styrene –vinylacetate copolymerization unfavorable