Describe the stage of polymerization through which a phenolic resin passes to become thermoset.

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Chemistry

Describe the stage of polymerization through which aphenolic resin passes to become thermoset.

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In aqueous solution formaldehyde exists in equilibrium withmethylene glycol Depending on the pH of the catalyst these monomers react to formone of two general resin types NOVOLAC RESINS and RESOLRESINSNovolac ResinsAn acidic catalyst and a molar excess of phenol to formaldehydeare conditions used to make novolac resins The followingsimplified chemistry illustrates the wide range of polymerspossible The initial reaction is between methylene glycol andphenolThe reaction continues withadditional phenol and splitting off of water The reaction creates a methylene bridge at either the orthoposition or the para position of the phenolic aromatic rings Therule of thumb is that the para position is approximately twice asreactive as the ortho position but there are twice as many orthosites two per phenol molecule so the fractions of orthoorthoparapara and orthopara bridges are approximately equalBranching occurs because reaction can occur at any of three siteson each ring As the reaction continues the random orientationsand branching quickly result in an extremely complex mixture ofpolymers of different sizes and structures The See Answer

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